Advances in contact angle, wettability and adhesion. Volume 4 9781119592549, 1291301321, 1119592542, 9781119593294, 1119593298

Table of contents :
Cover......Page 1
Title Page......Page 5
Copyright Page......Page 6
Contents......Page 7
Preface......Page 15
1.1 Introduction......Page 19
1.2 Theoretical Background......Page 21
1.3.1 Materials......Page 23
1.3.2 Experimental Apparatus and Procedures......Page 24
1.4 Results and Discussion......Page 25
References......Page 33
2.1 Introduction......Page 35
2.2 Intrinsic Wetting Properties of REOs......Page 38
2.3 Nanoscale Approach to Measuring Wettability......Page 43
2.4 On the Nature of Wettability of van der Waals Heterostructures......Page 46
2.5 Summary......Page 51
References......Page 52
3 Wettability of Wood Surfaces with Waterborne Acrylic Lacquer Stains Modulated by DBD Plasma Treatment in Air at Atmospheric Pressure......Page 59
3.1 Introduction......Page 60
3.2.2 Plasma Treatment......Page 61
3.2.3 Contact Angle (CA) Measurements and Surface Free Energy (SFE) Determination......Page 62
3.2.5 Application of Coatings on Sample Surfaces......Page 63
3.2.9 Cross-Cut Test......Page 64
3.3.2 Spreading of Colored Water Droplets on Untreated and Plasma Treated Wood Surfaces......Page 65
3.3.4 Contact Angles of Primer and Topcoat......Page 68
3.3.5 Adhesion Strength Determined by the Pull-Off Test Method......Page 70
3.4 Summary and Conclusions......Page 71
References......Page 72
4.1 Introduction......Page 75
4.2 Apparent Surface Free Energy Determination......Page 76
4.2.3 Equilibrium Contact Angle Approach......Page 77
4.3.1 Materials......Page 78
4.4.1 Surface Topography......Page 79
4.4.2 Contact Angle Measurements......Page 83
4.5 Conclusions......Page 88
References......Page 89
5 Determination of the Surface Free Energy of Solid Surfaces: Can the Best Model be Found......Page 91
5.1.1 Zisman Critical Surface Tension......Page 92
5.1.2 Neumann’s Method......Page 93
5.1.3 van Oss, Chaudhury and Good Approach......Page 95
5.1.4 Chen and Chang Model......Page 98
5.2.1 Statistical Methods......Page 100
5.2.2 Dalal’s Data......Page 103
5.3.1 Fittting of PVC Data......Page 104
5.3.2 Fitting of PMMA Data......Page 106
5.3.3 Assessing Which Model is Best......Page 110
5.4 Summary and Conclusions......Page 113
References......Page 114
6.1 Introduction......Page 117
6.2.1 vOCG Equation......Page 118
6.2.2 Contact Angle Measurements......Page 120
6.3.3 Capillary Rise Method......Page 122
6.3.5 Heat of Immersion Method......Page 123
6.4.1 Heat of Immersion......Page 124
6.4.2 Contact Angles......Page 125
6.4.3 Talc Surface Free Energy and Its Components......Page 128
6.5 Summary and Conclusions......Page 130
References......Page 131
7 Determination of the Surface Free Energy of Skin and the Factors Affecting it by the Contact Angle Method......Page 133
7.1 Introduction......Page 134
7.2 Experimental......Page 136
7.2.3 Preparation of Test Liquids for the Surface Free Energy Analysis of In Vivo and Ex Vivo Skin......Page 138
7.2.4 Determination of SFE of In Vivo and Ex Vivo Skin using the SFECA Method......Page 139
7.2.6 Determination of the Epidermic Hydration State by the SFECA Method......Page 141
7.3 Results and Discussion......Page 143
7.3.1 Determination of the SFE of Ex Vivo Skin by the SFECA Method......Page 144
7.3.1.1 Comparison between Surface Free Energy and Corneometric Data for the In Vivo Skin Hydration State Evaluation......Page 147
7.3.1.2 Determination of the Hydration State of In Vivo Skin......Page 148
7.3.2 Characterization of SFE, DC and PC of In Vivo Skin by the SFECA Method......Page 150
7.3.3 Determination of SFESKIN and Applicability of TVS Skin Test by the SFECA Method......Page 153
7.4 Summary and Conclusions......Page 157
References......Page 159
8 Determination of Surface Tension Components of Aqueous Solutions Using Fomblin HC/25 Perfluoropolyether Liquid Film as a Solid Substrate......Page 163
8.1 Introduction......Page 164
8.2 Materials Used......Page 169
8.4 Determination of Surface Free Energy (SFE)......Page 171
8.4.1 Determination of Surface Free Energy (SFE) of PermaFoam......Page 172
8.4.2 Determination of Surface Tension (ST) of MilliQ Water......Page 173
8.4.3 Determination of Surface Tension (ST) of Aqueous Solutions in DW......Page 176
8.4.3.1 Sodium Chloride Solutions......Page 178
8.4.3.2 Glycerol Solutions......Page 180
8.4.3.3 Sucrose Solutions......Page 181
8.4.3.4 Ternary Sugar Solutions......Page 185
8.5 Analysis of Correlations......Page 188
8.6 Summary and Conclusions......Page 189
List of Abbreviations......Page 192
References......Page 193
9 Enhancing the Wettability of Polybenzimidazole (PBI) to Improve Fuel Cell Performance......Page 197
9.1 Introduction......Page 198
9.2.3 X-Ray Photoelectron Spectroscopy (XPS)......Page 199
9.2.6 Thermal Gravimetric Analysis (TGA)......Page 200
9.3.1.1 XPS Quantitative Analyses and Contact Angle Measurements......Page 201
9.3.1.2 XPS Chemical State Analysis......Page 202
9.3.2 Surface Topography of PBI Treated with O Atoms......Page 203
9.3.3 TGA Analysis of PBI Samples Treated with O Atoms and Doped with H3PO4......Page 204
9.4 Discussion......Page 206
Acknowledgments......Page 207
References......Page 208
10.1.1 Long-Wear Foundation......Page 211
10.1.2 Wetting and Spreading......Page 213
10.2.2 Rheology of Foundation Samples......Page 214
10.2.4 Contact Angle Measurements......Page 215
10.3.1 Rheology of Foundation Samples......Page 216
10.3.2 Surface Roughness......Page 218
10.3.3 Surface Free Energy of Bio Skin Substrate and Foundation Films......Page 221
10.4 Contact Angles of Foundations with Water......Page 225
10.5 Contact Angles of Foundations with Sebum......Page 227
10.6 Effect of Sebum on Color Transfer and Film Integrity......Page 232
10.7 Summary and Prospects......Page 233
References......Page 235
11.1 Introduction......Page 241
11.2 Theoretical Background......Page 242
11.3.3 Surface Free Energy (SFE) Analysis......Page 246
11.3.6 Adhesion Tension Relaxation (ATR)......Page 247
11.4.1 Static Contact Angles and SFE Analysis......Page 248
11.4.3 Dynamic Contact Angle Hysteresis......Page 250
11.4.4 Adhesion Tension Relaxation (ATR)......Page 251
11.4.5 Peel Force......Page 253
11.5 Conclusion......Page 254
References......Page 255
12 The Potential of Surface Nano-Engineering and Superhydrophobic Surfaces in Drag Reduction......Page 257
Greek Letters......Page 258
12.1 Introduction......Page 259
12.2 Parameters Affecting the Slip Length......Page 264
12.3 Slip Length Measurement on Superhydrophobic Surfaces......Page 267
12.4.1 Wettability Parameters......Page 268
12.4.2.1 Turbulent Structure......Page 269
12.5 Effect of Superhydrophobicity on External Flow......Page 270
12.5.2 Bluff Body......Page 271
12.5.3 Superhydrophobic Streamline Body......Page 272
12.5.4 Partial Superhydrophobicity of NACA 0012 Hydrofoil......Page 273
References......Page 276
13 Laser Surface Engineering of Polymeric Materials for Enhanced Mesenchymal Stem Cell Adhesion and Growth......Page 285
13.1 Introduction......Page 286
13.2 Mesenchymal Stem Cells (MSCs)......Page 287
13.3 Poly(ether ether ketone)......Page 291
13.4 Laser Surface Engineering......Page 292
13.4.1 Laser-Induced Surface Patterning......Page 293
13.4.2 Pulsed Laser Deposition of Polymeric Biomaterials......Page 294
13.5 CO2 Laser Surface Engineering of Poly(ether ether ketone)......Page 295
13.5.1 Material Selection and Laser Surface Engineering......Page 296
13.5.2 Surface Roughness, Topography and Wettability Characteristics Analysis......Page 298
13.5.3 Surface Chemical Properties......Page 299
13.5.4 In Vitro Cell Experimentation......Page 300
13.6 Effects of CO2 Laser Surface Engineering on Surface Parameters of Poly(ether ether ketone)......Page 301
13.7 Effects of CO2 Laser Surface Engineering on Mesenchymal Stem Cell Response to Poly(ether ether ketone)......Page 303
13.8 Poly(ether ether ketone) and other Polymers as Bio-Composite Materials......Page 304
References......Page 308
14.1 Introduction......Page 317
14.2 Sustainable ‘Green’ Composites......Page 319
14.3 Sisal Fiber Composites......Page 320
14.4 Fiber/Resin Interface......Page 321
14.4.1 Sisal/Green Resin Interface Strength......Page 323
14.5 Modification of Cellulosic Fibers for Enhancing Fiber/Resin Interfacial Bonding......Page 325
14.6 Summary......Page 329
References......Page 330
Index......Page 337
EULA......Page 346

Citation preview

Advances in Contact Angle, Wettability and Adhesion Volume 4

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Adhesion and Adhesives: Fundamental and Applied Aspects The topics to be covered include, but not limited to, basic and theoretical aspects of adhesion; modeling of adhesion phenomena; mechanisms of adhesion; surface and interfacial analysis and characterization; unraveling of events at interfaces; characterization of interphases; adhesion of thin films and coatings; adhesion aspects in reinforced composites; formation, characterization and durability of adhesive joints; surface preparation methods; polymer surface modification; biological adhesion; particle adhesion; adhesion of metallized plastics; adhesion of diamond-like films; adhesion promoters; contact angle, wettability and adhesion; superhydrophobicity and superhydrophilicity. With regards to adhesives, the Series will include, but not limited to, green adhesives; novel and high-performance adhesives; and medical adhesive applications. Series Editor: Dr. K.L. Mittal P.O. Box 1280, Hopewell Junction, NY 12533, USA Email: [email protected] Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Advances in Contact Angle, Wettability and Adhesion Volume 4

Edited by

K.L. Mittal

This edition first published 2019 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2020 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 978-1-119-59254-9 Cover image: K.L. Mittal Cover design by Russell Richardson Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines Printed in the USA 10 9 8 7 6 5 4 3 2 1

Contents Preface

xiii

1 Contact Angle Determination of Talc Powders from Heat of Immersion Ismail Yildirim and Roe-Hoan Yoon 1.1 Introduction 1.2 Theoretical Background 1.3 Experimental 1.3.1 Materials 1.3.2 Experimental Apparatus and Procedures 1.4 Results and Discussion 1.5 Summary References

1 3 5 5 6 7 15 15

2

17

Surface Wetting at Macro and Nanoscale Meenakshi Annamalai, Saurav Prakash, Siddhartha Ghosh, Abhijeet Patra and T. Venkatesan 2.1 Introduction 2.2 Intrinsic Wetting Properties of REOs 2.3 Nanoscale Approach to Measuring Wettability 2.4 On the Nature of Wettability of van der Waals Heterostructures 2.5 Summary References

3 Wettability of Wood Surfaces with Waterborne Acrylic Lacquer Stains Modulated by DBD Plasma Treatment in Air at Atmospheric Pressure Jure Žigon, Marko Petrič and Sebastian Dahle 3.1 Introduction 3.2 Materials and Methods 3.2.1 Materials

1

17 20 25 28 33 34

41 41 43 43 v

vi

Contents 3.2.2 Plasma Treatment 3.2.3 Contact Angle (CA) Measurements and Surface Free Energy (SFE) Determination 3.2.4 Spreading Area Determination 3.2.5 Application of Coatings on Sample Surfaces 3.2.6 Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy 3.2.7 Confocal Laser Scanning Microscopy 3.2.8 Pull-Off Adhesion Strength of the Coatings 3.2.9 Cross-Cut Test 3.3 Results and Discussion 3.3.1 Contact Angles and Surface Free Energy 3.3.2 Spreading of Colored Water Droplets on Untreated and Plasma Treated Wood Surfaces 3.3.3 Surface Roughness 3.3.4 Contact Angles of Primer and Topcoat 3.3.5 Adhesion Strength Determined by the Pull-Off Test Method 3.3.6 The Results of the Cross-Cut Tests 3.4 Summary and Conclusions Acknowledgements References

4 Wettability of Ultrafiltration Membranes Konrad Terpiłowski, Małgorzata Bielska, Krystyna Prochaska and Emil Chibowski 4.1 Introduction 4.2 Apparent Surface Free Energy Determination 4.2.1 Contact Angle Hysteresis Approach 4.2.2 Neumann Equation-of-State Approach 4.2.3 Equilibrium Contact Angle Approach 4.2.4 van Oss, Chaudhury and Good Approach 4.3 Experimental 4.3.1 Materials 4.3.2 Methods 4.4 Results and Discussion 4.4.1 Surface Topography 4.4.2 Contact Angle Measurements 4.5 Conclusions References

43 44 45 45 46 46 46 46 47 47 47 50 50 52 53 53 54 54 57

57 58 59 59 59 60 60 60 61 61 61 65 70 71

Contents 5

Determination of the Surface Free Energy of Solid Surfaces: Can the Best Model be Found Frank M. Etzler 5.1 Introduction 5.1.1 Zisman Critical Surface Tension 5.1.2 Neumann’s Method 5.1.3 van Oss, Chaudhury and Good Approach 5.1.4 Chen and Chang Model 5.2 The Present Study 5.2.1 Statistical Methods 5.2.2 Dalal’s Data 5.3 Data Analysis 5.3.1 Fittting of PVC Data 5.3.2 Fitting of PMMA Data 5.3.3 Assessing Which Model is Best 5.4 Summary and Conclusions References

6 Surface Free Energy Characterization of Talc Particles Ismail Yildirim and Roe-Hoan Yoon 6.1 Introduction 6.2 Theoretical Background 6.2.1 vOCG Equation 6.2.2 Contact Angle Measurements 6.3 Experimental 6.3.1 Talc Samples 6.3.2 Liquids 6.3.3 Capillary Rise Method 6.3.4 Thin Layer Wicking Method 6.3.5 Heat of Immersion Method 6.4 Results and Discussion 6.4.1 Heat of Immersion 6.4.2 Contact Angles 6.4.3 Talc Surface Free Energy and Its Components 6.5 Summary and Conclusions References 7 Determination of the Surface Free Energy of Skin and the Factors Affecting it by the Contact Angle Method Davide Rossi and Antonio Bettero 7.1 Introduction

vii

73 74 74 75 77 80 82 82 85 86 86 88 92 95 96 99 99 100 100 102 104 104 104 104 105 105 106 106 107 110 112 113 115 116

viii

Contents 7.2 Experimental 7.2.1 Method for Preparation of Ex Vivo Skin 7.2.2 Preparation of Liposomal Dispersion by the Bettero/Gazzaniga Method 7.2.3 Preparation of Test Liquids for the Surface Free Energy Analysis of In Vivo and Ex Vivo Skin 7.2.4 Determination of SFE of In Vivo and Ex Vivo Skin using the SFECA Method 7.2.5 Evaluation of the Epidermic Hydration State by Corneometric Approach 7.2.6 Determination of the Epidermic Hydration State by the SFECA Method 7.2.7 Correlation Analyses and Mathematical Means 7.3 Results and Discussion 7.3.1 Determination of the SFE of Ex Vivo Skin by the SFECA Method 7.3.1.1 Comparison between Surface Free Energy and Corneometric Data for the In Vivo Skin Hydration State Evaluation 7.3.1.2 Determination of the Hydration State of In Vivo Skin 7.3.2 Characterization of SFE, DC and PC of In Vivo Skin by the SFECA Method 7.3.3 Determination of SFESKIN and Applicability of TVS Skin Test by the SFECA Method 7.4 Summary and Conclusions Acknowledgments References

8 Determination of Surface Tension Components of Aqueous Solutions Using Fomblin HC/25 Perfluoropolyether Liquid Film as a Solid Substrate D. Rossi, S. Rossi and N. Realdon 8.1 Introduction 8.2 Materials Used 8.3 Fomblin HC-25 Perfluoropolyether Liquid Film Preparation (Solid-Like Methodology) 8.4 Determination of Surface Free Energy (SFE) 8.4.1 Determination of Surface Free Energy (SFE) of PermaFoam

118 120 120 120 121 123 123 125 125 126

129 130 132 135 139 141 141

145 146 151 153 153 154

Contents 8.4.2 Determination of Surface Tension (ST) of MilliQ Water 8.4.3 Determination of Surface Tension (ST) of Aqueous Solutions in DW 8.4.3.1 Sodium Chloride Solutions 8.4.3.2 Glycerol Solutions 8.4.3.3 Sucrose Solutions 8.4.3.4 Ternary Sugar Solutions 8.5 Analysis of Correlations 8.6 Summary and Conclusions 8.7 Acknowledgements List of Abbreviations References 9 Enhancing the Wettability of Polybenzimidazole (PBI) to Improve Fuel Cell Performance Katerine Vega, Matthew Cocca, Han Le, Marc Toro, Anthony Garcia, Andrew Fleischer, Alla Bailey, Joel Shertok, Michael Mehan, Surendra K. Gupta and Gerald A. Takacs 9.1 Introduction 9.2 Experimental 9.2.1 Materials 9.2.2 Production of O Atoms 9.2.3 X-Ray Photoelectron Spectroscopy (XPS) 9.2.4 Contact Angle Goniometry 9.2.5 Atomic Force Microscopy (AFM) 9.2.6 Thermal Gravimetric Analysis (TGA) 9.3 Results and Discussion 9.3.1 XPS Analysis 9.3.1.1 XPS Quantitative Analyses and Contact Angle Measurements 9.3.1.2 XPS Chemical State Analysis 9.3.2 Surface Topography of PBI Treated with O Atoms 9.3.3 TGA Analysis of PBI Samples Treated with O Atoms and Doped with H3PO4 9.4 Discussion 9.5 Conclusions Acknowledgments References

ix

155 158 160 162 163 167 170 171 174 174 175 179

180 181 181 181 181 182 182 182 183 183 183 184 185 186 188 189 189 190

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Contents

10 Evaluation of Sebum Resistance for Long-Wear Face Make-Up Products Using Contact Angle Measurements Hy Si Bui, Mariko Hasebe and Jody Ebanks 10.1 Introduction 10.1.1 Long-Wear Foundation 10.1.2 Wetting and Spreading 10.2 Experiments 10.2.1 Foundation Samples and Bio Skin Plate 10.2.2 Rheology of Foundation Samples 10.2.3 Surface Roughness 10.2.4 Contact Angle Measurements 10.3 Results and Discussion 10.3.1 Rheology of Foundation Samples 10.3.2 Surface Roughness 10.3.3 Surface Free Energy of Bio Skin Substrate and Foundation Films 10.4 Contact Angles of Foundations with Water 10.5 Contact Angles of Foundations with Sebum 10.6 Effect of Sebum on Color Transfer and Film Integrity 10.7 Summary and Prospects Acknowledgements References 11 Contact Angle Hysteresis of Pressure Sensitive Adhesives due to Adhesion Tension Relaxation Naoto Shiomura, Takashi Sekine and Dehua Yang 11.1 Introduction 11.2 Theoretical Background 11.3 Experimental 11.3.1 Preparation of Samples and Experimental Conditions 11.3.2 Static Contact Angle Measurement 11.3.3 Surface Free Energy (SFE) Analysis 11.3.4 Dynamic Contact Angle as a Function of Time 11.3.5 Dynamic Contact Angle Hysteresis with the Wilhelmy Plate Method 11.3.6 Adhesion Tension Relaxation (ATR) 11.3.7 Peel Force Measurement 11.4 Results and Discussion 11.4.1 Static Contact Angles and SFE Analysis 11.4.2 Dynamic Contact Angle as a Function of Time

193 193 193 195 196 196 196 197 197 198 198 200 203 207 209 214 215 217 217 223 223 224 228 228 228 228 229 229 229 230 230 230 232

Contents 11.4.3 Dynamic Contact Angle Hysteresis 11.4.4 Adhesion Tension Relaxation (ATR) 11.4.5 Peel Force 11.5 Conclusion References 12 The Potential of Surface Nano-Engineering and Superhydrophobic Surfaces in Drag Reduction Ali Shahsavari, Amir Nejat and Seyed Farshid Chini Nomenclature Greek Letters Subscripts Superscript 12.1 Introduction 12.2 Parameters Affecting the Slip Length 12.3 Slip Length Measurement on Superhydrophobic Surfaces 12.4 Drag Reduction of Superhydrophobic Surfaces 12.4.1 Wettability Parameters 12.4.2 Reynolds Number and Shear Rate 12.4.2.1 Turbulent Structure 12.5 Effect of Superhydrophobicity on External Flow 12.5.1 Flat Plate 12.5.2 Bluff Body 12.5.3 Superhydrophobic Streamline Body 12.5.4 Partial Superhydrophobicity of NACA 0012 Hydrofoil 12.6 Conclusion References 13 Laser Surface Engineering of Polymeric Materials for Enhanced Mesenchymal Stem Cell Adhesion and Growth D.G. Waugh, D. Cosgrove, I. Hussain and J. Lawrence 13.1 Introduction 13.2 Mesenchymal Stem Cells (MSCs) 13.3 Poly(ether ether ketone) 13.4 Laser Surface Engineering 13.4.1 Laser-Induced Surface Patterning 13.4.2 Pulsed Laser Deposition of Polymeric Biomaterials 13.4.3 Laser-Induced Surface Chemistry Modification 13.5 CO2 Laser Surface Engineering of Poly(ether ether ketone) 13.5.1 Material Selection and Laser Surface Engineering

xi 232 233 235 236 237 239 240 240 241 241 241 246 249 250 250 251 251 252 253 253 254 255 258 258 267 268 269 273 274 275 276 277 277 278

xii

Contents

13.6 13.7 13.8 13.9

13.5.2 Surface Roughness, Topography and Wettability Characteristics Analysis 13.5.3 Surface Chemical Properties 13.5.4 In Vitro Cell Experimentation Effects of CO2 Laser Surface Engineering on Surface Parameters of Poly(ether ether ketone) Effects of CO2 Laser Surface Engineering on Mesenchymal Stem Cell Response to Poly(ether ether ketone) Poly(ether ether ketone) and other Polymers as Bio-Composite Materials Summary References

280 281 282 283 285 286 290 290

14 Sisal-Green Resin Interfaces in Green Composites A. N. Netravali 14.1 Introduction 14.2 Sustainable ‘Green’ Composites 14.3 Sisal Fiber Composites 14.4 Fiber/Resin Interface 14.4.1 Sisal/Green Resin Interface Strength 14.5 Modification of Cellulosic Fibers for Enhancing Fiber/Resin Interfacial Bonding 14.6 Summary References

299

Index

319

299 301 302 303 305 307 311 312

Preface In the Preface to Volume 3, I had drawn the attention of readers to three important and interesting topics: (1) The seminal papers published by A.B.D. Cassie and S. Baxter (1924) and R.N. Wenzel (1936) had their “Awakening” only in 2002 and 2003, respectively, after a very long hiatus. In modern times, these classical papers have garnered a huge number of citations, and are cited in almost every publication on the effect of surface structure on wetting behavior; (2) Many and variegated applications (some unique and intriguing) of contact angle technique; and (3) social implication of contact angle method. The Preface to Volume 2 pointed out that the study of contact angles had become quite prestigious and glamorous as 5 Noble Laureates had evinced interest in the study investigation of contact angle/wettability phenomena directly or indirectly and the work and tremendous contribution of Prof. Pierre-Gilles de Genes was specifically mentioned. Although much progress has been made, and it continues unabated, in the domain of wetting (wettability) but there is lack of consensus and there have been exothermic discussions regarding the following perennial questions: (1) What is the best and most appropriate method to measure contact angle? (2) What contact angle (equilibrium, advancing, receding) should be measured? (3) Is there an equilibrium contact angle on a real surface? (4) What contact angle should be used in determining the surface free energy (and its dispersion and polar components) as well as the acid-base characteristic of solid surfaces? (5) Should the geometric-mean approach (also known as the Owens-Wendt, Kaelble, or Owens-Wendt-Rabel-Kaelble) be used to determine the dispersion and polar components of surface free energy? The same applies to harmonic-mean approach. In this context the term “polar” is fraught with confusion and different connotations and many researchers feel that the term “polar component” has received much flak and they feel that it should be abandoned. Just to illustrate, Dr. F.M. Fowkes (a doyen of surface science) lamented on various occasions that the geometric-mean approach for polar components was not only wrong xiii

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Preface

but it was “illegal.” Also, on one occasion he remarked “Oh my God, the geometric means was bad enough and somebody extended to harmonic mean! Also, he took umbrage when people called the geometric-mean approach to polar component as the Extended Fowkes Equation. There is no problem with using the geometric-mean approach for dispersion components, but he abhorred its application to polar components. Also, here is another question: Is a superhydrophobic surface innately a self-cleaning surface? There has been much discussion on this topic. Hopefully, in the future these questions will be answered in a satisfactory manner and will generate consensus within the surface science community. Now coming to this volume which comprises 14 articles written by active and eminent researchers. The book is divided into three parts: Part 1: Contact Angle and Wettability Aspects; Part 2: Surface Free Energy and Surface Tension Determination; and Part 3: Applied Aspects. The topics covered include: contact angle determination of talc powders; surface wetting at macro and nanoscale; wettability of plasma treated wood surfaces; wettability of ultrafiltration membranes; discussion of various models to determine the surface free energy of solid surfaces with the hope to find the best model; determination of surface free energy of skin and factors affecting it; determination of surface tension components of aqueous solutions using liquid film as a solid substrate; wetting of polybenzimidazole (PBI) and fuel cell performance; evaluation of sebum resistance of make-up products using contact angle measurements; contact angle hysteresis of pressure-sensitive adhesives; potential of surface nano-engineering and superhydrophobic surface in drag reduction; laser surface engineering of polymeric surfaces to enhance cell adhesion; sisal-green resin interfaces in green composites. The chapters included in this book are primarily based on presentations made at the Eleventh International Symposium on Contact Angle, Wettability and Adhesion held at the Stevens Institute of Technology in Hoboken, NJ, June 13–15, 2018 under the aegis of MST Conferences. However, the authors were strongly urged to render their manuscripts more general and review in nature and scope. It should be recorded that all manuscripts were rigorously reviewed, revised (some twice or thrice) and properly edited before inclusion in this volume. Apropos. some manuscripts did not pass muster. So, the material presented in this book is of archival value and meets the highest standard of publication. I sincerely hope that the current Volume 4 will be received as warmly as its predecessors. As a matter of fact, to me personally it as been a satisfying experience to bring out books in this series as I have received many nice comments testifying that these the books have served a very useful

Preface

xv

purpose to researchers in this arena. This book should be very valuable to anyone interested in staying abreast of the latest exciting developments, which are happening unabatedly, and perspectives in the Wide World of Contact Angle, Wettability and Adhesion. Further, I hope the information consolidated in this volume will fuel further research and will serve as a font of new research ideas. Now comes the pleasant task of thanking all those who made this book possible. First and foremost, my sincere and heart-felt thanks go to the authors for their interest, enthusiasm, cooperation and sharing their valuable research results in the form of written accounts, without which this book would not have seen the light of day. Next I extend my thanks to Martin Scrivener (publisher) for his whole-hearted interest in this book as well as for supporting the initial idea of publishing this series of books. Kash Mittal P.O. Box 1280 Hopewell Jct., NY 12533 E-mail: [email protected] August 2019

1 Contact Angle Determination of Talc Powders from Heat of Immersion Ismail Yildirim and Roe-Hoan Yoon* Department of Mining and Minerals Engineering Virginia Polytechnic Institute and State University Blacksburg, VA, USA

Abstract Many industrial processes rely on controlling the wettability of particulate materials as in flotation. It is difficult, however, to measure the contact angles of powdered samples. To overcome this problem, a method of measuring heats of immersion in water and using the data to calculate water contact angles has been developed. The new method has been tested successfully with a series of talc samples, and the results are compared with those obtained using the capillary rise method. The contact angles measured using the heats of immersion method are more reproducible than the capillary rise method based on using the Washburn equation. The results obtained with the talc samples show that measured contact angles increase with decreasing particle size, indicating that under stress the layer-structured mineral particles tend to break preferentially along the basal surfaces that are naturally hydrophobic. Thermodynamic analysis of the data obtained in the present work shows that the Gibbs free energy of immersion (ΔGi) may be a more sensitive measure of wetting than the enthalpy of immersion (ΔHi) and the contact angles (θ) by themselves. Keywords: Heat of immersion, free energy of immersion, contact angle, capillary rise, wetting, talc

1.1 Introduction Many industrial processes depend on controlling the hydrophobicity of the solids involved. These include flotation, wetting, filtration, adhesion, *Corresponding author: [email protected] K.L. Mittal (ed.) Advances in Contact Angle, Wettability and Adhesion: Volume 4, (1–16) © 2020 Scrivener Publishing LLC

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Advances in Contact Angle, Wettability and Adhesion

etc. The most commonly used measure of hydrophobicity is water contact angle (θ). It can be readily measured by placing a drop of water on the surface of a solid of interest, and measuring the angle through the aqueous phase at the three-phase contact. In using this method, known as the sessile drop technique, it is necessary that the solid surface be flat and smooth. To meet these requirements, a mineral specimen is cut by a diamond saw and polished with an abrasive powder such as alumina. It is possible, however, that the mineral surfaces, particularly those of sulfide minerals, may undergo significant chemical changes and atomic rearrangements during polishing. Therefore, it would be more desirable to measure contact angles directly on powdered samples. In many cases, the solids of interest exist only in powdered form, in which case the sessile drop technique cannot be used for contact angle measurements. For powdered samples, capillary rise technique is widely used [1–3]. In this method, a powdered solid is packed into a glass tube, one end of which is subsequently immersed into a liquid of known surface tension (γLV). The liquid will rise through the capillaries formed in between the particles within the glass tube. The distance l traveled by the liquid as a function of time t is recorded. If the mean radius r* of the capillaries present in the tube is known, one can calculate the contact angle using the Washburn equation [4]:

l

2

LV

r *t cos , 2

(1.1)

where η is the liquid viscosity. One can determine r* with a liquid which completely wets the powder, i.e., θ=0. One problem with this technique is the uncertainty associated with determining r*. There is no guarantee that the value of r* determined with a completely wetting liquid is the same as that determined by a less than completely wetting liquid such as water. Also, the method of using the Washburn equation gives advancing angles rather than equilibrium angles. The Washburn equation is also used in thin layer wicking method [5, 6]. In this technique, a powdered sample is deposited on the surface of a glass slide and dried. One end of the slide coated with dry powder is immersed in water, and the rate at which the water rises along the height of the slide is measured. The contact angle of a powdered sample can also be measured by compressing it into a pellet. The measured values may vary depending on the roughness and porosity of the pellet. There is also a concern that the

Contact Angle Determination of Talc Powders 3 particles in the top layer of a pellet may be deformed during compression, which may also affect the measurement [7]. Another method of determining the contact angles of powders is to measure the heat of immersion in water. In this technique, a powdered sample is degassed to remove the pre-adsorbed water and then immersed in water [8, 9]. In general, the more hydrophobic a solid is, the lower the heat of immersion. Thus, one should be able to obtain the values of contact angles from the heats of immersion in water. Different investigators use different methods of calculating contact angles from the heat of wetting. Some of the methods reported in the literature used gross assumptions, which may be the source of inaccuracy in determining water contact angles. In the present work, an improved method of determining the contact angles of powdered materials has been developed. It is based on measuring the heats of immersion in water using a flow microcalorimeter and calculating contact angles using a rigorous thermodynamic relation. The method was tested on talc samples of different particle sizes.

1.2 Theoretical Background In the present work, a flow microcalorimeter was used to measure the heat effect (hi) created when a powdered sample was immersed in water. By dividing hi with the total surface area of the sample used in the experiment, one obtains the heat of immersion (-ΔHi) given in units of mJ/m2. In the wetting experiment, a powdered sample was dried by evacuation prior to immersion. Under this condition, the free energy (ΔGi) of immersion can be given by the following relation,

ΔGi

SL

(1.2)

SV

where γSL is the solid-liquid interfacial tension and γSV is the surface free energy of the solid, which is in equilibrium with its own vapor. The enthalpy of immersion (ΔHi) can be related to ΔGi as follows,

ΔHi

ΔGi T

dΔGi dT

(1.3) p

where T is the absolute temperature. Substituting Eq. (1.2) into Eq. (1.3), one obtains,

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Advances in Contact Angle, Wettability and Adhesion

ΔHi

SL

T

SV

d

SL

SV

(1.4)

dT

p

By substituting (γSL-γSV) with -γLVcosθ in accordance with the Young’s equation, Eq. (1.4) can be rewritten as follows,

ΔHi

LV

LV

cos

cos

cos

(

T

cos ) T

LV

LV

T

p

LV

T

cos T

LV

T cos

LV

p

T

LV

T

p

cos T

(1.5) p

, p

where θ is contact angle. Since the enthalpy of a liquid (HL) is given by

HL

LV

T

LV

T

,

(1.6)

p

Eq. (1.5) can be reduced to,

ΔHi

H L cos

LV

T

cos T

.

(1.7)

p

Solving Eq. (1.7) for cosθ, one obtains the following relation,

cos

1 HL

LV

T

cos T

ΔHi p

(1.8)

Contact Angle Determination of Talc Powders 5 which is a first-order differential equation with respect to cosθ. By multiplying -γLV to cosθ, one obtains ΔGi. Thus, the thermodynamic derivation given above pertains essentially to the conversion of ΔHi to ΔGi. Eq. (1.8) may be used to obtain the value of θ from the value of experimentally determined ΔHi. However, there are no analytical solutions. Numerical solutions are possible, but it is necessary to know the contact angle value at a particular temperature. Due to these mathematical difficulties, Malandrini, et al. [8] assumed that ΔGi=0.5ΔHi for talc. For very hydrophobic solids, Yan, et al. [9] and Spagnolo et al. [10] assumed that dγSV/dT = 0.07 mJm2deg-1 and dγSL/dT = 0. The approach taken in the present work is to solve Eq. (1.8) for θ from the values of ΔHi and ∂cosθ/∂T. The values of HL and γLV are readily available in literature. The values of ΔHi can be readily obtained from heat of immersion experiments, while those of ∂cosθ/∂T can be obtained using independent methods of determining contact angles. There are several ways of determining temperature coefficient of cosθ. First, one measures θ on polished talc samples as a function of temperature and determine ∂cosθ/∂T experimentally. An assumption made here is that although contact angle may change when the sample is pulverized, its temperature coefficient may remain the same. Second, the contact angle of a powdered sample is measured by pressing it into a pellet. Again, the pressed talc sample may have a different contact angle from that of loose powders. However, its temperature coefficient may be assumed to remain the same. Third, the contact angles of powdered samples are measured using the capillary rise technique. This technique gives advancing rather than equilibrium contact angles. If one uses this technique to determine ∂cosθ/∂T, an implicit assumption is that the temperature coefficients of the equilibrium and the advancing angles are the same.

1.3 Experimental 1.3.1 Materials A run-of-mine (ROM) talc sample from Montana was received from Luzenac America, Greenwood Village, CO. The purity of the talc sample was greater than 98%. It was crushed to finer than 50 mm using a handheld hammer. One part was kept for contact angle measurements using the sessile drop and the Wilhelmy plate techniques on flat surfaces, while the other part was ground using an agate mortar and pestle and screened to obtain -75+53 μm size fraction. Here the negative sign refers to ‘smaller

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Advances in Contact Angle, Wettability and Adhesion

than’ and the positive sign refers to ‘larger than.’ This size fraction was used for i) heat of immersion measurements using a flow microcalorimeter, and ii) contact angle measurements using the capillary rise technique. A number of powdered talc samples of different trade names were also obtained from Luzenac America. Due to proprietary reasons, these commercial products are referred to as Samples A, B, C, and D in this communication. The samples were used as received. All heat of immersion measurements were conducted using the Nanopure water produced from a Barnsted Nanopure II water purification system.

1.3.2 Experimental Apparatus and Procedures Heat of immersion measurements were conducted using a flow microcalorimeter from Microscal, United Kingdom. Figure 1.1 shows a schematic diagram of the calorimeter. A calorimeter cell, made of Teflon, was placed in a metal block, which was insulated from the ambient by mineral wool. Two glass-encapsulated thermistors were placed inside the cell to monitor Liquid from syringe pump

Teflon cell Metal block Teflon block Calibration coil Thermistors

Thermistors

Porous Teflon membrane

Talc powder Seals

To vacuum

Downstream

Figure 1.1 Schematic representation of the Microscal flow microcalorimeter used for the measurement of heats of immersion of talc powders.

Contact Angle Determination of Talc Powders 7 the changes in temperature of the sample, and two reference thermistors were placed in the metal block outside the cell. The calorimeter was calibrated by means of a calibration coil, which was placed in the sample bed. The entire unit was housed in a draft-proof enclosure to minimize the effect of temperature fluctuations in the ambient. In each measurement, a talc sample was dried overnight in an oven at 110oC. A known amount (usually 0.05 to 0.15 g) of the dried sample was placed in the calorimeter cell, and degassed for at least 30 minutes under vacuum ( 90o the heat effect becomes endothermic. Spagnolo et al. [10] actually showed that the heats of immersion of the two fluorinated hydrocarbon powders with water contact angles of 120o and 125o were endothermic.

1.5 Summary A method of determining water contact angles on powdered samples has been developed. It is based on measuring the heats of immersion, and calculating the contact angles using rigorous thermodynamic relations. The new method was tested on a series of talc samples from Luzenac America. The results are comparable to those obtained using the capillary rise method. It was found that the calorimetric methods gave more reproducible results. It was found also that the water contact angles of the talc samples increased with decreasing particle size, which may be attributed to the preferential exposure of the hydrophobic basal surface during breakage. A simple thermodynamic analysis suggests that the free energy of immersion is a more sensitive measure of particle hydrophobicity or wetting than the enthalpy of immersion. It is shown also that wetting is enthalpic rather than entropic, with the enthalpy change being associated with the H-bonding of water with the surface functional groups of a solid.

References 1. Bruil, H.G. and van Aartsen, J.J., The determination of contact angles of aqueous surfactant solutions on powders. Colloid Polym. Sci., 252, 32–38, 1974. 2. Hansford, D.T., Grant, D.J.W., Newton, J.M., Surface energetics of the wetting of a hydrophobic powder. J. C. S. Faraday Trans. I, 76, 2417–2431, 1980.

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3. Crawford, R., Koopal, L.K., Ralston, J., Contact angles on particles and plates. Colloids Surfaces, 27, 57–64, 1987. 4. Washburn, E.W., The dynamics of capillary flow. Phys. Rev., 1, 273–283, 1921. 5. van Oss, C., Giese, R.F., Li, Z., Murphy, K., Norris, J., Chaudhury, M.K., Good, R.J., Determination of contact angles and pore sizes of porous media by column and thin layer wicking. J. Adhesion Sci. Technol., 6, 413–428, 1992. 6. Wu, W., Griese, R.F., van Oss, C., Change in surface properties of solids caused by grinding. Powder Technol., 89, 129–132, 1996. 7. Neumann, A.W. and Good, R.J., Techniques of measuring contact angles, in Surface and Colloid Science, vol. 11, R.J. Good and R.R. Stromberg (Eds.), pp. 31–91, Plenum, New York, 1979. 8. Malandrini, H., Clauss, F., Partyka, S., Douillard, J.M., Interactions between talc particles and water and organic solvents. J. Colloid Interface Sci., 194, 183–193, 1997. 9. Yan, N., Maham, Y., Masliyah, J.H., Gray, M.R., Mather, A.E., Measurement of contact angles for fumed silica nanospheres using enthalpy of immersion data. J Colloid Interface Sci., 228, 1–6, 2000. 10. Spagnolo, D.A., Maham, Y., Chuang, K.T., Calculation of contact angle for hydrophobic powders using heat of immersion data. J Phys. Chem., 100, 6626–6630, 1996. 11. Adamson, A.W., Physical Chemistry of Surfaces, 5th ed., John Wiley and Sons, New Jersey, USA, 1990. 12. Weast, R.C. and Astle, M.J. (Eds.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1981.

2 Surface Wetting at Macro and Nanoscale Meenakshi Annamalai, Saurav Prakash, Siddhartha Ghosh, Abhijeet Patra and T. Venkatesan* NUS Nanoscience and Nanotechnology Initiative (NUSNNI), National University of Singapore, Singapore

Abstract Understanding the interaction of water molecules with the surfaces of different materials has become an important field of research. A large body of theoretical and experimental research work has been dedicated to understanding how interfacial water interacts with the surface structure/chemistry of materials. The underlying mechanism on which many applications are based is, in fact, the interaction of materials with liquids. This necessitates further investigations on wetting dynamics of materials. Motivated by the gaps in the knowledge in this domain, we have performed a systematic study of water contact angle and surface free energy determination on less explored oxide thin-films and two-dimensional (2D) van der Waals structures ranging from exotic rare-earth oxides (REOs) to graphene, MoS2, WS2 and their hetero-structures. Besides these macroscale measurements, we also show an atomic scale approach to locally probe the wetting properties of materials. Keywords: Thin films, rare-earth oxides, two-dimensional materials, surface wetting

2.1 Introduction Water/solid interfaces are an exciting field of research which has widespread implications ranging from everyday phenomena to advanced scientific and technological processes, such as protective coatings [1], self-cleaning surfaces [2], lubricants [3], micro-fluidics [4] and heterogeneous catalysis [5]. Transition metal oxides (TMOs) have been studied extensively due to

*Corresponding author: [email protected] K.L. Mittal (ed.) Advances in Contact Angle, Wettability and Adhesion: Volume 4, (17–40) © 2020 Scrivener Publishing LLC

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Advances in Contact Angle, Wettability and Adhesion

their exotic surface and interface properties [6–8]. Their thin-films can be readily fabricated using numerous techniques [6]. Many studies have been undertaken to explore and understand reversible wettability or enhancement of hydrophilicity/hydrophobicity at water/oxide (e.g. ZnO and TiO2) interfaces via ultraviolet (UV) and visible light illumination [9, 10], electro-wetting [11], structural modifications [12], thermal treatment [13] and laser irradiation [14]. Modification of wetting properties by variation of electronic structure of TMO thin-film surfaces has been the central theme in this field of research [14–17]. Other efforts aimed at understanding the enhanced hydrophilicity in TMOs, like ZnO and TiO2, have attributed it to removal of organic contaminants [18], increase of surface roughness [19], and change of surface chemical composition/hydroxyl content [20]. Photo-induced reversible wettability has gained significant attention due to its potential impact in practical applications. A number of studies have reported UV and visible light induced hydrophilicity/hydrophobicity in TiO2 [21], SnO2 [22], ZnO [23], WO3 [24] and V2O5 [25]. An interesting observation by Miyauchi et al. highlights that SrTiO3 does not show any evidence of photo-induced hydrophilicity unlike TiO2 [26] whose surface atomic structure closely resembles that of SrTiO3. This finding clearly indicates that wetting properties of an oxide surface strongly depend on the electronic structure at the surface, and not only on chemical property or morphology of surface [26]. Along with the study of water contact angle (WCA) on transition metal oxide (TMO) surfaces, the wettability behaviour of rare-earth oxide (REO) surfaces [27–29] has attracted significant attention recently. Hydrophobic nature of the entire series of REO surfaces was observed for the first time by Azimi et al. which opened up possibilities for their use in transportation, hydropower systems and aircraft engines, etc [28]. Hydrophobic coatings have important implications in drop-wise condensation [30]. Hydrophobicity of noble metals due to hydrocarbon adsorption has been used to promote drop-wise condensation for enhanced heat transfer applications. REOs being intrinsically hydrophobic (as reported) would serve as a low-cost alternative to noble metals in this regard. Following the initial work on REOs, several conflicting reports on non-wetting mechanisms of REOs were published [31–33]. We have performed a systematic study to explore the intrinsic wetting properties of REO thin films which has been discussed below. The thin film fabrication facility at NUSNNI-Nanocore was utilized to prepare ultra-smooth, phase pure (consisting of a single phase), single crystalline thin films of metal oxides [transition metal (TMO) as well as rare-earth (REO)] using Pulsed Laser Deposition (PLD) technique (refer to Figure 2.1). This enabled us to explore the fundamental aspects of

Surface Wetting at Macro and Nanoscale

19

Substrate Carousel

RHEED electron gun

E-beam RHEED CCD

LASER

Plume

(λ= 24

8 nm)

Focal lens

p um

P To Gas exhaust N2 Ar O2

Target Carousel Gas inlet

Figure 2.1 Schematic of a typical PLD system.

interfacial interaction at the thermodynamic phase boundary (of liquid droplet and solid metal oxide surface) free from any chemical (hydrocarbon contamination) or physical (roughness) imperfections. After the successful synthesis of the high Tc superconducting yttrium-barium-copper oxide (YBa2Cu3O7 or YBCO) on sapphire by laser ablation from a stoichiometric YBCO target (in 1986 at Bellcore) [34], PLD has become an exotic tool for the growth of oxide thin films [35, 36], hetero-structures [37, 38] and well-controlled interfaces [7, 8]. PLD is a physical vapour deposition thin film growth technique where a high power pulsed laser beam (energy density ~0.6–2.0 J/cm2) is focussed inside a vacuum chamber onto the target of the material to be deposited [39]. The laser pulse causes “ablation” producing a transient, highly luminous plasma plume that expands rapidly away from the target surface leading to film deposition on a substrate. The short pulse duration reduces thermal heating of the target which minimizes unintentional thermal evaporation from target, thereby maintaining the stoichiometry of the target material in the plume. PLD is thus a promising route for fabricating high quality stoichiometric thin films of complex materials. In order to bridge the knowledge gaps in this field of research, we have realized that a systematic approach is extremely crucial for meaningful scientific conclusion [40–42]. Our previous studies on oxide thin films have

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shown the effect of surface hydrocarbon contamination, surface roughness, oxygen vacancy and effect of substrate on WCA [41, 42]. We have also explored the wetting properties of van der Waals (vdW) based single and hybrid structures which is extremely important from both fundamental and application perspectives. Previous studies have mostly focussed on electronic properties and device applications of these materials and there were no reports on wetting properties of 2D heterostructures when we first reported [43]. We have also recently showcased the nano-scale correlation of the macro-scale WCA [40] – all of these research works have been discussed at greater length in the later part of this article and our findings have opened up new possibilities and challenges in this field of research.

2.2 Intrinsic Wetting Properties of REOs Hydrophobicity of REO ceramics has been a subject of debate since it was first reported by Azimi et al. in 2013 [44]. In this study, WCA measurements were done on pellets of rare-earth oxides (from ceria to lutecia) prepared by solid-state reaction route. This study proposed the REO ceramics to be intrinsically hydrophobic due to the unique electronic configuration of rare-earth cation (fully filled octet 5s2 5p6 shielding the empty 4forbitals) which deters hydrogen bonding with water molecule. However, the contact angle hysteresis (CAH) for all the pellets was around~50°. Such high values of CAH are expected as the measurements were done on bulk polycrystalline ceramic pellets of REOs, which most likely have inherent chemical and physical heterogeneities (polycrystallinity, grain boundaries and pores), high surface roughness and potential atmospheric contamination. Moreover, X-ray photoelectron spectroscopy (XPS) measurements were performed on ceramic samples after Ar+ sputtering which is a known method to remove hydrocarbon contamination from the surface. The presence of hydrocarbon overlayer on almost every surface including several oxides has been studied extensively [45–48]. Hydrocarbon overlayer cannot be overlooked as several studies have shown that less than a monolayer thick hydrocarbon contamination makes the surface of gold hydrophobic [46]. The schematic shown in the reported article [46] is hence misleading, as it shows direct interaction between the surface and water molecule, completely ignoring the hydrocarbon overlayer. The prospect of a robust intrinsic hydrophobic surface has captivated the wetting community and several investigations have followed [49]. Zenkin et al. did similar study on a host of oxide and nitride sputtered thin films of low electronegativity metal (including rare earth) [50]. They reported that the low Lewis acidity

Surface Wetting at Macro and Nanoscale

21

of the low electronegativity rare-earth cations caused intrinsic hydrophobicity. Another study performed on Atomic Layer Deposition (ALD) grown thin films of REOs also showed them to have hydrophobic behavior [51]. A contradicting study by Preston et al. followed these early reports, which explored the temporal wettability of REO ceramic pellets (similar to Azimi et al. [44]) after Ar plasma etching (known method to remove hydrocarbon contamination) [52, 53]. Using temporal XPS and WCA measurements, a clear correlation between the hydrocarbon contamination and increasing WCA was observed. The authors proposed that REOs are intrinsically hydrophilic (immediately after Ar plasma etching WCA ~ 0°) but the adsorption of hydrocarbon contaminants on the surface progressively makes it hydrophobic. Khan et al. argued surface stoichiometry and not hydrocarbon overlayer decides the wettability of REOs [27]. Using XPS, they showed O/Ce ratio was higher immediately for sputtered CeO2 films but relaxed to the ideal value when stored in a clean vacuum environment. This change in surface stoichiometry (O/Ce ratio) causes the surface to be hydrophilic to start with but becomes hydrophobic eventually. Azimi et al. also reported fabrication of super-hydrophobic surface (WCA=160°) from hydrophobic ceria pellet (WCA=102°) by the technique of laser texturing [29]. In order to clearly understand the intrinsic nature of REOs, we have performed a systematic study on thin films of REOs on lattice matched Yttria-stabilized Zirconia (YSZ) substrates prepared by PLD. In short, most of our thin films are single crystalline, epitaxial, ultra-smooth and stoichiometric as shown in Figure 2.2(a–b). Samples were then stored in a vacuum desiccator and static WCA measurements were performed after a few days (~6–7 days) [refer to Figure 2.2(c)]. The measured static WCAs for most of the REO films were found to be on the higher side but still lower than the technical hydrophobic bar of 90° (except for Dy2O3). In a different set of experiments, we studied the evolution of WCA of the REO thin films prepared by PLD immediately after removing them from the vacuum system. To contrast the behaviour of REOs with the transition metal oxides we also repeated the same for TiO2 single crystalline epitaxial thin film. The initial WCAs for both REOs (Lu2O3, Er2O3 and Dy2O3) and TiO2 thin film were very low (hydrophilic) ~20° and 10° respectively. These increased over weeks to saturation values of ~80° and 60° respectively. The temporal evolution behaviour of WCA is similar for both REOs and TiO2 (representative d-block oxide) as shown in Figure 2.2(d). The only difference is that the surface of TiO2 saturates at a lower WCA than REOs. This suggests that the process of lowering of surface free energy on exposure to ambient (dry cabinet) is identical but the extent of it is larger in the case of REOs. In addition to this, we also found no correlation of WCA with

Advances in Contact Angle, Wettability and Adhesion

RMS roughness (in nm)

(a) 10

2.00 1.75

Roughness Factor RMS roughness

8

1.50

6 1.25

4 1.00

2

0.75

0

1.0

WCA (in degree)

WCA (in degree)

(d)

60 40

0.4 IDEAL EXPT.

0.2

2 unit cell 10 unit cell 50 unit cell

60

80

0.6

O3 Lu 2 O3 2 Yb 3 O 2 Tm 3 O Er 2 O3 2 Ho 3 O 2 Dy 3 O 2 Gd O3 Eu 2O 3 2 Sm O2 Ce

100

0.8

0.0

0.50 O3 Lu 2 O3 2 Yb O3 2 Tm O3 Er 2 O3 2 Ho O3 2 Dy O3 2 Gd O3 Eu 2 O3 2 Sm O2 Ce

(c)

(b) Roughness Factor Elemental O in REO

22

60

40

40

20

20

20

TiO2 WCA 0

0 O3 Lu 2 O3 2 Yb O 3 2 Tm O3 Er 2 3 O 2 Ho O3 2 Dy 3 O 2 Gd O3 Eu 2O 3 2 Sm O2 Ce

0

0

0

150

300 450

150 300 450 Time (in hours)

600

750

600

Figure 2.2 (a) AFM studies showing roughness factor [Y-axis RHS] (as well as RMS roughness [Y-axis LHS]) of all Re2O3 thin-films grown by PLD. We found roughness factor values very close to 1.00 for all our PLD grown thin-film samples indicating atomically smooth sample surfaces. (b) Composition analysis of all the Re2O3 thinfilms grown by PLD using Rutherford Backscattering Spectrometry (RBS). The y-axis shows elemental O in REO system, i.e. O:(RE+O). (c) WCA measurements done on RE2O3 thin-films grown by PLD after a lapse of > 120 hour (5 days) showing technically hydrophilic behaviour in all cases. (d) Temporal evolution of WCA of 2, 10 and 50 unitcell thick Lu2O3 (001) on YSZ (001) and TiO2 (001) on SrTiO3 (001) [inset] (Adapted from Ref. [42]).

film thickness (Lu2O3 film of about 2 unit cells (~2nm thickness) was prepared using reflection high-energy electron diffraction (RHEED)) [refer to Figure 2.2(d)]. To characterize the wetting behavior of surfaces, both static and dynamic contact angles were measured. The static contact angle (θs) is the angle at which the contact area between liquid and solid is not changed externally, whereas dynamic contact angle is measured during wetting (advancing angle, θa) or de-wetting (receding angle, θr). Dynamic contact angles can either be measured by changing droplet volume, causing the droplet to expand (θa)/contract (θr) or by using a tilting cradle. The

Surface Wetting at Macro and Nanoscale

23

surface is tilted from 0° to 90° and the values are extracted when advancing angle (downhill) approaches a maximum value, and the receding angle (uphill) approaches a minimum value. The significance of dynamic contact angle (advancing and receding) has been extensively studied and the difference between the two arises from surface roughness and/ or heterogeneity. On smooth surfaces, the experimental advancing contact angle θa might be expected to be a good approximation of θs [54, 55]. Whereas on rough solid surfaces, there is usually no correlation between θa and θs. Our samples shown in this study are ultra-smooth phase-pure rare-earth oxides which is quite evident from the roughness factor [Figure 2.2(a)]. Therefore, static contact angle measurements can be chosen to measure contact angle when its variation as a function of time needs to be studied. However, to further demonstrate our rationale behind the choice of static contact angle measurement, we obtained advancing and receding contact angles of Dy2O3 using the tilt base method [Figure 2.3(a)]. The reason behind choosing Dy2O3 is that its roughness factor is on the higher side compared to most of the other rare-earth oxides we have studied. Although Dy2O3 shows a roughness factor higher than most of our other rare-earth oxides, the overlapping values of θa and θs from the graph above [Figure 2.3(a)], clearly show that the sample is very smooth and thus θs could be a good approximation of θa for all our samples. CAH was very low and static WCA was equal to advancing WCA for all time points [Figure 2.3(a)]. In addition to the conventional WCA measurement, the polar component of surface free energy was indirectly determined using the pull-off force

WCA (in degrees)

80

(a)

Dy2O3 Thin-film Sample

(b-i)

WCA – 73.9º

60

40 (b-ii) 20

0

WCA – 43.6º

Static CA Advancing CA Receding CA 0

30

60 90 120 150 180 Time (in hours)

Figure 2.3 (a) Temporal evolution of WCA of Dy2O3 thin-film surfaces (static as well as advancing & receding contact angles). (b) WCA of Lu2O3 thin-film changes from 73.9° (top) to 43.6° (bottom) upon annealing at 300°C. (Adapted from Ref. [42]).

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Advances in Contact Angle, Wettability and Adhesion

measurement approach [40]. The plot of pull-off force also shows a similar evolution with time. However, a faster saturation of pull-off force is seen unlike WCA measurements. We believe that this is due to the water droplet used to measure WCA. The droplet used repeatedly (at different time points) impedes the process of hydrocarbon adsorption, thus delays saturation. Two thin film samples of Lu2O3 were prepared simultaneously. Immediately after removing from vacuum chamber one was immersed in DI water and the other was not and transferred into an x-ray photoelectron spectroscopy (XPS) system. On XPS measurements we found that the hydrocarbon content was about ~2% lower for the sample immersed in DI water which corroborates our suggestion about water impedes hydrocarbon adsorption. We performed temporal XPS measurements to identify the changes on the surface of REO (Lu2O3) thin films. XPS measurements showed the presence of adventitious hydrocarbon and hydroxide species on the surface. The temporal results of XPS measurements are shown in Table 2.1. We have also used Selected-ion-flow-tube mass spectrometry (SIFT-MS) to identify the hydrocarbons adsorbed on the surface. The samples were heated to 300°C and the gases emanating from the surface were monitored in real time. A clear spike in alkanes (ethane, methane and propane) is seen. After this measurement the sample was cooled instantly and WCA was measured. The WCA was almost half the saturation value as shown in Figure 2.3(b). In summary, we have shown that single crystalline REO thin films prepared by PLD are intrinsically hydrophilic and become progressively hydrophobic with hydrocarbon contamination. The stabilized contact angle depends significantly on the nature of adsorbed contamination and hence the ambient. Our findings were corroborated, in parts, by other studies published around the same time [56–58]. A very recent study used DFT calculations and WCA measurements to show that freshly deposited CeO2 and Er2O3 films on Yttria-stabilized Zirconia (YSZ) Table 2.1 C1s, O1s and Lu4d elemental compositions of Lu2O3 thin film using XPS and their temporal evolution. Sample exposure time

C (%)

Lu (%)

O (%)

Oxide (%)

0 min

10.1

39.7

15.1

35.2

40 min

13.6

39.4

13.3

33.7

80 min

16.3

38.0

13.2

32.5

> 5 days

28.2

32.2

16.7

22.9

Surface Wetting at Macro and Nanoscale

25

substrates showed crystallographic dependence. Irrespective of the crystallographic orientation, all films eventually converged to WCA value of 80°, suggesting a non-intrinsic hydrophobicity caused by hydrocarbon contamination [59].

2.3 Nanoscale Approach to Measuring Wettability A limitation of the sessile drop method is the inability to provide meaningful contact angle and surface free energy information from nanoscale spatial regions. This is important for patterned surfaces with dissimilar materials. Evaluating “local” contact angles in these cases would require less than a picolitre droplet which is both difficult to produce and not reliable or reproducible easily. Hence, there is an unaddressed need for a technique which can evaluate local contact angles with nanoscale spatial resolution and avoid the limitations of the sessile drop method. We report, herein, a technique based on pull-off force measurement through an atomic force microscopy (AFM) setup [40]. The probe is first brought from non-interaction regime (showing no deflection of cantilever on detector) and pushed towards the sample (bending of cantilever due to repulsive forces showing positive deflection), and then retracted back to original position. Schematic representations of both nanoscale and macroscale measurement techniques are shown in Figure 2.4(a). This maximum pull-off force obtained from a variety of samples ranging from hydrophilic to hydrophobic (refer to Table 2.2 for sample details) is plotted against different parameters of macroscale contact angle measurements obtained through traditional sessile drop method (refer to Table 2.2) and Figure 2.4(b). It is observed that the maximum pull-off force decreases exponentially with the contact angle. Remarkably, the pull-off force increases exponentially with the polar component of surface free energy. No relation is seen with the dispersion component. These correlations make it possible to predict the universal trend of adhesion force with surface free energy. A very thin layer of water is always found to be condensed between the AFM tip and the surfaces in ambient conditions [62–64]. In prior literature, the resulting capillary forces have been used to account for the pull-off forces [65]. The adhesion forces determined through force spectroscopy, simulations and experimental results on mica surface have consistently found that the pull-off force is incommensurately high compared to the expected capillary forces [62–64, 66]. In general,

Fad = FvdW + Fcap + Fel + Fchem

Advances in Contact Angle, Wettability and Adhesion

26

where Fad is the total adhesion force which is measured through AFM, FvdW is van der Waals force, Fcap is the capillary force, Fel is electrostatic force and Fchem is chemical force (if covalent/ionic bonds need to be broken). Fel is zero because there is no electrical bias applied as well as tip and sample have remained in air for hours [66]. Again, in previous reports, it has been claimed that the surface of the tip and mica being saturated with chemical bonds implies no covalent or ionic bonds are expected to form upon contact [66]. Hence, Fchem is also zero. It has been shown that the maximum possible values of FvdW and Fcap cannot make up for the values of pull-off forces we observe [40]. When this observation is placed in conjunction with the correlation observed between the pull-off force and polar component of surface free energy, it becomes apparent that electrostatic dipole-dipole interaction is the dominant component of the observed force. Hence, we propose that

θ

30

3

45 6 7

30

20

E 1 5 4

15

32

6 7

20 60 0 40 Polar Component of Surface Energy (mJ/m2)

1. Mica 2. Vo2(B) 3. Vo2(M) 4. Hard Disk Media 5. STO 6. LU2O3 7. Lubricated Hard Disk Media

15 10

30

20

5

25

5

0 20 40 60 80 100 120 Contact Angle (degrees)

25

10

Pull-off Force (nN)

2

10 5

Pull-off Force (nN)

1

Pull-off Force (nN)

Pull-off Force (nN)

30

15

γsl

Solid

Native water layer

(b)

20

Liquid

25

7

2 65 4 3

0 20 60 80 40 Total Surface Energy (mJ/m2)

F

1 2

20 15 10 5

1

4 7

3 5 6

0 20 40 Dispersion Component of Surface 2 Energy (mJ/m )

30 25 Mica 20 15 Hard Disk Medium

10

Lubricated Hard Disk Medium

5 0

(d)

0

20

40

60

80

100 120

Contact Angle (degrees)

0.8

Deflection (Volts)

De

γnv

25

Vapour

γlv

Pull-off-Force (nM)

(c)

Push Pull

tec

tor

Laser

(a)

1

0.7 0.6 0.5 0.4

2

0.3

3

0.2 4

0.1

5

6

7

80

100

0.0 0

20

40

60

Contact Angle (degrees)

Figure 2.4 (a) Schematic of an AFM force spectroscopy and macroscale contact angle measurement (right). (b) The variation of pull-off force with measured WCA, total surface free energy, polar and dispersion components of surface free energy estimated by the Owens, Wendt, Rabel and Kaelble (OWRK) model [60, 61] (exponential functions have been fitted and no correlation was observed for pull-of force against dispersion component of surface free energy). (c) Pull-off-force versus water contact angle obtained on mica (0°), hard disk medium (66°), lubricated hard disk medium (107°). (d) Force spectroscopy data on all seven samples using a probe tip from NANOSENSORS (Type PPP-CONT). (Adapted from Ref. [40]).

5.5±0.2

49.3±0.1

0±0

18.4±1.5

25±1.5

52.7±0.2

60.5±0.4

77.8±0.2

101.7±0.3

Mica

VO2 (B)

VO2 (M)

Hard Disk Medium (CoCrPt Magnetic layer)

Strontium Titanate (STO)

Lu2O3

Lubricated Hard Disk Medium

89.0±0.3

63.1±0.3

15.7±0.4

16.0±0.6

20.4±0.6

Water

Label

Ethylene glycol

Contact angle (degrees)

87.7±0.4

49.5±0.4

31.7±0.1

44.0±0.3

20.5±0.6

27.3±0.2

45.7±0.3

Diiodomethane

14.0

33.1

47.4

42.8

60.3

61.6

65.9

Surface free energy total

11.1

28.1

34.8

22.4

27.4

24.0

15.8

Surface free energy dispersion component

2.9

5.0

12.6

20.4

32.9

37.6

50.2

Surface free energy polar component

0.97

0.84

0.99

0.86

0.91

0.90

0.93

Fit to OWRK model

Table 2.2 Contact angles and surface free energies (mJ/m2) of the samples determined through the traditional sessile drop method (Adapted from Ref. [40]).

Surface Wetting at Macro and Nanoscale 27

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Advances in Contact Angle, Wettability and Adhesion

the pull-off force is influenced by the number of hydrogen bonds that the tip needs to break to escape from the condensed water layer. The number of hydrogen bonds at the surface is decided by surface atomic structure as well as chemical nature of surfaces. It has also been known that tip geometry also influences capillary forces [67–69]. Precautions were taken to make sure that the geometry of the tip was not altered during the force spectroscopy measurements and this has been detailed in our earlier published article [40]. The entire study was repeated using probe tips of similar type and the data obtained on several new samples are shown in Figure 2.4(c–d). The data show the same trend as the original results shown in Figure 2.4(b) and the force values are comparable as fortunately the RH values were similar (50–60%), thus indicating the robustness and reproducibility of the methodology adopted. In conclusion, adhesion forces have been demonstrated to be an exponential function of macroscale water contact angle and surface free energy due to strong liquid-solid interaction involved in capillary forces. We can use this knowledge to locally evaluate surface free energies and probe contact angles for nano-patterned surfaces. Conventional water droplet methods would result in obtaining a value which is an average of all local values. Moving forward, it is possible to develop a catalogue of standard curves with various commonly used AFM tips such that it would become possible to predict the expected contact angle of a surface simply by measuring the pull-off force and looking up the corresponding standard curve for that tip material.

2.4 On the Nature of Wettability of van der Waals Heterostructures Initial WCA measurements on epitaxial graphene grown on hydrophilic silicon carbide showed hydrophobicity as well as thickness dependent wettability [70]. Following which, Kim et al. reported WCA of graphene transferred from copper and nickel to SiO2 to be around ~90° [71]. As the 2D systems are dominated by vdW forces, it was demonstrated that graphene is wetting-transparent to copper, gold and silicon [72]. It was also observed to be non-transparent to glass due to short range chemical bonding [72]. The intrinsic wettability of graphene was found to be affected by airborne contaminants by Li et al. and it was observed that when the samples are exposed to ambient conditions, due to hydrocarbon coverage they exhibit hydrophobicity [73]. Static contact angle and dynamic force spectroscopy measurements both indicated hydrophilicity of pristine graphene and eventual hydrophobicity on exposure to ambient conditions. Annealing and UV-O3

Surface Wetting at Macro and Nanoscale

29

treatment was found to be effective in removing adsorbed hydrocarbon species [73–75]. These initial studies on wettability of graphene produced contrasting claims which spurred continued interest in experimental as well as theoretical research to understand the surface – liquid interactions dominated by van der Waals forces in layered 2D nanomaterials which extend beyond just graphene. Recently, there have been few studies on the wetting properties of other 2D materials like graphene oxide, h-BN, transition metal dichalcogenide (TMDC), etc. as well [76–78]. In our work, we have investigated the wettability of graphene and graphene-like layered TMDCs (MoS2 and WS2) as individual and hybrid structures on h-BN and SiO2/Si substrates [Figure 2.5(a–b)] [79]. From earlier reports on wetting of 2D materials we also believe that airborne contaminants have a pronounced impact on wettability and can significantly modify the measured contact angles. It is found that annealing at 400 K recovers the intrinsic surface of graphene/BN system as evident from the zero charge neutrality [80]. It was also reported that 150°C annealed MoS2

80

M oS

2

WS 2 N h-B i /S SiO 2

Contact Angle (degrees)

(a)

(c)

75 70 80

65

75 70

60

65 60 55

55 50

50

0

0

50 100 150 200 250 300

1000 500 Time (min)

(b) (d) Θw=86.3º

WS2/h-BN/SiO2/Si

Θw=66.4º WS2/SiO2/Si

Θw=71.2º

MoS2/h-BN/SiO2/Si

Θw=61.6º MoS2/SiO2/Si

Θw=74.3º

MoS2/WS2/h-BN/SiO2/Si Θw=63.3º MoS2/WS2/SiO2/Si

Θw=89.9º

MoS2

Intensity (arb. Units)

Graphene/h-BN/SiO2/Si Θw=79.1º Graphene/SiO2/Si

1500

300

400

500

600

700

800

900

Raman Shift (cm-1)

Figure 2.5 (a) Schematic of 2D materials heterostructures. (b) WCAs obtained on various fabricated structures. (c) WCA of MoS2/SiO2/Si (thermally annealed at 150ºC) over time. (d) Raman spectrum of MoS2 after 150°C thermal annealing (Adapted from Ref. [43]).

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Advances in Contact Angle, Wettability and Adhesion

samples were very much identical to as prepared samples and the MoS2 was not significantly altered by mild annealing at 150°C [80]. Thus, annealing was performed at 150°C on all 2D single and hybrid structures to remove the adsorbed species before conducting contact angle measurements in a controlled class 10,000 cleanroom with 45% RH condition. As an example, aged MoS2 in a controlled cleanroom environment (up to ~24 hours) did not exhibit change in WCA [refer to inset in Figure 2.5(c)]. The contact angles of our samples were invariably restored over several runs of measurements after this simple thermal annealing to remove any adsorbed molecules on the surface and hence the reproducibility in measurements was ensured. In order to make sure this annealing step did not introduce oxidation of MoS2 and WS2 into MoO3 and WO3, we obtained Raman spectra to provide convincing evidences that this occurrence can be excluded. Any presence of MoO3 should be reflected as a strong Raman peak at ~820 cm-1 (refer to Figure 2.5(d)). From the surface free energy estimations through OWRK model (refer to Tables 2.3 and 2.4), it is found that all the vdW individual and heterostructures are intrinsically dispersive. This is due to the non-polar bonds that exist in these materials and the long-range London dispersion forces. All explored 2D materials in this study invariably possess hexagonal lattice structure [81–83]. Graphene possesses non-polar homonuclear C-C intralayer bonds [81–83]. Analogous to graphene, MoS2 and WS2 belong to the same family of layered TMDCs. They consist of S-X-S sandwiches (X = Mo or W) where S and X atoms are held together by strong covalent bonds which gives in-plane stability for realization in 2D forms and the monolayers are held together by van der Waals coupling [82]. The surface created by the covalent bonds tends to be hydrophilic, whereas the predominant vdW interactions give rise to dispersive nature in these materials [82]. Due to this these systems do not require a non-pairwise additive theory where Keesom (force between two permanent dipoles) and Debye (force between a permanent and a corresponding induced dipole) contributions to polar interactions are significant. The classical approaches based on Lifshitz and Hamaker to London dispersion forces can be applied to these systems instead [84]. However, we observed a small increase in the polar component of the surface free energy when the 2D materials or their heterostructures were transferred onto h-BN. h-BN has highly polar B-N bonds and hence the increase in polar component of surface free energy of the overlayer 2D materials could be attributed to the underlying polar h-BN. From our experiments we understand that the 2D materials are not completely wetting-transparent but there seems to be a considerable amount of influence on their wetting property by the underlying substrate and hence making these partially wetting-transparent. Our study shows that the surface free energy of all 2D layered materials is

SiO2/Si

BN/Cu

2

details

Sample

1

number

Sample

21.2±1.2

48.4±0.9

(degrees)

Water CA

25.9±0.6

16.7±0.1

49.2±0.9

CA (degrees)

(degrees)

43.0±0.1

Diiodomethane

glycol CA

Ethylene

22.2±0.1

58.27

44.68

30.28

24.69

27.99

19.99

0.8597

0.9129

model component component

total

(degrees) 48.10±0.6

OWRK

Fit to dispersion

energy

Surface free energy polar

Surface free

energy

free

Surface CA

Formamide

Table 2.3 Data obtained from wetting measurements with four liquids and calculated surface free energies (mJ/m2) of SiO2/Si and BN/Cu (Adapted from Ref. [43]).

Surface Wetting at Macro and Nanoscale 31

41.00 38.51 41.64

39.19 39.48

Graphene/h-BN/SiO2/Si

MoS2/h-BN/SiO2/Si

WS2/h-BN/SiO2/Si

MoS2/WS2/h-BN/SiO2/Si

Graphene/SiO2/Si

MoS2/SiO2/Si

WS2/SiO2/Si

MoS2/WS2/SiO2/Si

1

2

3

4

5

6

7

8

34.87

34.68

36.69

Sample details

Surface free energy total

Sample number

0.31

5.51

5.87

1.53

8.65

8.48

11.11

3.45

Surface free energy polar component

39.16

33.68

28.99

33.15

32.98

30.03

29.89

33.24

Surface free energy dispersion component

0.9240

0.9067

0.9429

0.9811

0.8556

0.8453

0.8202

0.9607

Fit to OWRK model

Table 2.4 Calculated surface free energies (mJ/m2) of the fabricated samples using the contact angles measured with four different liquids (Adapted from Ref. [43]).

32 Advances in Contact Angle, Wettability and Adhesion

Surface Wetting at Macro and Nanoscale

33

undoubtedly dominated by London dispersion forces with little contribution from dipole-dipole interactions. Following our findings, there has been interest in exploring the wetting properties of 2D materials [85–88]. Wetting-transparency of a few-layer graphene on Si/SiOx and mica substrates has been investigated using atomic force microscopy (AFM) and coordinated molecular dynamics (MD) simulations [85]. Recently, water wettability of graphene using DFT calculations, traditional CA measurement and force of adhesion using AFM have been reported [86]. Belyaeva et al. have demonstrated rupture-free contact angle measurements of water on graphene using ice and hydrogels as substrates [88]. Such substrates provide strong support for the overlayer graphene and at the same time mimic water. The wetting characteristics of millimetre sized suspended graphene have been probed to estimate the intrinsic nature of wettability using captive bubble method [89]. Tunable wettability from hydrophobicity to hydrophilicity and modification of electrochemical performance have been observed in vertically-oriented few-layered graphene through rotary plasma processing [90]. Peng et al. have detailed the influence of airborne hydrocarbons on the properties of 2D materials [91]. Intrinsic hydrophilicity and its implications on graphitic materials and recent developments in this area of research have been highlighted by Liu and Li [87]. All these reports open up further studies in both supported and suspended 2D individual and heterostructures providing an enabling route to extract the intrinsic properties of thin films in general.

2.5 Summary From our experimental findings on wettability of thin films ranging from oxides to 2D materials, it is well understood that a systematic approach is essential to precisely evaluate intrinsic wetting property of surfaces. Our past investigations have demonstrated that surface hydrocarbon contamination and roughness significantly affect the static contact angles measured. Our published reports in this area provide a guideline on the precautions to be taken in estimating the intrinsic wettability of surfaces. We have additionally developed a nanoscale approach to evaluate wetting property through correlation between the force of adhesion and WCA. By using an AFM based technique, we have been able to obtain unambiguous insight into the wetting properties of macro as well as nanopatterned surfaces without altering the surface property of the materials examined. Thus, nanoscale simulations of wetting property surface of surfaces could be scaled to large area WCA measurements.

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46. Smith, T., The hydrophilic nature of a clean gold surface. J. Colloid Interface Sci., 75, 51–55, 1980. 47. Song, W., So, S., Cao, L., Angular-dependent photoemission studies of indium tin oxide surfaces. Appl. Phys. A, 72, 361–365, 2001. 48. Li, Z., Wang, Y., Kozbial, A., Shenoy, G., Zhou, F., McGinley, R., Ireland, P., Morganstein, B., Kunkel, A., Surwade, S.P., Effect of airborne contaminants on the wettability of supported graphene and graphite. Nat. Mater., 12, 925–931, 2013. 49. Tian, Y. and Jiang, L., Wetting: Intrinsically robust hydrophobicity. Nat. Mater., 12, 291–292, 2013. 50. Zenkin, S., Kos, Š., Musil, J., Hydrophobicity of Thin films of compounds of low-electronegativity metals. J. Amer. Ceram. Soc., 97, 2713–2717, 2014. 51. Oh, I.-K., Kim, K., Lee, Z., Ko, K.Y., Lee, C.-W., Lee, S.J., Myung, J.M., Lansalot-Matras, C., Noh, W., Dussarrat, C., Hydrophobicity of rare earth oxides grown by atomic layer deposition. Chem. Mater., 27, 148–156, 2014. 52. Preston, D.J., Miljkovic, N., Sack, J., Enright, R., Queeney, J., Wang, E.N., Effect of hydrocarbon adsorption on the wettability of rare earth oxide ceramics. Appl. Phys. Lett., 105, 011601, 2014. 53. O’Kane, D.F. and Mittal, K.L., Plasma cleaning of metal surfaces. J. Vac. Sci., Technol., 11, 567–569, 1974. 54. Neumann, A., Contact angles and their temperature dependence: Thermodynamic status, measurement, interpretation and application. Adv. Colloid Interface Sci., 4, 105–191, 1974. 55. Lam, C., Wu, R., Li, D., Hair, M., Neumann, A., Study of the advancing and receding contact angles: Liquid sorption as a cause of contact angle hysteresis. Adv. Colloid Interface Sci., 96, 169–191, 2002. 56. Lundy, R., Byrne, C., Bogan, J., Nolan, K., Collins, M.N., Dalton, E., Enright, R., Exploring the role of adsorption and surface state on the hydrophobicity of rare earth oxides. ACS Appl. Mater. Interfaces, 9, 13751–13760, 2017. 57. Fu, S.-P., Rossero, J., Chen, C., Li, D., Takoudis, C.G., Abiade, J.T., On the wetting behavior of ceria thin films grown by pulsed laser deposition. Appl. Phys. Lett., 110, 081601, 2017. 58. Külah, E., Marot, L., Steiner, R., Romanyuk, A., Jung, T.A., Wäckerlin, A., Meyer, E., Surface chemistry of rare-earth oxide surfaces at ambient conditions: Reactions with water and hydrocarbons. Sci. Rep., 7, 43369, 2017. 59. Tam, J., Feng, B., Ikuhara, Y., Ohta, H., Erb, U., Crystallographic orientation–surface energy–wetting property relationships of rare earth oxides. J. Mater. Chem. A, 6, 18384–18388, 2018. 60. Etzler, F.M., Determination of the surface free energy of solids: A critical review. Rev. Adhesion Adhesives, 1, 3–45, 2013. 61. Etzler, F.M., Characterization of surface free energies and surface chemistry of solids, in: Contact Angle, Wettability and Adhesion, vol. 3, K.L. Mittal (ed.) pp. 219–264, CRC Press, Boca Raton, FL, 2003.

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62. Eastman, T. and Zhu, D.-M., Adhesion forces between surfacemodified AFM tips and a mica surface. Langmuir, 12, 2859–2862, 1996. 63. Sedin, D.L. and Rowlen, K.L., Adhesion forces measured by atomic force microscopy in humid air. Anal. Chem., 72, 2183–2189, 2000. 64. Xu, L., Lio, A., Hu, J., Ogletree, D.F., Salmeron, M., Wetting and capillary phenomena of water on mica. J. Phys. Chem. B, 102, 540–548, 1998. 65. Sirghi, L., Nakamura, M., Hatanaka, Y., Takai, O., Atomic force microscopy study of the hydrophilicity of TiO2 thin films obtained by radio frequency magnetron sputtering and plasma enhanced chemical vapor depositions. Langmuir, 17, 8199–8203, 2001. 66. Xiao, X. and Qian, L., Investigation of humidity-dependent capillary force. Langmuir, 16, 8153–8158, 2000. 67. Ando, Y., The effect of relative humidity on friction and pull-off forces measured on submicron-size asperity arrays. Wear, 238, 12–19, 2000. 68. Ata, A., Rabinovich, Y.I., Singh, R.K., Role of surface roughness in capillary adhesion. J. Adhesion Sci. Technol., 16, 337–346, 2002. 69. Biggs, S., Cain, R.G., Dagastine, R.R., Page, N.W., Direct measurements of the adhesion between a glass particle and a glass surface in a humid atmosphere. J. Adhesion Sci. Technol., 16, 869–885, 2002. 70. Shin, Y.J., Wang, Y., Huang, H., Kalon, G., Wee, A.T., Shen, Z., Bhatia, C.S., Yang, H., Surface-energy engineering of graphene. Langmuir, 26, 3798–3802, 2010. 71. Kim, K.S., Lee, H.J., Lee, C., Lee, S.K., Jang, H., Ahn, J.H., Kim, J.H., Lee, H.J., Chemical vapor deposition-grown graphene: The thinnest solid lubricant. ACS Nano, 5, 5107–5114, 2011. 72. Rafiee, J., Mi, X., Gullapalli, H., Thomas, A.V., Yavari, F., Shi, Y., Ajayan, P.M., Koratkar, N.A., Wetting transparency of graphene. Nat. Mater., 11, 217–222, 2012. 73. Li, Z., Wang, Y., Kozbial, A., Shenoy, G., Zhou, F., McGinley, R., Ireland, P., Morganstein, B., Kunkel, A., Surwade, S.P., Li, L., Liu, H., Effect of airborne contaminants on the wettability of supported graphene and graphite. Nat. Mater., 12, 925–931, 2013. 74. Lai, C.-Y., Tang, T.-C., Amadei, C.A., Marsden, A.J., Verdaguer, A., Wilson, N., Chiesa, M., A nanoscopic approach to studying evolution in graphene wettability. Carbon, 80, 784–792, 2014. 75. Kozbial, A., Li, Z., Sun, J., Gong, X., Zhou, F., Wang, Y., Xu, H., Liu, H., Li, L., Understanding the intrinsic water wettability of graphite. Carbon, 74, 218– 225, 2014. 76. Perrozzi, F., Croce, S., Treossi, E., Palermo, V., Santucci, S., Fioravanti, G., Ottaviano, L., Reduction dependent wetting properties of graphene oxide. Carbon, 77, 473–480, 2014. 77. Chow, P.K., Singh, E., Viana, B.C., Gao, J., Luo, J., Li, J., Lin, Z., Elías, A.L., Shi, Y., Wang, Z., Terrones, M., Koratkar, N., Wetting of mono and few-layered

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3 Wettability of Wood Surfaces with Waterborne Acrylic Lacquer Stains Modulated by DBD Plasma Treatment in Air at Atmospheric Pressure Jure Žigon*, Marko Petrič and Sebastian Dahle University of Ljubljana, Biotechnical Faculty, Department of Wood Science & Technology, Jamnikarjeva, Ljubljana, Slovenia

Abstract Better wettability of wood surfaces prior to bonding or coating processes can be achieved by treatments with gas discharges. One of the modification methods to achieve better penetration, spreading and wettability of applied liquid adhesives or coatings on wood surfaces is the treatment with a dielectric barrier discharge (DBD) plasma. Changes caused on wood surfaces are influenced by many internal and external parameters of the discharge process. We used a DBD plasma in ambient air at atmospheric pressure for surface treatment of wood. Special emphasis was placed on the study of water contact angles and evolution of spreading on untreated and treated surfaces. Based on the wettability results, the plasma pre-treatment process was optimized and interactions of the wood substrate with different liquids as well as some selected commercial waterborne acrylic lacquer stain coatings were studied. The results also showed changes in morphological characteristics of untreated and plasma treated wood surfaces. It was shown that plasma treatment increased the wettability of wood, which positively influenced the adhesion strength of the primer coating, but not of the topcoat. Keywords: Wood, DBD plasma, atmospheric air, coatings, adhesion, wettability

*Corresponding author: [email protected] K.L. Mittal (ed.) Advances in Contact Angle, Wettability and Adhesion: Volume 4, (41–56) © 2020 Scrivener Publishing LLC

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3.1 Introduction Wood and other ligno-cellulosic materials can be used for various types of products, for use in indoor or outdoor environments. The material, constituting a product, is exposed to different biotic and abiotic influences from the surroundings [1]. Such influences on unprotected wood initially cause damage only on its surface, but later can deteriorate also the properties of the bulk material. Therefore, when there is no other option for protection (e.g. constructional protection, by placing it under a roof and above the ground), such material has to be additionally surface protected with coatings [2]. There are numerous solvent- as well as waterborne coatings available on the market. The latter are nowadays more favourable, due to ecological reasons. For an adequate surface protection of wood, a good compatibility of the applied coating with the wood material is needed. Usually, commercial coatings are intended to be used as they are, and not to be significantly modified. On the other hand, the surface of wood to be coated can be modified in different ways before finishing process [3]. Indicators for the appropriate application of coatings and their sufficient compatibility with the surface of wood are good wettability, fast spreading on the substrate surface and adequate penetration into the substrate. Inherent properties of wood (e.g. chemical composition, moisture content) and environmental influences (e.g. oxidation in atmospheric air, aging due to exposure to ultraviolet light, pollutants) inactivate the wood surface and thereby render it nonamenable for interactions with waterborne coatings [4, 5]. Furthermore, thermal- and/or chemical modification of wood can render interactions with waterborne coatings more challenging [6]. To achieve better adhesion to wood surfaces, various treatment methods can be used [7, 8]. One method for modification of lignocellulosic materials is the treatment with gas discharges, commonly known as plasmas [9]. These can be ignited at low pressures, high pressures (75 to 7500 bar), or atmospheric pressure, but because of practical and energy consumption reasons, the last one is preferred for the purposes of materials surface treatment [10–12]. For similar reasons, generation of plasma in air at atmospheric pressure is mostly preferred. One of the so-called non-thermal plasmas is the dielectric barrier discharge (DBD) plasma [10] with a variety of embodiments. Here, two electrodes are separated with a defined gap. One is kept at ground potential and the other one is connected to a high voltage source. At least one of the electrodes is covered with a dielectric barrier. By application of high voltage of alternating polarity, electrical

Wettability of Wood Surfaces 43 breakdown occurs in the gap in the form of so-called streamers. In a discharge, a neutral gas is ionized by separation of individual electrons from atoms and gas molecules, thus forming free charge carriers. On exposure of a material to such conditions inside the gap, chemical reactions occur on its surface. Due to bombardment with reactive species, ions, free radicals, and other active species are formed, and thus its surface free energy is increased [13]. From the literature, it is well known that the treatment of surfaces of lignocellulosic materials with plasma prior to the application of liquid agents (paints, varnishes, adhesives) enhances their adsorption and increases their level of adhesion [10, 14]. In this research, we have investigated the wettability property of Norway spruce wood surfaces before and after treatment with DBD plasma in air at atmospheric pressure. Firstly, based on the investigations of colored water spreading performance, the optimal speed of treatment process was determined. The reasons for enhanced wettability after plasma treatment were investigated by determining the chemical and morphological properties of surfaces before and after treatment process. In the second part of the research, plasma treatments of wood were performed before the application of different coating systems. Commercially available waterborne coatings were used for the formation of coating systems: either only the primer, only the topcoat, or both together. Finally, mechanical properties of the cured coatings were investigated and evaluated according to standardized methods.

3.2 Materials and Methods 3.2.1 Materials Norway spruce (Picea abies (L.) Karst.) lamellae with radial orientation and dimensions (300 × 100 × 3) mm3 were prepared, stored and conditioned for 7 days in a standard climate with a relative humidity of 65% and 20°C temperature. An average wood moisture content of 11.2% was achieved. Waterborne coatings used were the primer “Bori impregnation – colorless” and the topcoat “Bori stain UV protection – white” (both from Helios TBLUS d.o.o., Količevo, Slovenia).

3.2.2 Plasma Treatment The dielectric barrier discharge (DBD) setup used for the generation of plasma in air at atmospheric pressure is presented in Figure 3.1. The setup

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Advances in Contact Angle, Wettability and Adhesion Plasma module High voltage electrode

Plasma streamers Wood specimen Metal pad (ground electrode)

High voltage generator

Figure 3.1 Schematic representation of DBD plasma treatment setup for treatment of wood surfaces used in the present research.

consists of a metal pad, where a wood specimen is positioned. The plasma module above the specimen moves at constant velocity. The power supply unit and the high-voltage generator with adjustable electrical voltage, current and frequency provide the electrical current to the plasma module. The high-voltage electrode is located inside a hollow ceramic (Al2O3, wall thickness 2.5 mm) tube. Plasma is ignited between the high-voltage electrode and the metal pad (Figure 3.1). An alternating high-voltage (~13 kV peak) pulse generator with a pulse repetition frequency of 16 kHz was connected to one electrode. The gap distance, i.e. the distance between the sample surface and the electrode, was adjusted to 1  mm. Plasma treatments were carried out after sanding the sample surface and prior to application of coatings.

3.2.3 Contact Angle (CA) Measurements and Surface Free Energy (SFE) Determination In many investigations, it was shown that plasma treatments increased the surface free energy of wood and, therefore, increased its wettability [15]. Better penetration and final adhesion of applied and cured coatings are attributed to better wettability of the treated surfaces [16]. Surface free energy determination by means of contact angle (CA) measurements is one of the techniques for demonstrating and evaluating the efficiency of plasma treatments on surface wettability [17]. Sessile drop CA measurements on untreated and treated surfaces using distilled water, glycerol, and diiodomethane were performed. Droplets were deposited on 10 different places on the sample surface, on the areas with the same visually assessed wood structure as possible (i.e. the proportion of early wood and late wood). After application of a droplet with a volume 5 μL, photographs of deposited droplets were taken after 1, 2, 5, 10, 20, 30 and 45 s, using the DataPhysics (DataPhysics Instruments GmbH, Filderstadt, Germany) contact angle meter. For the determination of average CAs, 10 replicate measurements were performed.

Wettability of Wood Surfaces 45 There are many approaches to use contact angle measurements for determination of SFE [18, 19]. Here the SFE was determined from contact angle measurements using the Owens–Wendt approach on the basis of Young’s equation (3.1) [20].

γS = γSL + γL cosθ,

(3.1)

where γS is the surface free energy of the solid in mJ/m2, γSL is the interfacial free energy between solid and liquid, γL is the surface tension of the liquid, and θ is the contact angle. The total surface free energy (γtot) is divided into a polar component (γP) and a dispersion component (γD), as shown in equation (3.2):

γtot = γD + γP

(3.2)

The following test liquids were used to determine surface free energy [17]: - distilled water (γP = 50.2 mN/m, γD = 22.6 mN/m), - glycerol (γP = 26.4 mN/m, γD = 37.0 mN/m), and - diiodomethane (γP = 0.0 mN/m, γD = 50.8 mN/m).

3.2.4 Spreading Area Determination The course of the droplet’s spreading and area on wood surfaces was observed after application of 5 μL green-colored water. The droplets were deposited on the area, with the same wood structure as possible or the same annual growth. The wetted area and its time evolution were measured with a Leica EZ4D (Leica, Wetzlar, Germany) microscope via top views of the droplets, under 8-fold magnification. The perimeters of the stain were obtained using the Fiji software (ImageJ 1.46d, Bethesda, Maryland, USA). Final droplet spreading area was measured after 24 hours.

3.2.5 Application of Coatings on Sample Surfaces The samples were coated with a commercial state-of-the-art waterborne coating as an example for application in construction and exterior usage. Before every coating application process, the specimen surfaces were sanded with a sandpaper (grit P120) or in the cases of intermediate sanding between the primer and additional topcoat application with a finer sand-paper (P240). The coatings were applied manually with standard

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quadruple coating applicators. The wet coating thickness of the applied primer was 60 μm and of the topcoat was 180 μm.

3.2.6 Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy Chemical changes to sample surfaces due to plasma treatment were assessed by ATR-FTIR spectroscopy using a Bruker Alpha Platinum-ATR spectrometer. The spectra were corrected for the background and the relevant absorption bands were interpreted.

3.2.7 Confocal Laser Scanning Microscopy Three-dimensional surface topographies were determined using a Keyence VK-X200 (Keyence, Osaka, Japan) confocal laser-scanning microscope with VK-X200K controller. The images and measurements were obtained at 500 × magnification. A laser with a wavelength of 408 nm was used for the illumination.

3.2.8 Pull-Off Adhesion Strength of the Coatings Adhesion of the coating systems on both plasma treated and untreated specimens was evaluated by the pull-off test on coatings from the substrates 10 days after the application of the coatings according to the standard [21]. On the surface of each specimen, aluminium dollies with a diameter of 20 mm were glued on the coated wood surface with a two-component epoxy resin. After 24 h of curing, the films of the coating around glued dollies were carefully cut off until reaching the substrate, to prevent propagation of failures out of the tested area. The adhesion strengths of cured coatings were measured using the pull-off testing machine (DeFelsko, Ogdensburg, USA) until separation of the dolly from the specimen surface occurred. If the separation between the substrate and the coating occurs (at least 60 %), an adhesion strength is determined, otherwise the strength is considered as a cohesive one (either of the substrate or of the coating layer, depending on the predominant cohesive failure in the substrate or in the coating system, respectively).

3.2.9 Cross-Cut Test In order to assess adhesion of coatings, additional cross-cut tests following [22] were carried out. The cross-cut tests were performed on samples with

Wettability of Wood Surfaces 47 complete coating system comprising of primer and topcoat. A special cutting tool with a blade spacing of 1 mm was used to cut a network of squares into the coating down to the substrate.

3.3 Results and Discussion 3.3.1 Contact Angles and Surface Free Energy CAs of 5 μL water (a), glycerol (b), and diiodomethane droplets (c) determined on samples treated at different moving rates of plasma treatment module above the treated sample are presented in Figure 3.2. For comparison, CAs on untreated surfaces are also presented. From Figure 3.2a it can be seen that the lowest CAs of water droplets were achieved after plasma treatment at a moving speed of 3 mm/s. Because of the lowest CA in this case, we decided to continue our experiments in a way where the substrates will be treated with this speed of moving plasma module. Figure 3.3 depicts a bar chart of calculated SFEs with dispersion and polar components calculated according to the Owens-Wendt approach. The equilibrium CAs were taken 1 s after deposition of the droplets, which we believe is sufficient for our purposes of SFE calculations. It can be seen that the total SFE is increased at all speeds of the plasma treatment, while the highest total SFE was calculated for surfaces plasma-treated at a speed of plasma module of 6 mm/s along the specimen. The lowest total SFE was calculated for untreated wood surfaces. Similar findings on spruce wood treated by DBD plasma in air at atmospheric pressure were observed by Avramidis and co-workers [6]. In Figure 3.4, water CAs on surfaces prepared according to different sample preparation steps are presented. Measurements of water droplet CAs on coated surfaces showed that lowest CAs were for surfaces with topcoat applied. Surfaces covered with primer seemed to be more hydrophobic than those covered with topcoat.

3.3.2 Spreading of Colored Water Droplets on Untreated and Plasma Treated Wood Surfaces The time-dependent evolution of stain formation after application of green-colored water (stain) droplets and their final appearance are presented in Figure 3.5.

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(a) 50 40 30 20 10 0

0

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UT PT_1 mm/s PT_3 mm/s PT_6 mm/s PT_9 mm/s 40 50

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(b)

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30 (c) 25 20 15 10 5 0

0

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UT PT_1 mm/s PT_3 mm/s PT_6 mm/s PT_9 mm/s 50 40

Time [s]

Figure 3.2 CAs of water (a), glycerol (b), and diiodomethane droplets (c) on untreated (UT) and plasma treated (PT) samples, with plasma module movement speed of 1 mm/s (PT_1 mm/s), 3 mm/s (PT_3 mm/s), 6 mm/s (PT_6 mm/s) and 9 mm/s (PT_9 mm/s).

Wettability of Wood Surfaces 49 80 Surface free energy [mJ/m2]

SFE polar component SFE dispersion component

60

40

20

0 PT_1 mm/s PT_3 mm/s PT_6 mm/s PT_9 mm/s Sample series

UT

Figure 3.3 Polar and dispersion components of surface free energy of untreated and plasma treated wood surfaces.

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50 40 30 20

Untreated + primer Untreated + topcoat Treatment + primer Treatment + topcoat Treatment + primer + topcoat

10 0

0

10

20

30 Time [s]

40

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Figure 3.4 CAs of water droplets on untreated samples with the primer applied, untreated samples with the topcoat applied, plasma treated sample with the primer applied, plasma treated sample with the topcoat applied, and plasma treated sample with both the primer and topcoat applied.

When the water drop was deposited onto the wood surface, a linear expansion along the grain occurred. It is clear that applied colored water drop on treated samples stained larger areas compared to untreated samples. Based on these observations, we decided to use plasma treatment process with sample feed speed of 3 mm/s for pull-off and cross-cut tests to achieve the best spreading and wettability of the coatings.

Perimeter of applied stain droplet [mm]

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Type of Stain appearance 24 h samples after liquid deposition

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5 mm

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Figure 3.5 Time-dependent evolution of perimeter of applied green-colored stain droplet (left) and its appearance 24 hours after deposition on untreated and differently plasma treated surfaces.

No changes were observable in the ATR-FTIR spectra, which is most probably due to its information depth of approx. 1 μm being much larger than the penetration depth of the plasma treatment [23].

3.3.3 Surface Roughness The effect of the plasma treatment on the surface roughness of Norway spruce wood was determined by confocal laser-scanning microscopy, and different roughness parameters were measured to evaluate surface roughness. The determined parameters showed that the surface roughness was decreased after plasma treatment process. In other words, the surface became smoother. Vander Wielen and co-workers [24] came to similar findings, who observed surface smoothness of pulp fibers after treatment with DBD plasma in air at atmospheric pressure. 3D profiles and corresponding optical images of the observed spots before and after plasma treatment are presented in Figure 3.6. Surface roughness parameters determined on crosswise section profiles of surfaces in the middle of the observed area on the sample are presented in Table 3.1.

3.3.4 Contact Angles of Primer and Topcoat CAs of primer and topcoat droplets on untreated and plasma-treated (module moving speed 3 mm/s) samples are presented in Figure 3.7.

Wettability of Wood Surfaces 51

26.5μm 0.0

25.0 μm

24.4μm 0.0

20.0

10.0

277.8 200.0

277.8 200.0

15.0

194.4

5.0 0.0

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100.0

100.0

100.0

100.0

0.0μm 0.0μm

0.0μm 0.0μm

Figure 3.6 3D profiles and corresponding optical images of the observed spot on a sample surface before (left) and after (right) plasma treatment at module moving speed of 3 mm/s.

Table 3.1 Maximum height of surface roughness profile determined on a segment on sample surface before and after plasma treatment. Maximum height of roughness profile Sample surface

Segment

(Rz [μm])

Untreated

Section line

16.38

Whole surface

26.52

Section line

13.99

Whole surface

24.41

Treated

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UT, primer PT_3 mm/s, primer UT, topcoat PT_3 mm/s, topcoat

Coating contact angle [º]

100 80 60 40 20 0 0

10

30

20

40

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Time [s]

Figure 3.7 CAs of primer and topcoat droplets on untreated samples (UT) and plasma treated (PT) samples at module movement speed of 3 mm/s (PT_3 mm/s).

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By comparing CAs of primer and topcoat droplets it can be seen that the primer droplets formed much lower contact angles than the droplets of the topcoat. Such a result is actually expected, since the use of primer and its application is intended to improve the wetting and adhesion on certain types of wood [25]. Interestingly, here plasma treatment did not really have an effect on CAs of both the primer and the topcoat. This could be expected, since plasma treatment made surfaces more hydrophilic as shown before, and since both primer and topcoat are water-based.

3.3.5 Adhesion Strength Determined by the Pull-Off Test Method The adhesion strengths of the coating systems on untreated and plasma treated wood surfaces determined by the pull-off test method are presented in Figure 3.8. At all measurements performed, a predominant interfacial failure between the substrate and the coating system occurred. The adhesion strength of the primer coating on the plasma treated surface, when compared to that on untreated surface, on average improved by 0.26 MPa. However, in the case of the samples with additional topcoat applied on the cured primer, the adhesion strength of the complete coating system slightly decreased in comparison to treated samples with applied primer or topcoat. The adheison strengths on untreated and plasma pretreated samples with topcoat applied were almost equal. Overall, from the results presented in Figure 3.8, it can be deduced that the adhesion of

Pull-off strength [MPa]

3.00 2.80 2.60 2.40 2.20 2.00

Untreated + primer

Untreated + topcoat

Treatment + primer

Treatment + Treatment + topcoat primer + topcoat

Sample series

Figure 3.8 Pull-off strengths of coatings on different samples.

Wettability of Wood Surfaces 53 primer was improved after plasma treatment. The adhesion of the topcoat applied on plasma pre-treated wood surfaces was not enhanced at all.

3.3.6 The Results of the Cross-Cut Tests Evaluation of the appeared damage to the topcoat after performing the cross-cut test showed that in all cases the delaminated area was between 35 and 65%, which corresponds to grade 3 or 4 [22]. There were no visible differences in the failure appearance of coated untreated and coated plasma pre-treated samples.

3.4 Summary and Conclusions Besides for many different purposes, plasma can also be used as a treatment technique for surface modification of various materials. When generated in air and at atmospheric pressure, it does not require vacuum conditions for its generation. In addition, treatment of a material with atmospheric plasma is an environmentally-friendly technique, without energy-intensive drying processes or chemical waste generated prior to finishing, since better results for coated surfaces (i.e. higher adhesion strength, durability and resistance to weathering) are achieved without needing any special solvents. The influence of plasma treatment of Norway spruce wood surfaces on their wettability, surface free energy and spreading of selected liquids was assessed. The results showed that Norway spruce wood became more hydrophilic after treatment with DBD plasma, generated in air at atmospheric pressure. We investigated also the influence of the plasma module moving speed on wettability of plasma treated wood and it was found that the greatest observed wettablity effect was achieved for treatment at plasma module moving speed of 3 mm/s along the specimen’s surface. The highest calculated total surface free energies were determined for wood surfaces treated at a plasma module moving speed of 6 mm/s. The ATR-FTIR spectra did not indicate any chemical changes on plasma treated surfaces of wood. The reason for this most probably is in the sampling depth of infrared beam exceeding the thickness of the plasma activated layer, since treatment of wood surfaces with plasma has an influence only to a few nm in depth. Confocal laser-scanning microscopic observations of the sample surface area showed that its initial roughness decreased after treatment with plasma. The value of the maximum height of the roughness profile (Rz)

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was in the case of untreated surface 26.52 μm, but after the plasma treatment process it decreased to 24.41 μm. Finally, the pull-off and cross-cut tests indicated that plasma treatments of the substrates increased the adhesion strength of the primer coating (from 2.45 MPa to 2.71 MPa), while the adhesion strength of the topcoat on the substrate was not influenced by plasma treatment.

Acknowledgements The authors greatfully acknowledge the financial support of the COST FP1407 programme and the Slovenian Research Agency (research core funding No. P4–0015, “Wood and lignocellulose composites”). Special thanks to Dr. Kash Mittal and Dr. Robert H. Lacombe for the invitation to prepare this contribution.

References 1. Cogulet, A., Blanchet, P., Landry, V., Morris, P., Weathering of wood coated with semi-clear coating: Study of interactions between photo and biodegradation. Int. Biodeterior. Biodegrad., 129, 33–41, 2018. 2. Nejad, M. and Cooper, P., Exterior wood coatings. Part-1: Performance of semitransparent stains on preservative-treated wood. J. Coat. Technol. Res., 8, 449–458, 2011. 3. Evans, D.P., Michell, J.A., Schmalzl, J.K., Studies of the degradation and protection of wood surfaces. Wood Sci. Technol., 26, 151–163, 1992. 4. Šernek, M., Frederick, A., Kamke, A.F., Glasser, G.W., Comparative analysis of inactivated wood surface. Holzforschung, 58, 22–31, 2004. 5. Petrič, M. and Oven, P., Determination of wettability of wood and its significance in wood science and technology: A critical review. Rev. Adhesion Adhesives, 3, 121–187, 2015. 6. Avramidis, G., Militz, H., Avar, I., Viöl, W., Wolkenhauer, A., Plasma treatment of wood and wood-based materials to generate hydrophilic or hydrophobic surface characteristics. Wood Mater. Sci. Eng., 4, 1–2, 52–60, 2012. 7. Acda, N.M., Devera, E.E., Cabangon, J.R., Ramos, J.H., Effects of plasma modification on adhesion properties of wood. Int. J. Adhesion Adhesives, 32, 70–75, 2012. 8. Petrič, M., Surface modification of wood: A critical review. Rev. Adhesion Adhesives, 1, 216–247, 2013. 9. Baltazar-y-Jimenez, A., Bistritz, M., Schulz, E., Bismarck, A., Atmospheric air pressure plasma treatment of lignocellulosic fibres: Impact on mechanical

Wettability of Wood Surfaces 55

10. 11. 12.

13.

14.

15.

16. 17.

18. 19.

20. 21. 22. 23.

24.

25.

properties and adhesion to cellulose acetate butyrate. Composites Sci. Technol., 68, 215–227, 2008. Conrads, H. and Schmidt, M., Plasma generation and plasma sources. Plasma Sources Sci. Technol., 9, 441–454, 2000. Thomas, M. and Mittal, K.L. (Eds.), Atmospheric Pressure Plasma Treatment of Polymers: Relevance to Adhesion, Wiley-Scrivener, Beverly, MA, 2013. Klarhöfer, L., Frerichs, M., Maus-Friedrichs, W., Kempter, V., Viöl, W., Investigation of pure and plasma treated spruce with surface analytical techniques, in: Proc. of The Second European Conference on Wood Modification (ECWM2), pp. 339–345, University of Göttingen, Göttingen, 2005. Kogelschatz, U., Dielectric-barrier discharges: Their history, discharge physics, and industrial applications. Plasma Chem. Plasma Process, 23, 23–26, 2003. Wolkenhauer, A., Avramidis, G., Hauswald, E., Loose, S., Viöl, W., Investigation on the drying behaviour of adhesives on plasma-treated wood materials. Wood Res., 54, 59–66, 2009. Huang, H., Wang, J.B., Dong, L., Zhao, M., Wettability of hybrid poplar veneers with cold plasma treatments in relation to drying conditions. Drying Technol., 29, 323–330, 2011. Mittal, K.L., The role of the interface in adhesion phenomena. Polym. Eng. Sci., 17, 467–473, 1977. Wolkenhauer, A., Avramidis, G., Hauswald, E., Militz, H., Viöl, W., Sanding vs. plasma treatment of aged wood: A comparison with respect to surface energy. Int. J. Adhesion Adhesives, 29, 18–22, 2009. Etzler, F.M., Determination of the surface free energy of solids: A critical review. Rev. Adhesion Adhesives, 1, 3–45, 2013. Etzler, F.M., Characterization of surface free energies and surface chemistry of solids, in: Contact Angle, Wettability and Adhesion, vol. 3, K.L. Mittal (Ed.), pp. 219–264, CRC Press, Boca Raton, FL, 2003. Owens, D.K. and Wendt, R.C., Estimation of the surface free energy of polymers. J. Appl. Polym. Sci., 13, 1741–1747, 1969. Paints and varnishes - Pull-off test for adhesion, EN ISO, pp. 11, 4246, 2016. Paints and varnishes - Cross-cut test, EN ISO, pp. 14, 2409, 2013. Král, P., Ráhel, J., Stupavská, M., Šrajer, J., Klímek, P., Mishra, K.P., Wimmer, R., XPS depth profile of plasma–activated surface of beech wood (Fagus sylvatica) and its impact on polyvinyl acetate tensile shear bond strength. Wood Sci. Technol., 49, 319–330, 2015. Vander Wielen, C.L., Elder, T., Ragauskas, J.A., Analysis of the topochemical effects of dielectric–barrier discharge on cellulosic fibers. Cellulose, 12, 185–196, 2005. Prieto, J. and Jürgen, K., Wood Coatings: Chemistry and Practice, Vincentz Network GmbH & Co. KG, Hannover, Germany, 2018.

4 Wettability of Ultrafiltration Membranes Konrad Terpiłowski1*, Małgorzata Bielska2, Krystyna Prochaska2 and Emil Chibowski1 1

Department of Physical Chemistry – Interfacial Phenomena, Faculty of Chemistry, Maria Curie Sklodowska University, Lublin, Poland 2 Institute of Chemical Technology and Engineering, Faculty of Chemical Technology, Poznan University of Technology, Poznań, Poland

Abstract Hydrophobic properties of three polymeric ultrafiltration membranes: Cellulose acetate (CA), Polysulfone (PS), Poly(vinylidene fluoride) (PVDF) were determined by contact angle measurements and their surface free energy evaluation. Advancing and receding contact angles were measured by the sessile-drop method. The contact angles were measured for water and surfactants solutions (Sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), oxyethylated coconut fatty acid methyl esters (OMC-10). Moreover, also binary mixtures of each ionic surfactant with the non-ionic one were used. Surface free energy was calculated using the contact angle hysteresis (CAH) approach and Neumann’s equation-of-state. The components of apparent surface free energy have been evaluated using the van Oss, Chaudhury and Good approach. The contact angles were also calculated from Tadmor’s equation, and they may be considered as the equilibrium ones. Measurements of the contact angle and calculated apparent surface free energy enable characterization of ultrafiltration membrane properties in terms of hydrophobicity. Keywords: Membrane, surfactant, wettability, apparent surface free energy

4.1 Introduction Micellar-enhanced ultrafiltration (MEUF) is a hybrid process combining the classical membrane technique and the ability of surface-active *Corresponding author: [email protected] K.L. Mittal (ed.) Advances in Contact Angle, Wettability and Adhesion: Volume 4, (57–72) © 2020 Scrivener Publishing LLC

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compounds solubilization. The basis of the MEUF process is the solubilization of the molecules of the separated substance in micelles formed by the surfactant (at its appropriate concentration), introduced into the feed phase [1–11]. The diameter of micelles is usually larger than that of membrane pores. As a result of ultrafiltration, the micelles with solubilized material remain in the retentate, whereas the permeate contains nonsolubilized molecules of the separated compounds and some amounts of surfactant monomers. Therefore, the principal problem in separation using the ultrafiltration technique is the proper choice of the surfactant, as well as its concentration. It should be kept in mind that a surface active compound adsorbs on solid surfaces including that of membrane [12]. As a result, a gradient of its concentration can be formed on both sides of membrane causing diffusion of surfactant molecules into the permeate along the walls of membrane pores. Moreover, surfactant adsorption can increase retention of separated compounds due to reduction in the diameter of membrane pores. However, the adsorption can lead to retention decrease when the membrane porosity is suitable but the surfactant adsorption on the membrane surface results in a decrease of its concentration in the solution. Thus, using the MEUF process before the main filtration, preliminary studies should be carried out which allow to determine the adsorption property of the surfactant as well as its interaction with the material of membrane. Adsorption property of the surfactants applied in MEUF is significant for the course of process and the capacity of membrane separation. Type and structure of the surfactant molecules determine both the micelles size and their solubilization capacity. Moreover, the kind of surfactant affects the amount of active surface agents penetrating through the membrane. Therefore, proper micellar ultrafiltration process requires complex studies of the surfactant properties on the surface which is planned to be used in the MEUF process. The results should make it possible to select the optimal surfactant structure and its concentration for the substance to be separated and the kind of membrane. It is obvious that from ecological and economical viewpoints the low permeability of the surfactants into the filtrate fraction determines the usefulness of the MEUF method.

4.2 Apparent Surface Free Energy Determination There are many theoretical approaches in the scientific literature [13, 14] that use the measured contact angles of probe liquids to calculate the

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apparent total surface free energy of a material. In this paper, the apparent surface free energy was calculated using the three methods described below; also van Oss, Chaudhury and Good [15, 16] approach was used to determine the surface free energy components and parameters.

4.2.1 Contact Angle Hysteresis Approach Chibowski proposed a quantitative interpretation of the contact angle hysteresis (CAH) [17, 18]. He related the total surface free energy of solid (γstot) to three measurable parameters Eq. (4.1): advancing (θa) and receding (θr) contact angles and the liquid surface tension (γL) used for the contact angle measurements. The apparent surface free energy of solid evaluated in this way also, to some extent, depends on the kind and strength of the liquid/solid interactions occurring at the interface, i. e. on the kind of probe liquid used.

(1+cos a )2 (2+cos r +cos a )

tot s

L

(4.1)

4.2.2 Neumann Equation-of-State Approach In the Neumann [19] equation-of-state approach the apparent surface free energy is determined from the liquid advancing contact angle, its surface tension and constant β (0.000125) as:

cos

S

1 2

a

2

e

L

S

(4.2)

L

4.2.3 Equilibrium Contact Angle Approach Determination of equilibrium Young [20] contact angle is still an open problem [21]. Here, Tadmor’s [22] equation (Eq. 4.3) was used for this purpose. B)

1/3

A)

sin a

2 3 cos

C) 0

arccos

a

1/3

3 a a

cos

cos

;

3 a

a

r

a

r

cos

r

sin r

2 3 cos

3 r r

cos3

; r

(4.3)

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Where: Γa - the advancing angle weight coefficient, Γr - the receding angle weight coefficient. Using the contact angles calculated from (Eq. 4.3 C) and assuming that at the equilibrium contact angle θ0, θa = θr = θ0 [16–18] (Eq. 4.1) transforms to (Eq. 4.4).

S

1 2

L

(1 cos 0 )

(4.4)

4.2.4 van Oss, Chaudhury and Good Approach Based on this model the work of adhesion is expressed as follows:

WA

l

(1 cos ) 2

LW s

LW l

2

s

l

2

s

l

(4.5)

where: γl is the liquid surface tension: θ is the contact angle; LW is the s LW Lifshitz-van der Waals component of solid surface free energy; 1 is the Lifshitz-van der Waals component of liquid surface tension; s is electronacceptor parameter of the solid; s is electron-donor parameter of the solid; 1 is electron-acceptor parameter of liquid; and 1 is electrondonor parameter of liquid. For a solid sLW component and s , s parameters of its surface free energy can be determined from equation (4.5) by measuring the contact angles of three probe liquids whose surface tension components are known: lLW , l and l . Solving simultaneously three equations (4.5) with three unknowns ( sLW , s , s ) allows determination of the surface free energy components and finally the total surface free energy value of the solid. However, such determination of solid surface free energy components depends on the kind of probe liquids used [15, 16].

4.3 Experimental 4.3.1 Materials Cetyltrimethylammonium bromide (CTAB) (Merck), sodium dodecylsulfate (SDS) (Merck), oxyethylated coconut fatty acid methyl esters of an average oxyethylation degree equal to 10 (OMC-10) (Institute of Heavy Organic Synthesis, Kedzierzyn Kozle, Poland), and the binary mixtures of each ionic surfactant with the nonionic one were used as surfactants.

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Deionized water from reverse osmosis was used as a solvent. The critical micelle concentrations (cmc) in deionized water were 0.34, 2.4, 0.35, 0.16 and 0.58 g/dm3 for CTAB, SDS, OMC-10, and binary mixtures of CTAB with OMC-10 and SDS with OMC-10, respectively. The concentration ratio of the ionic to nonionic surfactant in the binary mixture was 2.5 cmc: 2.5 cmc. The total concentration of the surfactants used singly or in a binary mixture was always equal to 5 cmc in deionized water. The surface tensions of surfactant solutions at the cmc were 34.4, 32.5, 31.4, 33.4 and 32.6 mN/m for CTAB, SDS OMC-10, and binary mixtures of CTAB with OMC-10 and SDS with OMC-10, respectively. Prior to measurements, the membranes (CA: cellulose acetate, PS: polysulfone, PVDF: poly(vinylidene fluoride) were conditioned in deionized water for 24 hours.

4.3.2 Methods GBX Contact Angle Meter (France) was used to measure contact angles. The advancing contact angle of water or surfactant solution was measured by settling 6μl droplet on the permeate side of membrane. Then, 2μl of the liquid was sucked from the droplet into the syringe and the receding contact angle was measured. To determine the wettability of textile surfaces used as membranes, some researchers have suggested that a better way is the Wilhelmy method [23]. This is because of the hydrophilic nature of surface, the droplet always penetrates into the pores in the textile and this effect prohibits the measurements. In the case of hydrophobic surfaces, the measured apparent contact angle will always differ from the true contact angle. Capillary effects occur even on hydrophobic substrates and compete with evaporation of the liquid [23]. However, the membranes investigated in this study have different wetting properties on each side. Therefore, it is impossible to measure their wettability using the Wilhelmy method. The samples during measurements were placed in a closed chamber with constant humidity about 50% RH.

4.4 Results and Discussion 4.4.1 Surface Topography The membranes roughness was determined using a Veeco Optical profilometer. No special preparation of the membranes was necessary to obtain images [24]. Figure 4.1 shows the 3D profilometry images of the membrane surfaces of size 1290 x 900 μm. This size of image corresponds to the membrane

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(a)

(b)

(c)

Figure 4.1 3D profilometry images, size 1290x900μm of (a) CA (cellulose acetate); (b) PS (polysulfone); (c) PVDF (poly(vinylidene fluoride)) membranes.

surface occupied by the liquid droplet during the contact angle measurement. The roughness profiles of the membrane surfaces were also taken and are shown in Figures 4.2–4.4. Profiles were taken along the white line seen in the images. The values of RRMS (root mean squared), Ra average roughness, and Rt the peak-to-valley distance were calculated by Veeco Instrument Inc.,

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10 8 6

Z [μm]

4 2 0 -2 -4 -6 -8 -10 100 200 300 400 500 600 700 800 900 1000 1100 1200 [μm]

Figure 4.2 CA (cellulose acetate) membrane roughness profile along the white line shown in (Figure 4.1 (a)).

4 3 2

Z [μm]

1 0 -1 -2 -3 -4 -5 100 200 300 400 500 600 700 800 900 1000 1100 1200 [μm]

Figure 4.3 PS (polysulfone) membrane roughness profile along the white line shown in (Figure 4.1 (b)).

Vison 4.20 software and are given in Table 4.1. The images show that the smoothest is the surface of PVDF membrane (Figure 4.1(c)) with RRMS = 1.6μm, whose average roughness Ra is 1.2 μm and the peak-to-valley distance is 15.8 μm. The cracks visible on this surface profile are quite narrow. The surface topography of PS membrane is similar to PVDF membrane with RRMS = 1.9 μm and average roughness 1.6 μm. CA membrane has significantly different surface parameters RRMS = 4.2 μm and average roughness Ra is

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Z [μm]

0 -1 -2 -3 -4 -5 100 200 300 400 500 600 700 800 900 1000 1100 1200 [μm]

Figure 4.4 PVDF (poly(vinylidene fluoride)) membrane roughness profile along the white line shown in (Figure 4.1 (c)).

about 3.3 μm. The peak-to-valley distance is two times higher in comparison to PVDF membrane and it is 30.4 μm (Table 4.1). For CA membrane, Bowen and Doneva [25] using AFM technique obtained average roughness of 2.98 μm. Dietz et al. [24] investigated many types of PS membranes but generally the size of image was 10 to 50 nm. However, there is a big difference between the obtained AFM and profilometry images. Bowen and Doneva [25] using AFM technique analysed very small area, only 30 x 30 nm. Comparing their results with those obtained from the profilometry it is clearly seen that they analysed (Figure 4.1(a) and Figure 4.2) only top of the roughness peaks or bottom of the roughness valleys. Dietz et al. [24] investigated samples of 1500 nm x 1500 nm size but this area was still 52x104 times smaller than the area analyzed using Table 4.1 Surface roughness parameters. Membrane

Ra[μm]

RRMS[μm]

Rt[μm]

CA

3.3

4.2

30.4

PS

1.6

1.9

23.8

PVDF

1.2

1.6

15.8

Ra – average roughness as calculated over the entire measured array RMS – root – mean – squared roughness calculated over the entire measured array Rt – peak-to-valley distance calculated over the entire measured array

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65

profilometry technique. In this paper [24] the membrane surface properties were also investigated via contact angle measurements. As mentioned above the investigated surface topography size corresponded to the surface area occupied by the droplet on the surface.

4.4.2 Contact Angle Measurements Based on the measured advancing water contact angles (Figures 4.5–4.7) hydrophobic property of the membranes changes in the sequence: CA (42.4°±2.8) < PS (65.8°±4.2) < PVDF (67.4°±3.9) (Figures 4.5–4.7). The results correspond with the surface roughness changes in the sequence: CA (3.3μm) 0. 5. γsv is not influenced by liquids. It is significant to note that only a few authors investigate the validity of the above assumptions in their work. According to Kwok and Neumann [9], contact angle can be expressed as a function of LV, the interfacial free energy at the liquid-vapor interface, and SV, the interfacial free energy at the solid-vapor interface only. Thus, LV

cos( ) = f (

SV

–

SL

=f(

,

LV

,

LV

)

SV

(5.2)

)

(5.3)

SV

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Advances in Contact Angle, Wettability and Adhesion SL

=

SV

–f(

LV

,

)=F(

SV

,

LV

)

(5.4)

SV

where f and F are appropriate functions. Kwok and Neumann have observed smooth monotonic dependence of LV cos( ) on LV consistent with Eqn. 5.1 when liquid-solid pairs conform closely to the assumptions listed above. For arbitrary solid-liquid pairs such a plot may show considerable scatter because the measured contact angles deviate significantly from the true Young’s contact angle. Stick-slip behavior in advancing contact angles, time-dependent contact angles, and liquid surface tension changes during the course of the experiments each result from physico-chemical interactions not consistent with the use of Young’s equation for determining surface free energies. The function F in Eqn. (5.4) has been historically modeled in several ways. Antonow [16] stated that: SL

LV

(5.5)

SV

or combining with Young’s equation

cos( )

1 2

SV

(5.6)

LV

Alternatively, Berthelot’s rule [17] has been used such that

SL

LV

SV

2(

LV

SV

)1/2

(

LV

)1/2 (

SV

)1/2

2

(5.7)

and again combining with Young’s equation

cos( )

1 2

SV

(5.8)

LV

Eqn. (5.7) would be identical to that used in the van Oss, Chaudhury and Good model (see discussion below) if the solid and liquid have only LW LW Lifshitz-van der Waals interactions such that LV LV . Here LV is the interfacial free energy at the liquid-vapor interface due only to Lifshitz - van

Surface Free Energy of Solid Surfaces 77 der Waals (LW) interactions. If Eqns. (5.2) and (5.4) were adequate descriptors then calculated values for SV would be independent of the choice of probe liquid used. This is unfortunately not the case. Li and Neumann [18] have considered a modified Berthelot equation such that

SL

LV

SV

2(

SV

LV

)1/2 e

2 SV )

( LV

(5.9)

and 1/ 2

cos( )

1 2

SV

e

( LV

SV

)2

(5.10)

LV

Empirically it has been shown that

0.0001247

(5.11)

and that the determined solid surface free energy using this choice for β is nearly independent of the choice of liquid. Eqn. (5.10) can be used in two ways. First, β can be taken from Eqn. (5.11) and a suitable contact angle can be used to determine SV. Second, β and SV can be treated as adjustable parameters. Least squares analysis using contact angles measured for several liquids is then used to determine the best fit values for β and SV. The second approach would seem to be preferable. The approach used by Kwok and Neumann [9] has the advantage of requiring only two adjustable parameters. Some may find the model unsatisfactory as the model provides no molecular interpretation of the resultant value.

5.1.3 van Oss, Chaudhury and Good Approach Lifshitz - van der Waals interactions between molecules result from interaction between the corresponding electron orbitals in each molecule. The principal non-bonding interactions result from induced dipole-induced dipole (London), dipole-induced dipole (Debye) and dipole-dipole (Keesom) interactions. van Oss, Chaudhury, and Good (vOCG) (See [19, 20] for further discussion) choose to express surface free energy in terms of two principal components : Lifshitz-van der Waals (LW) and Lewis acid-base (AB) components.

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The work of adhesion due to Lifshitz-van der Waals interactions is estimated using the geometric mean rule discussed above (see Etzler [2, 3] for a more complete discussion). Thus

WaLW

2(

LW 1/ 2 2

LW 1

)

(5.12)

The use of the geometric mean approximation with regard to Lifshitzvan der Waals interactions is not unique to the van Oss, Chaudhury and Good approach and is used by Chen and Chang as well as by Owens and Wendt [21] (see discussion below). According to the van Oss, Chaudhury and Good model [19, 20] the Lewis acid-base parameter is modeled as follows AB i

1/ 2

2

i

i

(5.13)

where + is the Lewis acid parameter and – the Lewis base parameter. van Oss, Chaudhury and Good further choose i

0

i

(5.14)

for alkanes, methylene iodide and α-bromonaphthalene which presumably interact only through Lifshitz-van der Waals interactions. For water H 2O

25.5 mJ/m 2

H 2O

(5.15)

Based on the above numerical choices (Eqns. (5.14 and 5.15)), γ+ and γ have been experimentally determined for a variety of liquids. van Oss [20] has compiled and reviewed the determination of these values (also see Etzler [2, 3]). Earlier Owens and Wendt [21] described surface free energy in terms of two components which were called dispersion γd and polar γp. Thus −

=

d

+

p

(5.16)

While it is generally recognized that γd ≈ γLW, the meaning of γp is perhaps hopelessly confused in the literature. According to Fowkes [22, 23], γp should

Surface Free Energy of Solid Surfaces 79 refer to dipole-dipole (Keesom) interactions. In the van Oss, Chaudhury and Good model such interactions are incorporated into γLW. Good [24] does not recommend the use of γp. Good’s argument follows in the next paragraph. Eqn. (5.13) reminds us that for monopolar materials (γ+ or γ− = 0) γAB = 0. On the other hand, for two interacting dipolar (γ+, γ− ≠ 0) materials, γp > 0; thus γAB ≠ γp. For example, the surface tensions of carbon tetrachloride and chloroform are nearly identical yet their interfacial tensions with water are 45.0 mN/m and 31.6 mN/m respectively. Because chloroform is a monopolar acid (γ+ ≠ 0, γ− = 0 γAB = 0) a descriptor such as γp is inadequate to describe the difference in the observed interfacial tensions as γAB = 0 for these two substances. Instead Eqn. (5.13) is a better descriptor. From a practical point of view, reported values of γd and γp can be regarded as γLW and γAB respectively. The values, however, should be interpreted in terms of the van Oss, Chaudhury and Good model. The Owens and Wendt model will not be explored in the work. According to the van Oss, Chaudhury, and Good approach

=

+

,

LW l

LW 1 2 s

(5.17)

and

Wa

l

1 cos

2(

)

2(

l

s

)1 2 2(

l

s

)1 2 (5.18)

If the Owens and Wendt approach is used, the above equation becomes,

Wa

l

1 cos

2(

LW l

LW 1 2 s

)

2(

AB AB 1 2 l s

)

(5.19)

If the van Oss, Chaudhury and Good parameters are known for at least three liquids and the contact angles of these liquids on a solid are measured, then Eqn. (5.18) can be used to determine the van Oss, Chaudhury and Good parameters for the surface free energy of the solid. van Oss [20] has reviewed the numerous publications which have reported the determination of the van Oss, Chaudhury and Good parameters for various liquids. Dalal [25] discussed the choice of liquid sets used to determine surface free energy parameters. While Dalal’s discussion addresses the older Owens and Wendt [21] model, much of the discussion applies directly to the van Oss, Chaudhury and Good model as well. Because the Owens-Wendt

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model has only two parameters, it is only necessary, in principle, to measure contact angles for two liquids. Dalal noted that the calculated values for the surface free energy components depended on the choice of liquids. The use of dissimilar liquid pairs (e.g. water, methylene iodide) minimized the dependence of the calculated results on the precise choice of probe liquids. Dalal recommended that many liquids be used and that the contact angle results from this overdetermined set of liquids be used to find the best-fit surface free energy components. In the present work, it will be shown that the use of an overdetermined set of probe liquids is absolutely essential for the estimation of surface free energy components. The parameters sj ,( j = LW,+,−) in Eqn. (5.18) have been interpreted in two ways for fitting purposes. The first method involves determination of sj directly from Eqn. (5.18). In this first case one will find sj 0 and j j Wa ≥ 0. In the second case investigators let csj s cs are the adjustable parameters used for fitting purposes. The second choice allows csj to become negative during the fitting process and thus Wa may also be negative. The first choice is the correct van Oss, Chaudhury and Good (See [19, 20] for details) model. In the present discussion it will be assumed that all γ’s are positive. In the present work all surface free energy components in all models will be considered to be positive numbers (except the Chen and Chang model where negative numbers are permissible). It is important to the reader of the literature to realize that various authors have calculated so-called van Oss, Chaudhury and Good parameters using various numerical methods which may influence the calculated result significantly. The van Oss, Chaudhury and Good model expressed in Eqn. (5.18) requires Wa > 0 and sj lk 0. Furthermore, lk 0 for all investigated liquids (j, k = +,−). Again, tabulated values for van Oss, Chaudhury and Good parameters have been compiled (see Etzler for the appropriate tables [2, 3]). The best estimates of the van Oss, Chaudhury and Good components require the use of an overdetermined set of probe liquids and carefully and properly measured contact angles. The set of chosen liquids should contain liquids that interact exclusively via Lifshitz-van der Waals interactions as well as liquids that also interact through Lewis acid-base interactions.

5.1.4 Chen and Chang Model The Chen and Chang model [26, 27] for interfacial free energy is largely based on the same principles which govern the van Oss, Chaudhury and Good model. Both models treat Lifshitz-van der Waals interactions in the same way. Calculation of the surface free energy components for each

Surface Free Energy of Solid Surfaces 81 model uses the same experimental data. The two models, however, differ in the way that Lewis acid-base interactions are modeled. Recall that

Wa WaLW WaAB

(5.20)

and

=

+

(5.21)

The Chen and Chang model uses the same geometric mean approximation for WaLW as does the van Oss, Chaudhury and Good model. Thus

WaLW WaL

2

LW 1

LW 2

12

P1L P2L

(5.22)

where

Pi L

2

L i

12

(5.23)

Pi L is the dispersion parameter. The superscript L is equivalent to LW. Like the van Oss, Chaudhury and Good model, the acid-base interaction is modeled using two parameters. These parameters, Pia and Pib , are referred to as principal values. The acid-base work of adhesion can be represented using the following relation:

WaAB

ΔGaAB

( P1a P2b P1b P2a )

(5.24)

The surface free energy of the material is thus LW

AB

1 L P 2

2

PaPb

(5.25)

Tabulated Pia and Pib values are substituted into Eqn. (5.24) such that the work of adhesion is maximized and the free energy of adhesion is minimized (see Etzler [2, 3] for the appropriate tables).

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The acid-base character of a material is characterized by the signs of Pi and Pib. If Pia Pib 0 then the material is neutral (no acid-base character). If Pia and Pib are both positive then the material is monopolar acidic and if both negative then the material is monopolar basic. If Pia and Pib are of opposite sign then the material is amphoteric. Despite some similarities to the van Oss, Chaudhury and Good model, the Chen and Chang model differs from the former model in a number of ways. The Chen and Chang model applies the geometric mean rule to only Lifshitz-van der Waals interactions. When determining values for Pia and Pib for a particular solid, interactions involving only n-alkanes are assumed to have exclusively Lifshitz-van der Waals interactions. The van Oss, Chaudhury and Good model, for instance, assumes that both methylene and α-bromonaphthalene also interact exclusively by Lifshitz-van der Waals interactions. A major difference is that the Chen-Chang model allows for WaAB both attractive and repulsive interactions. In other words, AB 0 . The Lewis whereas in the van Oss, Chaudhury and Good model Wa acid-base concept is general enough to include traditional ion-ion and dipole-dipole repulsions and thus it may not be unreasonable to suggest the existence of repulsive interactions [26]. Furthermore, entropic effects may contribute to the overall repulsion. Etzler [2, 3], for instance, provides a table of principal values for the Chen and Chang model. a

5.2 The Present Study In this study we explore the relative merits of the above models to data first collected by Dalal [25]. Specifically, contact angle data collected for various liquids on poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) are explored. The statistical principles explored here have also been discussed in an earlier paper [28].

5.2.1 Statistical Methods The relative quality of fits of data to several cohort models can be assessed using the following procedure. The Wolfram Mathematica version 11.3 procedure “NonLinearModelFit” was used to fit contact angle data to the Zisman; Kwok and Neumann; van Oss, Chaudhury and Good as well as the Chen and Chang models. Simulated data are based on Dalal’s measurements. The simulated data were generated by adding a normally distributed random number selected from a population with a mean of zero and a standard deviation of 1 to Dalal’s original data.

Surface Free Energy of Solid Surfaces 83 For each probe liquid-polymer combination 5 data points were generated. In an earlier work [28], it was shown that increasing the number of replicates increases the statistical power of subsequent statistical analysis. “NonlinearModelFit” provides four values for each fitted parameter : the parameter value, the parameter value standard error, the t value and the corresponding p value where,

t

Parameter value Parameter Standard Error

(5.26)

and t

p 1

Pt

, x dx

(5.27)

t

where Pt (v,x) is Student’s t distribution for ν degrees of freedom. The number of degrees of freedom is the number of data points minus the number of fitting parameters. Values of p > 0.05 indicate that parameter estimate is indistinguishable from zero and thus can be excluded from the analysis. If p > 0.05 the value of the fitted parameter should be considered to be zero. If some of the fitted parameters are indistinguishable from zero and thus not significant, data are refitted under the assumption that qualifying parameter(s) have a value of zero. The corrected fits represent the best possible fits for the chosen model. Historically, the goodness of fit of a function has often been assessed by the value of r2 where ^

2

y y r

2

1

2

(5.28)

y y Here y is the value of the response. In the present examples the value of Wa or 1+ cos θ calculated from the experimental contact angle is the response. y is the calculated value of the response using the fitting function and y is the average of all the experimental responses. Although

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r2 is a popular measure of goodness of fit, it is unsuitable for nonlinear examples [29]. More appropriate measures are the Akaike Information Criterion (AIC) and the corrected AIC (AICc) [29, 30]. AICc is an appropriate statistic for determining the best model from a cohort of candidate models which may include models with different numbers of adjustable parameters. The calculation of AIC and AICc is part of the output from the “NonLinearModelFit” procedure in Mathematica version 11.3. AIC is calculated as follows

AIC = 2p – 2lnL

(5.29)

where p is the number of adjustable parameters in the model and

ln L

1 2

N

N ln 2

xi2

1 ln N ln

(5.30)

i

Here N is the number of data points used in the model and xi is the residual of the ith data point or

xi

yi

y

(5.31)

ln L is referred to as the maximum log-likelihood of the estimated model. (The likelihood function describes the probability for obtaining the observed responses from the fitted parameters. Log-likelihood is the natural log of the likelihood function.) For small sample sizes, as is the present case, it is more appropriate to use AICc where

AICc

AIC

2 p 1 ( p 2) N

p 2

(5.32)

The best fit of the cohort set of proposed models is the model with the lowest value of AIC or AICc. Eqn. (5.31) is misstated by Spiess and Neumeyer [29] but correctly stated elsewhere [31, 32]. The relative probability, wi(AICc), that a given model, in the cohort set of models, is correct is calculated using the following expression.

Surface Free Energy of Solid Surfaces 85

1 Δi AICc 2

Exp wi ( AICc )

K

Exp k 1

(5.33)

1 Δ k AICc 2

where i and k are the model numbers and k is the total number of models in the cohort set of models. Also,

Δi(AICc) = AICci – AICcmin

(5.34)

and AICcmin is the smallest AICc of the cohort set of models. It is important to note that AICc does not imply the correctness of a model. It only provides a statistical comparison of the selected models.

5.2.2 Dalal’s Data The probe liquids studied by Dalal [25] are listed in Table 5.1. Note that Dalal explored the Owens and Wendt model [21] in his study. The Owens and Wendt parameters thus appear in this table. (N.B. γd = γLW and γp = γAB) The contact angles measured by Dalal are given in Table 5.2.

Table 5.1 Surface tension components (mN/m) of liquids as reported by Dalal [25]. Liquid

γT

γLW

γAB

Water (WT)

72.8

22.5

50.3

Diiodomethane (MI)

50.8

48.5

2.3

Formamide (FA)

58.2

39.5

18.7

α-Bromonaphthalene (BN)

44.6

44.6

0

Glycerol (GL)

63.4

37.0

26.4

Tricresyl phosphate (TP)

40.9

39.2

1.7

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Table 5.2 Contact angles (deg) of the probe liquids listed in Table 5.1 on the tested polymers as measured by Dalal [25]. Polymer

WT

MI

FA

BN

GL

TP

PVC, poly(vinyl chloride)

87

36

66

11

67

14

PMMA, poly(methyl methacrylate)

80

41

64

16

69

19

5.3 Data Analysis Simulated data sets based on Dalal’s data (Table 5.2) have been generated by adding a random number selected from a normal distribution with a mean of zero and a standard deviation of 1. This simulates a contact angle measurement error of ± 1 degree. The analysis has been performed for PVC (poly(vinyl chloride)) and PMMA (poly(methyl methacrylate)). The models investigated here include those proposed by Zisman [7], Kwok and Neumann [9], Chen and Chang [26, 27] as well as that by van Oss, Chaudhury and Good (see [19, 20] for further discussion). The results of the statistical fitting are listed in the tables below and discussed in the following sections of this paper. Note that the van Oss, Chaudhury and Good parameters for tricresyl phosphate are unknown; thus, contact angle data for this liquid are not used for fitting to the van Oss, Chaudhury and Good model.

5.3.1 Fittting of PVC Data Table 5.3 shows the simulated data generated from the data collected earlier by Dalal. The data in Table 5.3 are used for the statistical fits to the data. The results of the statistical fits are given in Table 5.4. Table 5.3 Contact Angles (deg) of various liquids on PVC (simulated data set with 5 replicates). Water

86.6328

87.4475

87.6749

87.7468

85.8667

Methylene iodide

35.7592

35.3206

35.0301

37.4964

35.7068

Formamide

66.3729

63.9801

66.5197

67.3053

67.2407

α-Bromonaphthalene

10.7907

10.7226

10.7724

10.8405

11.3088

Glycerol

66.4824

66.8431

68.1215

68.1838

65.9727

Tricresyl phosphate

13.5502

13.9736

13.9258

15.6109

13.1308

Simulated data are expressed here to a much greater precision than experimentally possible.

Surface Free Energy of Solid Surfaces 87 Table 5.4 Fitting parameters for the various models using PVC. Zisman γc

Std. Error

t

p

AICc

42.16

0.64

65.8

3.0 x 10-32

69.62

β

Std. Error

t

p

-0.03081

0.00116

26.5

2.2 x 10-21

γsv

Std. Error

t

p

AICc

41.64

0.61

68.0

1.2 x 10-32

71.77

β

Std. Error

t

p

0.000389

0.000040

9.80

1.4 x 10-10

PL

Std. Error

t

p

AICc

8.19

0.12

69.3

5.9 x 10-32

131.404

Pa

Std. Error

t

p

-1.50

0.20

7.55

4.0 x 10-8

Pb

Std. Error

t

p

-1.96

0.12

16.2

2.0 x 10-15

γsv

γLW

γAB

30.62

33.57

-2.95

γLW

Std. Error

t

p

AICc

40.30

0.91

44.2

5.5 x 10-23

135.639

γ-

Std. Error

t

p

1.67

0.51

3.25

0.0037

Kwok-Neumann

Chen-Chang

vOCG

(Continued)

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Table 5.4 Fitting parameters for the various models using PVC. (Continued) γ+

Std. Error

t

p

0.171395

0.0999573

1.71

0.10

γT

γAB

40.83 LV

+ -

LW

AB

0.53 T

Units of γc ,γsv ,γ ,γ γ ,γ , γ , γ are mJ/m2. Units of PL,Pa,Pb are (mJ/m2)1/2.

In Table 5.4 four quantities are listed for each fitted parameter. These quantities are the parameter estimate, the standard error of the estimate, Student’s t value for comparing the parameter value to zero, and the corresponding p value calculated from the value of t. If t > 0.05 then the parameter estimate is statistically equal to zero and can be excluded from the model. The results in Table 5.4 show that estimates of γT, the total surface free energy, for PVC are similar except that calculated using the ChenChang model. The Chen-Chang estimate for γT is considerably smaller than that calculated using the other models. For the van Oss, Chaudhury and Good model, the estimate for γ+ is not significantly different from zero. Table 5.4 also lists the value of AICc for each model. The smaller the value of AICc the better the fit. The values of AICc are also smaller for models with fewer adjustable parameters. The results obtained using the van Oss, Chaudhury and Good model indicate that γ+ = 0 (statistically). For this reason, the data were refitted using the assumption that γ+ = 0. It was found that NonlinearFit would not successfully converge when both methylene iodide (MeI) and α-bromonaphthalene (BrN) were included in the data set. For this reason, separate fits excluding either MeI or BrN were made. The results are seen in Table 5.5. The two fits give very similar estimates of the surface free energy parameters.

5.3.2 Fitting of PMMA Data The contact angles of the various probe liquids on PMMA are listed in Table 5.6. The results of fitting the contact angle data to the various models are listed in Table 5.7. The format of Table 5.7 is the same as that for the corresponding table using PVC as the substrate material (Table 5.4). This table indicates that all of the fitted parameters are significantly different from zero except for the value of γ+ for the van Oss, Chaudhury and Good model. For γ+ the corresponding p value is greater than 0.05 indicating that

Surface Free Energy of Solid Surfaces 89 Table 5.5 Fitting parameters for the corrected van Oss, Chaudhury and Good model using PVC. PVC corr Van Oss

γLW

Std. Error

t

p

AICc

No MeI

43.94

1.01

43.4

1.2 x 10-19

108.624

γ−

Std. Error

t

p

2.51

0.52

4.8

0.00014

γ+

Std. Error

t

p

0 PVC corr Van Oss

γLW

Std. Error

t

p

AICc

No BrN

42.69

1.01

42.4

1.7 x 10-19

110.342

γ−

Std. Error

t

p

3.15

0.60

5.21

0.000059

γ+

Std. Error

t

p

0 + -

LW

Units of γ γ ,γ

are mJ/m2.

Table 5.6 Contact angles (deg) of the various liquids on PMMA (simulated data set with 5 replicates). Water

79.0895

79.3086

81.0110

79.9275

78.2330

Methylene iodide

42.1580

40.1154

41.8189

40.0557

41.4993

Formamide

64.1944

65.4147

64.5412

62.7165

63.0938

α-Bromonaphthalene

16.4010

15.7675

14.5401

14.6793

16.1287

Glycerol

69.0859

69.6309

68.5057

67.5686

67.6797

Tricresyl phosphate

19.4604

19.2464

17.5211

19.5954

20.4538

Simulated data are expressed here to a much greater precision than experimentally possible.

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Table 5.7 Fitting parameters for the various models using PMMA. Zisman γc

Std. Error

t

p

AICc

40.37

0.68

59.7

4.5 x 10-31

80.02

β

Std. Error

t

p

-0.02658

0.00098

-27.20

1.1 x 10-21

γsv

Std. Error

t

p

AICc

39.98

0.55

72.1

2.3 x 10-33

71.77

β

Std. Error

t

p

0.0002448

0.000027

9.1

7.3 x 10-10

PL

Std. Error

t

p

AICc

8.27

0.15

56.1

1.7 x 10-29

144.615

Pa

Std. Error

t

p

-1.30

0.25

-5.22

0.000016

Pb

Std. Error

t

p

-2.75

0.15

-18.2

1.0 x 10-16

γsv

γLW

γAB

30.62

34.20

-3.58

γLW

Std. Error

t

p

AICc

31.08

2.01

15.5

2.6 x 10-13

181.687

γ−

Std. Error

t

p

8.04

2.83

2.84

0.0096

Kwok-Neumann

Chen-Chang

vOCG

(Continued)

Surface Free Energy of Solid Surfaces 91 Table 5.7 Fitting parameters for the various models using PMMA. (Continued) γ+

Std. Error

t

p

0.53

0.44

1.20

0.242

γT

γAB

33.15 LV

+ -

LW

AB

2.07 T

Units of γc ,γsv ,γ ,γ γ ,γ , γ , γ are mJ/m2. Units of PL,Pa,Pb are (mJ/m2)1/2.

the value of γ+ is statistically equal to zero. For this reason, the van Oss, Chaudhury and Good model was refitted under the assumption that γ+ = 0. This is the same approach as used for PVC. As it was not possible to achieve a satisfactory fit using both methylene iodide and α-bromonaphthalene, data sets without the contact angles from one of these liquids were fitted. Table 5.8 Fitting parameters for the corrected van Oss, Chaudhury and Good model using PMMA. PMMA corr vOCG

γLW

Std. Error

t

p

AICc

No MeI

42.41

0.38

110.77

5.8 x 10-27

70.40

γ−

Std. Error

t

p

6.33

0.32

19.84

1.1 x 10-13

γ+

Std. Error

t

p

0 PMMA corr vOCG

γLW

Std. Error

t

p

AICc

No BrN

40.31

0.47

85.25

6.4 x 10-25

81.28

γ−

Std. Error

t

p

7.44

0.45

16.60

2.3 x10-12

γ+

Std. Error

t

p

0 Units of γ+γ-,γLW are mJ/m2.

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These corrected results are shown in Table 5.8. In Table 5.7 which shows the results for the uncorrected van Oss, Chaudhury and Good model, it is noted that values of γT, the total surface free energy, differ substantially between models. The Chen-Chang as well as the van Oss, Chaudhury and Good models show lower values of γT (γT = γsv = γc) than those for the Zisman and Kwok-Neumann models. The corrected values of γT for the van Oss, Chaudhury and Good ,shown in Table 5.8, are, however, similar to those determined using the Kwok-Neumann and Zisman models. The quality of the fit to the Chen-Chang and the van Oss, Chaudhury and Good models is poorer than that to the Kwok-Neumann or the Zisman models.

5.3.3 Assessing Which Model is Best Table 5.9 shows the values of AICc for the fits of the PVC data to the various models. Table 5.10 shows the corresponding values for fits to the PMMA data. The smallest value of AICc in a set of cohort models indicates the statistically best model consistent with the data. Eqn. (5.33) can be used to calculate the relative probability, w, that a given model is the statistically correct model. In Tables 5.9 and 5.10 two comparisons are made. In Columns 3 and 4 all of the statistically significant models are compared. In Columns 5 and 6 only the Chen-Chang as well as the van Oss, Chaudhury and Good models are compared. According to Eqns. (5.29 and 5.30) the value of AIC depends, in part, on the value of the difference between the calculated response and the actual data. In the present work, the response quantity for the Zisman and KwokNeumann models is 1+ Cos (θ) while for the Chen-Chang as well as the van

Table 5.9 AICc, Δ(AICc) and w for fits to PVC data. AICc

Δ (AICc)

w

Δ (AICc)

w

Zisman

69.62

0

0.75

Kwok-Neumann

71.77

2.15

0.25

Chen-Chang

131.404

61.78

3 x 10-14

22.78

8 x 10-6

vOCG

135.639

66.07

3 x 10-15

27.02

1 x 10-6

vOCG corr No MeI

108.624

39.00

3 x 10-9

0

0.70

vOCG No BrN

110.342

40.70

1 x 10-9

1.7

0.30

Surface Free Energy of Solid Surfaces 93 Table 5.10 AICc, Δ(AICc) and w for fits to PMMA data. AICc

Δ (AICc)

w

Δ (AICc)

w

Zisman

80.02

9.62

0.005

Kwok-Neumann

71.77

1.37

0.33

Chen-Chang

144.62

74.22

5 x 10-17

57.13

4 x 10-13

vOCG

181.69

111.29

2 x 10-40

74.44

7 x 10-17

vOCG corr No MeI

70.40

0

0.66

0

.995

vOCG No BrN

81.28

10.88

0.003

10.88

.004

Oss, Chaudhury and Good models it is γL[1 + Cos (θ)]. The responses in the two cases are significantly different in magnitude, thus making a direct comparison far less reliable. Tables 5.9 and 5.10 both indicate that KwokNeumann and Zisman models have at least a fair probability of being correct. When comparing the Chen-Chang model to the van Oss, Chaudhury and Good model it appears that the van Oss, Chaudhury and Good model is the better one. It appears that the exclusion of contact angles determined using methylene iodide is the better choice. It is also noteworthy that the Zisman, Kwok-Neumann and the corrected van Oss, Chaudhury and Good models yield remarkably similar values for γT, the total surface free energy. Some caution in interpreting the AICc should be used as the data sets are not completely identical. The response variables for the Zisman and Kwok-Neumann models differ from those of the Chen-Chang and van Oss, Chaudhury and Good models. There are also different numbers of liquids used in the various models. Principally, the van Oss, Chaudhury and Good surface free energy components for tricresyl phosphate are unknown and thus these contact angles cannot be used in the fitting to the van Oss, Chaudhury and Good model. Figures 5.1 and 5.2 are plots of 1 + Cos θ versus γL. The plots show both the experimental data and the calculated values of 1 + Cos θ for various models. It is clear from the figures that the experimental variations of the contact angle measurements are not the major cause of lack of fit. Rather errors associated with the functional form of the models or in the case of the Chen-Chang and the van Oss, Chaudhury and Good models the values of the surface tension parameters for the probe liquids may be the cause for any lack of fit. It is also clear that the data cannot be used to justify

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1+ Cos θ

1.8 1.6 1.4 1.2 1.0

45

50

55

60

65

70

γL

Figure 5.1 1+ Cos (θ) vs γL (mN/m) for PVC. Solid dots (orange) - the data. Solid line (blue) - fit to Zisman model. Dashed line (red) - fit to Kwok-Neumann model. Triangles (green) - predicted values from the Chen-Chang model. Open squares (black) - predicted values using the corrected van Oss, Chaudhury and Good model without MeI. Open triangles (blue) - the corrected van Oss, Chaudhury and Good model without BrN.

2.0

1+ Cos θ

1.8 1.6 1.4 1.2 45

50

55 γL

60

65

70

Figure 5.2 1+ Cos (θ) vs γL (mN/m) for PMMA. Solid dots (orange) - the data. Solid line (blue) - fit to Zisman model. Dashed line (red) - fit to Kwok-Neumann model. Triangles (green) - predicted values from the Chen-Chang model. Open squares (black) - predicted values using the corrected van Oss, Chaudhury and Good model without MeI. Open triangles (blue) - the corrected van Oss, Chaudhury and Good model without BrN.

more than two adjustable fitting parameters. The Zisman, Kwok-Neumann and the corrected versions of the van Oss, Chaudhury and Good models have two adjustable parameters. Each of these models, furthermore, yields nearly identical values of surface free energy. Perhaps Occam’s razor would favor the Kwok-Neumann model.

Surface Free Energy of Solid Surfaces 95

5.4 Summary and Conclusions In this chapter the relative merits of various models used to calculate the surface free energy of solid surfaces are explored. Specifically, models discussed earlier by Zisman, Kwok-Neumann, Chen and Chang as well as van Oss, Chaudhury and Good are explored. Contact angle data by Dalal [25] have been used to generate a simulated data set. The simulated data set is constructed by adding a random number taken from a normal distribution of values that has a mean of zero and standard deviation of 1 to the contact angles experimentally measured earlier by Dalal. In this way, a data set consisting of 5 replicate determinations for each of the probe liquids is generated. Etzler [28] has previously discussed the increased statistical power provided by the use of individual replicate values of contact angles. The simulated data have been fitted to the models discussed in this paper and the results have been statistically evaluated. Contact angles of several probe liquids on poly(vinyl chloride) and poly(methyl methacrylate) have been investigated. The statistical evaluation shows that the Zisman and Kwok-Neumann models give nearly identical results. (See Figures 5.1 and 5.2) The fit to the Chen-Chang and van Oss, Chaudhury and Good models is generally inferior to that of the Zisman and Kowk-Neumann models. It is important to note that factors other than the experimental variability in the contact angle measurements appear to contribute to the lack of fit. Many estimates for the value of 1+ Cos (θ) lie outside the cluster of experimental values. For the van Oss, Chaudhury and Good model it is observed that γ+ is statistically equal to zero. Elimination of this variable from the analysis improves the fit substantially. Two corrected models using the van Oss, Chaudhury and Good model were investigated. One model excluded contact angle data from methylene iodide and the other the data from α-bromonaphthalene. While both corrected models give similar estimates of the surface free energy parameters of the solid, the exclusion of methylene iodide results in a better fit. Methylene iodide has been shown to exhibit stick-slip behavior on some surfaces [33]. If stick-slip behavior occurs, the contact angles are not appropriate for surface free energy analysis. Measurements using axisymmetric drop shape analysis are required to determine if stickslip behavior occurs [33]. The corrected van Oss, Chaudhury and Good model yields a similar value of γT, the total surface free energy, to that found for the Zisman and Kwok-Neumann models. It is, furthermore, important to note that

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the present analysis suggests that models with more than two adjustable parameters are statistically inferior. From this study, it can be seen that a careful statistical evaluation of the fitting process is a necessary part of the determination of the surface free energy of solids. In the past, statistical analysis had not been a part of the process of determining the surface free energy of solids. It is hoped that future studies will include statistical analysis as part of the critical evaluation of surface free energy calculations. This will allow for a more critical evaluation of surface free energy determinations.

References 1. Hubbe, M.A., Gardner, D.J., Shen, W., Contact angles and wettability of cellulosic surfaces: A review of proposed mechanisms and test strategies. Bioresources, 10, 8657–8749, 2015. 2. Etzler, F.M., Characterization of surface free energies and surface chemistry of solids, in: Contact Angle Wettability and Adhesion, Vol.3, K.L. Mittal (Ed.), pp. 219–264, CRC Press, Boca Raton, FL, 2003. 3. Etzler, F.M., Determination of the surface free energy of solids: A critical review. Rev. Adhesion Adhesives, 1, 3–45, 2013. 4. Etzler, F.M. and Gardner, D.J., Surface free energy determination of powders and particles with pharmaceutical applications: A critical review. Rev. Adhesion Adhesives, 6, 329–367, 2018. 5. Thompson, W., On the thermal effect of drawing out a film of liquid. Proc. Roy. Soc. (London), 9, 255– 256, 1858. 6. Gibbs, J.W., Collected Works, vol. 1, Longmans, Green, New York, 1928. 7. Zisman, W.A., Relation of equlibrium contact angle to liquid and solid constitution, in: Contact Angle, Wettability and Adhesion, Adv. Chem. Ser. No. 43, pp. 1–51, American Chemical Society, Washington, D.C. 1964. 8. Mittal, K.L., The role of the interface in adhesion phenomena. Polym. Eng. Sci., 17, 467– 473, 1977. 9. Kwok, D.Y. and Neumann, A.W., Contact angle measurements and contact angle interpretation: Relevance to the thermodynamics of adhesion, in: AcidBase Interactions: Relevance to Adhesion Science and Technology, vol. 2, K.L. Mittal (Ed.), pp. 91–166, CRC Press, Boca Raton, FL, 2000. 10. Baxter, S. and Cassie, A.B.D., The water repllency of fabrics and a new water repellency test. J. Textile Inst., 36, T67–90, 1945. 11. Cassie, A.B.D., Contact angles. Disc. Faraday Soc., 3, 11–16, 1948. 12. Cassie, A.B.D. and Baxter, S., Wettability of porous surfaces. Trans. Faraday Soc., 40, 546–551, 1944. 13. Wenzel, R.N., Resistance of solid surfaces to wetting by water. Ind. Eng. Chem., 28, 505–514, 1936.

Surface Free Energy of Solid Surfaces 97 14. Grundke, K., Bogumil, T., Gietzelt, T., Jacobash, H.-J., Kwok, D.Y., Neumann, A.W., Wetting measurements on smooth, rough and porous solids. Prog. Colloid Polym. Sci., 101, 58–68, 1996. 15. Adamson, A.W., The Physical Chemistry of Surfaces, 4th Ed., John Wiley, New York, 1990. 16. Antonow, G., Surface tension limit of two layers. J. Chim. Phys., 5, 372–385, 1907. 17. Berthelot, D., Sur le melange des gaz. Compt. Rend., 126, 1857, 1898. 18. Li, D. and Neumann, A.W., A reformulation of the equation of state for interfacial tensions. J. Colloid Interface Sci., 137, 304–307, 1990. 19. Good, R.J., Contact angle, wetting and adhesion: A critical review, in: Contact Angle, Wettability and Adhesion, K.L. Mittal (Ed.), pp. 3–36, VSP, Utrecht, The Netherlands, 1993. 20. van Oss, C.J., Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994. 21. Owens, D.K. and Wendt, R.C., Estimation of the surface free energy of polymers. J. Appl. Polym. Sci., 13, 1741–1747, 1969. 22. Fowkes, F.M., Determination of interfacial tensions, contact angles, and dispersions forces in surfaces by assuming additivity of intermolecular interactions in surfaces. J. Phys. Chem., 66, 382, 1962. 23. Fowkes, F.M., Calculation of the work of adhesion by pair potential summation. J. Colloid Interface Sci., 28, 493–505, 1968. 24. Good, R.J., On the acid/base theory of contact angles, in: Acid-Base Interactions: Relevance to Adhesion Science and Technology, vol. 2, K.L. Mittal (Ed.), pp. 167–172, CRC Press, Boca Raton, FL, 2000. 25. Dalal, E.N., Calculation of solid surface tensions. Langmuir, 3, 1009–1015, 1987. 26. Chang, W.V. and Qin, X., Repulsive acid-base interactions: Fantasy or reality, in: Acid-Base Interactions: Relevance to Adhesion Science and Technology, vol. 2, K.L. Mittal (Ed.), pp. 3–54, CRC Press, Boca Raton, FL, 2000. 27. Chen, F. and Chang, W.V., Applicability study of a new acid - base interaction model in polypeptides and polyamides. Langmuir, 7, 2401–2404, 1991. 28. Etzler, F.M., Determination of the surface free energy of solid surfaces: Statistical considerations, in: Advances in Contact Angle, Wettability and Adhesion, Vol.3, K.L. Mittal (Ed.), pp. 301–330, Wiley-Scrivener, Beverly, MA, 2018. 29. Spiess, A.-N. and Neumeyer, N., An evaluation of r2 as an inadequate measure for nonlinear models in pharmacological and biochemical research: A Monte Carlo approach in. BMC Pharmacol., 10, 2010, Article 6. 30. Aho, K., Derryberry, D., Paterson, T., Model selection for ecologists: The worldviews of AIC and BIC. Ecology, 95, 631–636, 2014. 31. Anderson, D.R. and Burnham, K.P., AIC model selection in overdispersed capture-recapture data. Ecology, 75, 1780–1793, 1994.

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32. Hurvich, C.M. and Chih-Ling, T., Regression and time series model selection in small samples. Biometrika, 76, 297–307, 1989. 33. Kwok, D.Y., Gietzelt, T., Grundke, K., Jacobasch, H.-J., Neumann, A.W., Contact angle measurements and contact angle interpretation. 1. Contact angle measurements by axisymmetric drop shape analysis and a goniometer sessile drop technique. Langmuir, 13, 2880–2894, 1997.

6 Surface Free Energy Characterization of Talc Particles Ismail Yildirim and Roe-Hoan Yoon* Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA

Abstract Free energy of interaction between particles in water can be calculated if one knows the values of appropriate interfacial tensions, which, in turn, can be determined from the surface free energies of the particles involved. The latter can be determined using the van Oss-Chaudhury-Good (vOCG) equation. This method requires the measurement of contact angles of well-defined liquids on the surfaces of the particles of interest. In the present work, the contact angles were determined using the heat of wetting, capillary rise, and thin-layer wicking methods. The results are compared with each other, and the utility of the surface free energy data obtained from the contact angles is discussed. Keywords: Surface free energy, acid-base theory, vOCG theory, contact angle, heat of immersion, talc, Lifshitz-van der Waals energy

6.1 Introduction Talc is an inherently hydrophobic mineral. Particles of talc have the shape of platelets due to the layered structure of the mineral. It is well known that the basal surfaces are hydrophobic, while the edge surfaces are hydrophilic. The hydrophobicity of the basal surfaces arises from the fact that the atoms exposed on the surface are linked together by siloxane (Si-O-Si) bonds which do not form strong hydrogen bonds with water. On the other hand, the edge surfaces are hydrophilic, because the atoms exposed on *Corresponding author: [email protected] K.L. Mittal (ed.) Advances in Contact Angle, Wettability and Adhesion: Volume 4, (99–114) © 2020 Scrivener Publishing LLC

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the surfaces can form strong H-bonds with surrounding water molecules. Thus, the hydrophobicity of talc particles as a whole may change with the basal-to-edge surface ratio (or aspect ratio), which, in turn, changes with particle size and crystallinity. The most commonly used measure of hydrophobicity of a solid is the contact angle (θ) of water on the surface. In general, the higher the contact angle is, the more hydrophobic the solid surface becomes. However, it would be more useful to derive information on surface free energy from the contact angle data. The acid-base theory developed by van Oss and coworkers [1–3] is useful to characterize solids in terms of their surface free energies and surface free energy components. Such information is useful for predicting various surface interactions. Talc is widely used in the paper industry to remove sticky substances present in wood pulp. It is possible to predict whether a sticky material can be collected by talc particles or not (and how well), if appropriate surface free energy data are available. Therefore, developing a convenient method of determining surface free energies and their components of particulate materials is important. In the present work, the contact angles of various liquids on powdered talc samples were determined using three different methods, i.e., capillary rise technique, thin layer wicking, and heat of immersion. The data were used to determine the surface free energies and their components using the van Oss-Chaudhury-Good equation [1–3]. Etzler [4, 5] reviewed various models for determining surface free energies of solids from contact angles, and Chibowski et al. [6] stressed the importance of measuring the contact angles on powders for many industrial applications. It is believed that the surface free energy data obtained in the present work will be useful for predicting various surface interactions involving fine particles of talc and other minerals.

6.2 Theoretical Background 6.2.1 vOCG Equation Fowkes [7] proposed that the surface tension of a material i can be subdivided into separate components: j i

i j

(6.1)

Surface Free Energy Characterization of Talc 101 where j refers to the type of surface tension component, e.g., dispersion, H-bonding, and metallic interactions. Thus, LW i

i

AB i

(6.2)

where iLW and iAB refer to the Lifshitz-van der Waals (non-polar) and acid-base (polar) components, respectively. The latter represents interactions between Lewis acids (electron acceptors) and bases (electron donors) on the surface. van Oss, Chaudhury, and Good (vOCG) [1–3] defined: AB i

2

i

(6.3)

i

where +i is the acidic component of the surface tension (or surface free energy) and i is the basic component. Therefore, Eq. (6.2) becomes, i

LW i

2

i

(6.4)

i

It can be readily shown also that the free energy change (ΔGSL) associated with the adhesion of a liquid to a solid is given by:

ΔGSL

LW SV

2

LW LV

SV

LV

SV

(6.5)

LV

where iLW is the Lifshitz-van der Waals component of the surface free energy, i is the acidic component, and i is the basic component of the interacting solid (represented by subscript SV) and liquid (represented by subscript LV) surfaces. Combining this with the Dupre equation [8]:

ΔGSL = γSL – γSV – γLV

(6.6)

one obtains:

SL

SV

LV

2

LW SV

LW LV

SV

LV

SV

LV

, (6.7)

which can be used to obtain interfacial tensions (or interfacial free energies).

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Combining Eq. (6.7) with Young’s equation [9],

γLV cos θ = γSV – γSL,

(6.8)

where θ is contact angle, one obtains the vOCG equation [1–3]:

1 cos

LV

LW LW SV LV

2

SV LV

(6.9)

SV LV

which is useful for characterizing a solid surface in terms of its surface free + energy components, i.e., LW SV , SV , and SV. To determine these values, it is necessary to measure contact angles of three different liquids of known properties (in terms of +LV , LV, LW LV ) on the surface of the solid of interest. One can then set up three equations with three unknowns, which can + be solved to obtain the values of LW SV , SV , and SV. Table 6.1 gives a list of liquids that can be used for the contact angle measurements, along with the + values of γLV, LW LV , LV , and LV.

6.2.2 Contact Angle Measurements The thin layer wicking, capillary rise and heat of immersion techniques can be used to obtain the values of contact angles on powdered samples. In the thin layer wicking technique, a powdered sample is deposited on a microscopic glass slide in the form of aqueous slurry [10, 11]. After drying the sample, one end of the glass slide is immersed vertically in a liquid. The liquid will start to rise along the height of the slide through the capillaries formed in between the particles deposited on the glass surface. The driving Table 6.1 Values of the surface tension components (in mJ/m2) of the liquids used in the experiments. Liquid

γLV

γLVLW

γLVAB

γLV+

γLV−

α-Bromonaphthalene

44.4

44.4

0

0

0

Methylene iodide

50.8

50.8

0

0

0

Ethylene glycol

48.0

29.0

19.0

1.92

47.0

Formamide

58.0

39.0

19.0

2.28

39.6

Water

72.8

21.8

51.0

25.5

25.5

γLVAB: acid-base component of liquid surface tension.

Surface Free Energy Characterization of Talc 103 force for the penetration of the liquid into a dry layer of particles on the glass surface is the capillary pressure across the liquid-vapor interface. Therefore, the liquid enters the capillaries spontaneously if the contact angle is less than 90o. The velocity at which a liquid creeps up the slide is measured, and then converted to a contact angle using the Washburn equation [12]:

2l 2 LV r t

cos

(6.10)

where l is the length of capillary rise in time t, γLV is liquid surface tension, η is the liquid viscosity, and r* is the effective radius of the pores formed between particles. The value of r* can be determined using a completely spreading non-polar liquid such as heptane, octane, decane, or dodecane. In this case, it is considered that cos θ = 1. Once the value of r* is known, the contact angles of non-wetting liquids such as water and formamide can be determined using Eq. (6.10). Alternatively, capillary rise technique can be used to obtain the contact angles on powdered surfaces using the Washburn equation [13, 14]. One problem with this technique is the uncertainty associated with determining r*. There is no guarantee that the value of r* determined with a completely wetting liquid would be the same as that determined by a less than completely wetting liquid. Reproducibility and repeatability of test results also depend on the shape and size of the particles. It has been stated that monosized and spherical particles give more reproducible results [11]. However, particle bed disturbances and skewing may be observed when the particles are extremely fine and platy in shape. Note here that both the thin layer wicking and capillary rise methods give advancing contact angles rather than equilibrium angles. Contact angles of powdered samples can also be determined from heat of immersion (-ΔHi). In the present work, the heats of immersion were measured using a flow microcalorimeter, which was used to calculate θ using the following relationship:

cos

1 HL

LV

T

cos T

ΔHi

(6.11)

p

where HL is the enthalpy of liquid. The values of HL and γLV are readily available in literature. The values of -ΔHi are obtained from heat of immersional wetting experiments, while the values of ∂cosθ/∂T are obtained using the capillary rise technique.

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6.3 Experimental 6.3.1 Talc Samples A run-of-mine (ROM) talc sample from Montana was received from Luzenac America, Greenwood Village, CO. The purity of the talc sample was greater than 98%. It was crushed to finer than 50 mm using a handheld hammer. The crushed sample was ground subsequently to less than 150 μm using an agate mortar and pestle and screened to obtain -150+53, -75+53 and -53 μm size fractions. Here the negative sign means ‘finer than’ and the positive size means ‘larger than.’ These size fractions were used for i) heat of immersion measurements using a flow microcalorimeter, and ii) contact angle measurements using the capillary rise and thin layer wicking techniques. A number of powdered talc samples were also obtained from Luzenac America. Due to proprietary reason, these commercial products are referred to as Samples A (d50 = 12.5 μm), B (d50 = 3.5 μm), C (d50 = 3.0 μm), and D (d50 = 3.4 μm) in the present work. They were used for characterization studies as received.

6.3.2 Liquids Contact angle measurements were conducted using the following nonpolar and polar liquids: methylene iodide and α-bromonaphthalene, water, formamide, and ethylene glycol. The organic liquids used in the present work were HPLC grade (>99.5% purity). They were dried overnight over Davidson 3-A molecular sieves (3-12 mesh) before use. α-Bromonaphthalene and methylene iodide were obtained from Aldrich Chemical Company while formamide and ethylene glycol were from Fisher Scientific. All experiments were conducted using Nanopure water produced with a Barnstead Nanopure II water purification system.

6.3.3 Capillary Rise Method A known amount of powder was placed in a glass tube of 6.5 mm inner diameter by manual tapping. The bottom part of the tube was closed with a glass frit to hold the particles in place. The glass tube filled with a particulate sample was placed in upright position in a wetting liquid, and the height of the liquid rising along the column of particles was measured as a function of time. The measurements were conducted three to five times with a given sample and a liquid, and the results were averaged.

Surface Free Energy Characterization of Talc 105 The effective capillary radius (r* in Eq. (6.10)) was determined using heptane, which was found to completely wet the talc samples. The value of r* determined as such was used to calculate the contact angle of a liquid which may not wet the talc sample completely using Eq. (6.10). In the present work, contact angles of three different liquids, i.e., α-bromonaphthalene, water and formamide, were determined using the capillary rise technique.

6.3.4 Thin Layer Wicking Method A sample was dispersed in water at 5 w/v%. Approximately 3 ml of the suspension was withdrawn by means of a pipette and sprayed over a glass slide (1x3 inches). The glass slide coated with the slurry was allowed to dry in air at ambient temperature. The sample was dried further in an oven at 110°C to remove residual water that may be left in the pores formed between the particles. The glass slide coated with a dry talc powder was placed vertically in a liquid whose contact angle on the talc sample was to be determined. The slide was immersed 5 mm into the liquid, while the rest of the height was exposed to ambient. Before each experiment, the coated glass slide was kept inside a closed container for about one hour so that the powder would come to equilibrium with the liquid vapor [11]. The velocity at which the liquid rose along the height of the coated surface of the glass slide was measured. With each liquid, the test was repeated at least three times and the results were averaged. For each sample, the value of r* in Eq. (6.10) was determined using non-polar liquids, i.e., heptane, octane, decane and dodecane, which completely wet the talc sample.

6.3.5 Heat of Immersion Method Heats of immersion measurements were conducted using a flow microcalorimeter from Microscal, United Kingdom. A calorimeter cell, made of Teflon, was placed in a metal block, which was insulated from the ambient by mineral wool. Two glass-encapsulated thermistors were placed inside the cell to monitor the changes in temperature of the sample, and two reference thermistors were placed in the metal block outside the cell. The calorimeter was calibrated by means of a calibration coil, which was placed in the sample bed. The entire unit was housed in a draft-proof enclosure to reduce the effect of temperature fluctuations in the ambient. In each measurement, a talc sample was dried overnight in an oven at 110°C. A known amount (usually 0.05 to 0.15 g) of the dried sample was placed in the calorimeter cell, and degassed for at least 30 minutes under

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vacuum (90° of WmQ demonstrated a dry state of SC due to a low presence of water (hydrophobic state). This suggested to us to apply the liposomal dispersion (skindecoder ) on the forearm’s skin area in order to achieve the best skin functionality and evaluate the influence of the liposomal dispersion on the SFE of skin. In light of the results shown in Figure 7.14, we hypothesized that the behaviour of the CAs of PFPEd after 10s can be related to different factors such as the film of liposomal dispersion previously nebulized, water, typical polar and dispersion components of SFE of skin and SC, etc. The complex system formed by these components led us to define the activated skin as an “Integrated Chemical Cutaneous System (ICCS)” with appropriate SFE, DC and PC and expressed by Equation (7.4). SKIN

(

d A

d B

d C

d D

......

d n

) (

p A

p B

p C

p D

......

p n

) (7.4)

Contact angle (deg)

100 90 80 70 60 50 40 30 20 10 0

120 110 100 90 80 70 60 50 40 30 20 10 0

Water Basal

1

0

1

y = -3.425x + 60.125 R2 = 0.9788

0

R2 = 0.9344

y = -5.6895x + 69.676

3

3

R2 = 0.0182

y = 0.1x + 108.1

7

5 Time (sec) 7

PFPEd activated skin

PFPEdBasal

9 10

y = -3.745x + 82.205 R2 = 0.5207

y = -0.34x + 82.76 R2 = 0.0124

9

R2 = 0.577

10

PFPEd Basal

y = -3.265x + 101.79

Test subject SR

Time (sec)

5

PFPEd activated skin

Contact angle (deg)

120 110 100 90 80 70 60 50 40 30 20 10 0

120 110 100 90 80 70 60 50 40 30 20 10 0

0

0

1

Water Basal

1

y = -4.315x + 110.74 R2 = 0.8387

Water Basal

3

3

7

5 Time (sec)

PFPEd Basal

7

10

9

10

y = -8.242x + 93.508 R2 = 0.9447

y = -9.4105x + 121.79 R2 = 0.9316

y = 0.721x + 90.891 R2 = 0.5431

PFPEd Basal

9

y = -4.195x + 55.405 R2 = 0.7527

y = -1.63x + 109.12 R2 = 0.8274

PFPEd activated skin

Test subject HM

5 Time (sec)

PFPEd activated skin

Test subject SM

Figure 7.14 Behaviour of CAs of water and PFPEd with time (10s) measured on the in vivo skins of N=4 test subjects before (basal) and after activation with liposomal dispersion (activated skin).

Contact angle (deg)

Test subject AB

Contact angle (deg)

Water Basal

Surface Free Energy of Skin 133

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Average contact angle (deg)

where γSKIN is Surface Free Energy of skin defined as an Integrated Chemical Cutaneous System (ICCS), d is dispersion component, p is polar component, A is the liposomal dispersion, B and C are polar and non-polar substances normally present on untreated skin, D the SC, and n other compounds present on skin surface. The γSKIN represents the real cutaneous surface system because the nebulization of liposomal dispersion improves the skin barrier and, consequently, the hydration state of SC. The only parameter of difference between untreated and treated skin should be the CA of water (WmQ), which represents the increase of hydration state of skin after the activation with skindecoder liposomal dispersion. On the basis of these considerations, the OWRK approach for the evaluation of the applicability of the surface free energy model developed for the calculation of SFE of in vivo activated skin confirms that the pair of test liquids PFPEd (ST=18.1 mN/m, DC=18.0 mN/m, PC=0.1 mN/m) and WmQ (ST=72.8 mN/m, DC=21.8 mN/m, PC=51.0 mN/m) is ok for SFE characterization of skin. The PFPEd led to repulsion forces at the interface, while the contact between WmQ and skin generates adhesion forces. The “negative adhesion” of PFPEd is mainly due to its repulsive characteristics and this makes possible to consider it as a reference test liquid for the determination of SFE of in vivo activated skin. For these reasons, these two liquids were chosen as the pair for the study of the SFE of skin activated with liposomal dispersion (ICCS) also. In Figure 7.15 are reported the average CAs of PFPEd measured on activated skin and the CAs of water (WmQ) measured on the surface of untreated skin. Figure 7.15 shows clearly that CAs of PFPEd measured on the activated skins of different kinds of test subjects are higly reproducible (31.8±2.4). 119 112 105 98 91 84 77 70 63 56 49 42 35 28 21 14 7 0 Before treatment (WmQ) After treatment (PFPEd) Test liquids

Figure 7.15 Average CAs of PFPEd measured on skin after treatment with liposomal dispersion (activation) and those of water (WmQ) measured before treatment.

Surface Free Energy of Skin 135 However, the CAs of WmQ measured after 10s on untreated skin showed high variability because of the different hydration states of the skins of the test subjects (96.2±11.3).

7.3.3 Determination of SFESKIN and Applicability of TVS Skin Test by the SFECA Method The surface tension method for the evaluation of SFESKIN was applied by measuring the CAs (t=10s) of PFPEd before and after the application of liposomal dispersion (skindecoder ) on 50 test subjects. In Figure 7.16 is shown the comparison between the average values of SFESKIN, DC and PC of the group, the values for the test subject with less skin hydration (S31) and the surface free energy parameters of the more hydrated (S17) test subject after activation with skindecoder . Figure 7.16 shows clearly the different kinds of responses in terms of SFESKIN of the test subjects 17 and 31 with respect to the average value, after activation with skindecoder . The different responses are due to the different chemical compositions of skin surfaces after activation with liposomal dispersion. The components of the skin surface after treatment are constituted mainly of liposomal dispersion, water with its solutes, and SC. Some of these components change after nebulization of the liposomal dispersion (skindecoder ) increasing the SC hydration and, consequently, the selective permeability of the epidermal barrier is enhanced due to the presence of liposomes. In Figure 7.17 are reported the comparisons between

Activated skin (skindecoder R ): average N=50 SFE (mJ/m2) 70 60 50 Test subject 31 40 Test subject 17 30 Average N=50 20 10 0

PC (mJ/m2) Test liquid Water (WmQ) PFPEd

DC (mJ/m2)

Contact angle (deg) Subject 31 Subject 17 Average (N=50) 55.0 24.0 81.4 28.5 32.7 34.5

Figure 7.16 Variations in surface free energy (SFE) and its DC and PC of skin in relation to two different test subjects after activation with skindecoder (activated skin) and average (N=50).

Contact angle (deg) Subject 31 Subject 17 Average (N=50) 45.1 87.0 68.5 55.7 50.7 35.3

Water (WmQ) PFPEd

Test liquid

Contact angle (deg) Subject 31 Subject 17 Average (N=50) 24.0 55.0 81.4 32.7 28.5 34.5

DC (mJ/m2)

Water (WmQ) PFPEd

Test liquid

DC (mJ/m2)

Test subject 31 basal Test subject 31 activated

Contact angle (deg) Contact angle (deg) Test liquid Subject 31 Subject 17 Average (N=50) Subject 31 Subject 17 Average (N=50) 24.0 Water (WmQ) 87.0 68.5 81.4 45.1 55.0 34.5 32.7 28.5 50.7 PFPEd 35.3 55.7

PC (mJ/m2)

0

10

20

30

Figure 7.17 Comparison between surface free energy (SFE) and its DC and PC of in vivo skins of test subjects 17, 31, and average (N=50) before (basal) and after treatment (activation) with liposomal dispersion (skindecoder ).

Water (WmQ) PFPEd

Test liquid

PC (mJ/m2)

Test subject 17 basal Test subject 17 activated

SFE (mJ/m2) 40

Subject 31 Subject 17 Average (N=50) 55.0 24.0 81.4 34.5 28.5 32.7

Contact angle (deg)

DC (mJ/m2)

SFE (mJ/m2) 70 60 50 40 30 20 10 0

Water(WmQ) PFPEd

Test liquid

In vivo skin before (basal) and after activation: test subject 31

Contact angle (deg) Subject 31 Subject 17 Average (N=50) 68.5 87.0 45.1 55.7 35.3 50.7

PC (mJ/m2)

Average basal N=50 Average activated N=50

In vivo skin before (basal) and after activation: test subject 17

Water(WmQ) PFPEd

Test liquid

SFE (mJ/m2) 50 40 30 20 10 0

in vivo skin before (basal) and after activation: average N=50

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Surface Free Energy of Skin 137 the surface free energy data of activated in vivo skin and the values of S17, S31 and average before treatment with liposomal dispersion (basal values). Figure 7.17 demonstrates the influence of the test subject variables on the activation of the skin by the liposomal dispersion. The average values demonstrated that the SFESKIN, its DC and PC increase with activation by skindecoder , confirming the improvement in the hydration and, consequently, in the functionality of the skin. The test subject 17 confirms the behaviours of average surface free energy parameters, however the increases in SFESKIN and PC are less than average levels.The surface free energy parameters of test subject 31 confirm the behaviour of test subject 17 and average data (increase of all surface free energy parameters); however it shows an anomalous increase in DC after activation with liposomal dispersion. We hypothesize that this strong increase of DC could be attributed to a large amount of liposomes present on the SC surface. The liposomes probably were not absorbed completely by the skin after 10 of nebulization, or the amount of liposomal dispersion delivered on the skin was in excess (see Experimental section). We assumed that the variations in SFESKIN would depend on the different capabilities of test subjects to recover the hydration state of their skins (increase of PC) and consequently the functionality of their cutaneous systems (CA10s PFPEd=32.8±1.9 deg, CA10s WmQ=55.0±19.0 deg). The high variability in the CA of WmQ, measured after nebulization of the liposomal dispersion (CA10s WmQ=55.0±19.0 deg) influences the PC values that depend on the different capability of each test subject to recover its skin functionality in terms of hydration state. The low variability of CA of PFPEd (CA10s PFPEd=32.8±1.9 deg) measured after activation with skindecoder mainly reflects the low repulsive forces generated at the interface with the liposomal film that still remains on the surface of skin after 10 from its nebulization. The development of the method for determining the SFESKIN led to the development of TVS skin test. The TVS skin test is capable to determine the SFE of skin using the OWRK model and using the CA value of PFPEd as a constant (CA10s PFPEd=32.8 degree). This test is based essentially on the measurement of the variations in the value of the CA10s of WmQ after the application of the liposomal dispersion (skindecoder ). Here is reported an example of a wide application of the test on 501 test subjects performed in collaboration with Veneto Region (Venice, Italy). The research was carried out by the Tenskinmeter Versus Skin (TVS) Observatory (established with the patronage of Veneto Region/prot. 2228/2002) aiming to monitor the epidermal functional state in different conditions. The work was held under the project “University and Territory together for the promotion of water for a better life quality”. The study was coordinated by the Chemical Cosmetology Centre of the

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University of Padova (Italy) (1988–2005) Department of Pharmaceutical Sciences, now Department of Pharmacological and Pharmaceutical Sciences. In this study the subjective variables (gender, age range, skin type), the environmental influence (seaside, mountain, educational and professional areas) and the influence of the cutaneous application (mud therapy and cosmetics) were considered. The monitoring activity was carried out under controlled conditions on healty volunteers. The population of 501 test subjects was composed of a female/male ratio of 2.6 (M=140; F=361), a prevalent age G˝) with high elasticity (δ < 450) while the FD3 was a liquid (Gʹ< G˝) with dominant viscous behavior (δ > 450). To represent the viscoelastic property or consistency of the three foundations as a function of phase angle, the complex moduli G* and tanδ values were selected at 0.1% strain which was in the linear viscoelastic regime as shown in Figure 10.3b. The complex modulus (G*) of these foundations increased in the order below:

FD3 (G* = 10.4 Pa) < FD2 (G*= 148.9 Pa) < FD1 (G*=273.5 Pa) Figure 10.4 shows similar order for the shear viscosity at which the FD1 and FD2 foundation samples exhibited high viscosity of 547 Pa.s and 364 Pa.s, respectively at a shear rate of 0.01 (1/s). In contrast, the FD3 had very low viscosity of 31 Pa.s at the same shear rate. In the high shear rate regime (100 1/s – 1000 1/s), all of these foundations showed a good shear-thinning such that the viscosity dropped nearly 2 to 3 decades at high shear rate, indicating good spreading of these products upon application

Evaluation of Sebum Resistance for Make-Up 199 (a) 103

103 FD 2 Storage modulus G’ (Pa) Loss modulus G” (Pa)

Storage modulus G’ (Pa) Loss modulus G” (Pa)

FD 1

102

102

101

101

100

10-1 10-1

100

103 100 101 102 Oscilliation strain γ (%)

104

10-1 10-1

103 100 101 102 Oscilliation strain γ (%)

104

101 Storage modulus G’ (Pa) Loss modulus G” (Pa)

FD 3

100

10-1 10-1

100

101 103 102 Oscilliation strain γ (%)

104

(b) 250 FD1

G* (Pa)

200

150 FD2 100

50 FD3 0

0

10

20

30 δ (degree)

40

50

60

Figure 10.3 (a) Viscoelastic modulus versus % oscillation strain of the three foundations: FD1 and FD2 show solid-like behavior with G’> G’’ while FD3 shows liquid-like with G’ G˝) under small deformation. However, the FD3 behaved as a viscoelastic liquid (Gʹ< G˝) and showed a low shear viscosity of 31 Pa.s. The rheological data from complex modulus and viscosity suggested that the differences among these 3 foundations might come from the dispersed phase (water) volume fraction, droplet size and size distribution, the emulsifiers, solid particle contents, and the interaction between solid particles and droplets in the water-in-oil (W/O) emulsion foundations [21–27]. In addition, the role of gelling agents such as clay and silica in the continuous phase also contributed to the viscoelasticity of these formulations [28].

10.3.2

Surface Roughness

The topography and morphology of skin surface are important in the application of cosmetic products. The roughness of the human skin was reported to vary significantly, depending on anatomical location and age [29–32]. The skin surface roughness on human face such as forehead, cheek and nose was observed to be different ranging from 2 to 5 μm. Aging also increased skin surface roughness, for example, about 2 μm difference for

Evaluation of Sebum Resistance for Make-Up 201 forehead but around 24 μm difference for the hand, measured on people from 20 years old to greater than 60 years old [31]. It was not easy to evaluate the effect of roughness on the wetting, spreading and deposition of products on the human skin directly. Therefore, the bio skin plate made of polyurethane elastomer with similar roughness and elasticity was selected for use in this study. It is known that the surface roughness of a solid substrate can have a strong effect on the wetting and spreading behavior of liquid droplets on that substrate. This factor will have effect on the contact angle measurement as described by Wenzel through equation below [33]

cos θA = r.cos θ

(10.6)

Where θA is an apparent contact angle observed for the real rough surface, and is an ideal contact angle for a smooth surface, where r is the ratio of areas of rough surface to ideal smooth surface. This equation was based on the assumption that the liquid penetrates into the roughness grooves. Foundation formulation is a complex W/O emulsion containing multiple ingredients ranging from solids such as TiO2, fillers, pigments to gels as thickeners. After solvents and water have evaporated, these ingredients would arrange into different nano- and micro-scale domains on surface of the solid deposit. Figure 10.5 shows the scanning electron

BIO SKIN

*200X 2015/04/27

14:01

L

FD3

*500X 15:09

500 um

FD2

FD1

2015/04/27

x200

L

x500

200 um

*500X 2015/04/27

L

x500

200 um

*500X 2015/04/27

14:24

L

x400

200 um

Figure 10.5 Scanning electron microscopy images with magnifications of the bio skin plate and the three foundations deposited on bio skin.

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microscopy images of the bare bio skin and the 3 foundations deposited on the bio skin. The images of these samples show that the FD3 has less particles/fillers deposited on its surface compared to foundations FD1 and FD2. The roughness profiles of these samples determined from a 3D laser confocal microscope are shown in Figure 10.6. Figure 10.6a shows the optical images for the studied samples with the valleys (dark area) and height (bright area). Figure 10.6b is the 3D height color map with maximum (red) and minimum (blue) heights of samples. Figure 10.6c shows the 3D roughness profiles with the maximum roughness Rz which is defined as the absolute vertical distance between the maximum peak height and maximum valley depth along sampling length [20]. For the surface roughness evaluation, the mean square roughness Ra value is commonly used and gives more information about surface topography.

(a)

(b)

(c)

66.6μm

BIO SKIN Ra = 13.67 μm

40.0 0.0μm 0.0μm

0.0 100.0 200.0

82.2μm

FD1 Ra = 11.93 μm

40.0 0.0

0.0μm 0.0μm 100.0 200.0

FD2 Ra = 13.51 μm

82.1μm 40.0 0.0

0.0μm 0.0μm 100.0 200.0

FD3 Ra = 13.55 μm

70.3μm 40.0 0.0

0.0μm 0.0μm 100.0 200.0

Figure 10.6 3D images of the bio skin plate and the three foundations. (a) optical images, (b) height maps with blue (minimum height) and red (maximum height), (c) 3D surface roughness.

Evaluation of Sebum Resistance for Make-Up 203 It was observed that the bio skin plate had many groove channels similar to the human skin texture. The bio skin plate had the highest roughness with a mean Ra of 13.67 μm which was close to the Ra of forehead, cheek and nose (16 μm -33 μm) for people from 20–60 years old [31]. The mean surface roughnesses Ra of the 3 foundation samples deposited on the bio skin plate were obtained and ranked in the following order:

FD3 (13.55 μm) > FD2(13.51 μm) > FD1(11.93 μm) The order of sample mean surface roughness was observed to be dependent on the solid contents and rheology of the foundation formulation applied on the surface of the bio skin. The higher viscosity or higher solid contents contributed to a thicker deposit and smoother surface as seen from Rz and Ra values. The Ra of FD2 was observed to have similar roughness as of FD3 even though its viscosity was much higher. In addition, the foundation deposits had many grooves with different channel patterns on the surfaces depending on the compositions of these foundations. Therefore, the size and shape of fillers could be other factors that would affect the surface roughness and the groove networks in the foundation films. In conclusion, after solvent had evaporated, the film thickness and uniformity of the foundation product on the bio skin surface were strongly dependent on the solid contents, size and shape of fillers and flow property of the product formulation. Therefore, the surface roughness of these foundations on bio skin substrate would influence the contact angle measurements that we conducted here on these products [34].

10.3.3

Surface Free Energy of Bio Skin Substrate and Foundation Films

The contact angle measurement can help to understand the wetting of a solid substrate and the spreading of liquid on such substrate. However, the contact angle alone is not sufficient to explain the wetting and spreading behaviors of a liquid on a substrate. Therefore, it is necessary to determine the surface free energy of the substrate using the contact angle measurements [35, 36]. In order to determine the surface free energy (SFE) of the bio skin plate and the foundation films in this study, we utilized the Owens, Wendt, Rabel  and Kaelble (OWRK) method from Attension software. The OWRK model is commonly used for calculating the SFE of polymeric materials [37]. This approach was used to calculate the dispersion

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and polar components of the surface free energy using the following equation [37, 38]: L

(1 cos 2

Y

)

D S

D L

P S

P L

(10.7)

Here, θY is Young’s static contact angle, γL is the liquid surface tension, and DL and PL are the dispersion and polar components of liquid surface tension, respectively. The terms DS and SP are the dispersion and polar components of solid surface free energy, and they are unknown. By using 2 different liquids with known DL and PL , the equation system with 2 variables can be solved. Thus, based on contact angle measurements using different liquids, the dispersion and polar components of polymer surface free energy are obtained. In this study, three different probe liquids with known surface tensions with varying polarity such as water (γ = 72.5 mN/m), ethylene glycol (γ = 47.7 mN/m), and n-hexadecane (γ = 27.5 mN/m) were used to determine the surface free energy of the bio skin and the 3 foundation films (Table 10.1). From the contact angles of substrates with these liquids, SFE of substrates was computed from the Attension software, using the Owens, Wendt, Rabel and Kaelble (OWRK) approach above. Table 10.2 shows the results of solid surface free energy (SFE), the dispersion DS and polar SP components of the bio skin and the three foundation films deposited on the bio skin. The surface free energy (SFE) of the bio skin plate (polyurethane elastomer) used as substrate was determined to be 45 +/- 0.6 mJ/m2 with a high value of dispersion component and a low value of polar component. The SFE of this bio skin was close to the reported SFE of polyurethanes of 37.6 mJ/m2 – 51.5 mJ/m2 [39–42]. It was reported that the SFE values of polyurethane (PU) elastomers were Table 10.1 Liquids used in surface free energy (SFE) determination by contact angle measurements.

Liquid

Surface tension γL (mN/m)

Dispersion component D L  (mN/m)

Polar component P L (mN/m)

Deionized water

72.8

22.1

50.7

Ethylene glycol

47.7

30.1

17.6

n-Hexadecane

27.5

27.5

0

Evaluation of Sebum Resistance for Make-Up 205 Table 10.2 SFE of the bio skin and long-wear foundations determined from OWRK model.

Solid substrate

Surface free energy, γS (mJ/m2)

Dispersion component, D 2 S (mJ/m )

Polar component, P 2 S (mJ/m )

Bio Skin

45

41.85

3.15

FD1

24.3

21.052

3.34

FD2

27.4

26.1

1.3

FD3

27.8

23.1

4.75

dependent on the chemistry of the soft and hard segments as well as on their chemical composition in the elastomers. For example, the polyurethane (PU) containing 25% urethane hard segment and 75% ester soft segment had a SFE of 41 mJ/m2 [41]. For PU/PDMS cross-linked films, the SFE of PU decreased from 37.6 mJ/m2 to 29.7 mJ/m2 with increasing percentage of soft segment poly(dimethylsiloxane) (PDMS) in the PU elastomer [43]. With a high amount (60%) of PDMS, the SFE of PDMS-PU was low in the range of 24 mJ/m2–26 mJ/m2 [44]. These results indicated the microphase separation of the soft segments with the low surface free energy to the surface. In this study, the obtained SFE of the bio skin was similar to the SFE of human skin (forehead) when exposed to sebum (46 mJ/m2) but higher than the skin after cleansing with soap (34.5 mJ/m2) [45, 46]. Thus the SFE of the bio skin plate could be used to mimic the wetting of the foundations on human skin in this study, especially for oily skin type (Table 10.3).

Table 10.3 Summary of skin surface free energy before and after washing on volar forearm and forehead. Surface free energy mJ/m2 (before washing)

Surface free energy mJ/m2 (after washing)

Location

Reference

38.5

32.4

forearm

Mavon et al. [58]

44.8 (20°C) 42.8 (35°C)

36 34.7

forearm

Krawczyk [59]

42–46

34.5

forehead

Mavon et al. [45]

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For the foundation films deposited on the bio skin, the FD2 and FD3 had similar surface free energy values of 27.4 ± 1.2 mJ/m2 and 27.8 ± 0.8 mJ/ m2 respectively, while the FD1 had the lowest SFE value of 24.3 ± 1.3 mJ/ m2 (Figure 10.7). These results suggested that the silicone resins or silicone film-formers migrated or stratified to the surface during film formation and thus reduced the surface free energy of these foundations. In this study, the obtained SFE values of these foundations were similar to the reported SFE values of the poly(dimethylsiloxane) (PDMS) substrate of 24 mJ/m2–26 mJ/ m2 [47]. The similarity of SFE for the FD2 and FD3 foundations suggested that these formulations contained the same type of silicone resins, and their SFEs were not much influenced by the other chemical ingredients in the formulations due to high percentage of silicone on the film surface. In contrast, the low SFE of FD1 foundation might be due to the contribution of a different silicone resin/film-former type such as silicone acrylate with a high density of silicone molecules oriented at solid/air interface. As a result, the surface of the FD1 was smoother with lower surface roughness Ra as seen previously. With such low SFE, these foundation films had strong adhesion to the high SFE bio skin plate and modified its surface roughness.

50

Surface Free Energy (mJ/m2)

40

30

20

10

0 Bio skin

FD3

FD2

FD1

Figure 10.7 Surface free energy of bio skin plate and the foundation products FD1, FD2, FD3 deposited on bio skin using the OWRK approach.

Evaluation of Sebum Resistance for Make-Up 207

10.4 Contact Angles of Foundations with Water It is well known that wetting behavior depends on the surface chemical composition, the surface orientation in contact with water, the surface molecular mobility, and surface roughness. The static contact angles (CAs) of water recorded at 10 s at room temperature for the bio skin and 3 foundations containing silicone resin/film-former are shown in Figure 10.8. The obtained water contact angle (CA) on the bio skin was 62°. This water CA was lower than CAs of water droplets on smooth surfaces of different types of polyurethane elastomer/polymer of 70°, 78° and 74° with SFE of 51.5 mJ/m2, 46 mJ/m2 and 37.6 mJ/m2 respectively [41–43]. According to the Wenzel equation (10.6), the roughness factor gives hydrophilic surfaces a lower contact angle and hydrophobic surfaces a larger contact angle. Therefore, the lower water CA on the bio skin should be due

120

Contact angle of water (degree)

100

80

60

40

20

0

FD1

FD1

FD2

FD2

FD3

FD3

Bio skin

Bio skin

Figure 10.8 Contact angles of water droplets on the bio skin plate and the foundation products deposited on bio skin.

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to the chemis try of polyurethane and the surface roughness. However, our CA value at 10 s was still higher than the CA values on various PU artificial skins reported by Shimizu and Nonomura [48] for water CAs determined at 5 min being in the range of 35°–46° for various surface roughnesses Ra from 7.6 μm to 15.5 μm and with different micro-textures. This suggested that their low CA values might be due to water evaporation during 5 min measurement or our water droplet continues to spread over the surface and flow to the grooves of the bio skin plate after 10 s. The FD1 foundation exhibited good water repellency with highest water CA of 107.6° ± 2° which was attributed to the surface orientation of silicones to the solid/air interface. This result was close to the CA of water droplet on silicone PDMS film (103.1°± 2.5°) with a surface free energy of 26 mJ/m2, reported by Rios et al. [47]. The higher water CA of foundation FD1 compared to the water CA of a smooth PDMS surface was due to its surface roughness which enhanced the hydrophobicity according to Wenzel’s equation. In addition, the water CA of this foundation was also close to the advancing water CA on highly hydrophobic surfaces such as paraffin wax and Teflon (110° and 112°) reported by El-Shimi and Goddard [49]. The poor wetting behavior of this FD1 foundation with water was consistent with its low surface free energy (24.3 ± 1.3 mJ/m2), indicating the incompatibility of water with its silicone surface. However, the other foundations FD2 and FD3 showed lower CAs of water of 94° and 83.1° respectively, even they had the same magnitude of surface free energy of 27 mJ/m2 as well as surface roughness. It was observed that the SFE polar component of FD3 SP 4.75 mJ/m 2 had a higher value than the polar component of FD2 SP 1.3 mJ/m 2 , therefore foundation FD3 is more hydrophilic. The high polar contribution in FD3 could be from the high density of silanol (Si-OH) or hydroxyl groups exposed to the surface and formed hydrogen bonding with water, which consequently reduced the water CA as observed from silica surface modified with hydroxyl functional groups [50, 51] or from PDMS modified with PEG (poly (ethylene glycol)) [52, 53] or oxygen plasma treated PDMS [54, 55]. Also, the water CA on modified polar PDMS could be reduced from 116.7° to 40° depending on the polar component concentration [56]. The foundations containing film-formers, TiO2, silicone elastomers, etc. created micro-domains on deposited surface with various roughnesses and groove channels that could have the strong effect on water CA as discussed for the bio skin plate above. In conclusion, the foundation substrates FD1 & FD2 were hydrophobic with water CA being greater than 90° while the foundation substrate FD3 was hydrophilic with water CA being less than 90°. Overall, the CAs of these 3 foundations with water were higher than the water CA on the

Evaluation of Sebum Resistance for Make-Up 209 PU elastomer bio skin (62°) and the water CA on the forehead rich in sebum (57°–73°) [46]. In addition, the water CA of FD1 was greater than the CA of silicone acrylate (103.3°) and CA of FD2 was similar to the CA of silicone  resin MQ (100.4°) [57]. Therefore, the type of silicone filmformer/resins as well as the density of polar groups on the surface will have a strong effect on the wetting and spreading of water on these foundations. Other factors such as thickeners, surfactants, fillers and treated pigments in the foundation film could alter the interaction with water at the solid/ liquid interface.

10.5 Contact Angles of Foundations with Sebum Generally, the skin surface is hydrophobic and has low critical surface tension and low surface free energy. The recorded surface free energy values of skin are in the range of 38.5 mJ/m2–46 mJ/m2 depending on skin location and skin type [45, 46, 58, 59]. The outer layer of the human skin, the stratum corneum (SC), is comprised of mostly sebum and dead skin cells. The function of sebum is to hydrate and protect the skin from the environment. Sebum is a mixture of lipids comprised of triglycerides, free fatty acids, wax esters, squalene, cholesterol and cholesterol ester [60]. Typically, the greatest amount of sebum is found on the forehead, nose and chin also known as the “T-zone” area of the face. The surface tension of sebum is 24.9 mN/m (between 26.5°C and 31°C) [61] which is lower than the skin surface free energy, thus sebum is easy to spread on the skin. With high lipophilicity and low surface tension, sebum will also affect the spreading of the cosmetic formulation as well as the adhesion of the long-wear foundations on the skin. To mimic human sebum for in-vitro study, a number of artificial sebum formulations and their physical properties were reported [61–64]. In these artificial sebum formulations, the fatty acids such as C14, C16 and C18 were commonly used [61]. For the present study, the artificial sebum was prepared in the lab, containing ester and hydrocarbon oils, oleic acid, and alcohol. The surface tension γ of the prepared artificial sebum composition was determined from the pendant drop method to be 23.4 ± 0.6 mN/m which was close to the surface tension of the natural sebum reported at 25 mN/m. Sebum is easy to spread on silicone resin/film-former with a low contact angle as reported [57]. Therefore, the degree of spreading of the sebum is a good indicator of the affinity of the foundation with sebum.  To evaluate the spreading and wetting of sebum, the static CAs of artificial sebum droplets on the three foundations were monitored at 10 s, 120 s,

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180 s, 300 s and 600 s at room temperature. From the sebum CA results in Figure 10.9, two regions of spreading were observed: 1. Region 1: for time less than 180 s, the CAs of these foundations at 10 s were similar with average values of 36°. Then, these sebum CAs decreased at a fast rate. The CA of foundation FD1 reduced to a lower value of 32° at 60 s and then became nearly constant at 180 s, while the CAs of FD2 and

Contact angle of Sebum (degree)

45 40 35 FD1

30 FD2

25 20

FD3

15 10

Fast spreading

slow spreading

5 0 0

Samples

100

10 sec

200

300 Time (sec)

3 min

400

500

600

10 min

FD3

FD2

FD1

Figure 10.9 Time-dependence of contact angles of sebum droplets on the foundation products deposited on the bio skin plate.

Evaluation of Sebum Resistance for Make-Up 211 FD3 still continued decreasing to lower values of 25.9° and 22°, respectively. The sebum CA values for the 3 foundations deposited on the bio skin plate were lower than the reported sebum CA for the silicone resin or silicone film-former alone on the glass slide which were in the range of 49°- 34° at 120 s [57]. This phenomenon could be from the effect of surface roughness and the groove structures of the foundations in which the sebum flowed quickly into the valleys under capillary force. As a result, the diameter of the sebum droplet increased quickly with time during this period as shown in Figure 10.10. The decreasing of CA in this region was mainly due to wetting and spreading of sebum on the rough substrate and grooves with minimal adsorption of liquid on the surface (Figure 10.11). 2. Region 2: for times longer than 180 s, the sebum CA on FD1 remained constant while the CAs of FD2 and FD3 continued decreasing at a slower rate for up to 10 minutes. On the FD1 substrate, the sebum drop spread slowly and reached a CA of 30° at 10 min. In contrast, the sebum spread much faster on the FD2 and FD3 substrates and the CAs at 10 min were 22.4° and 16.6°, respectively as shown in Figure 10.9. The abrupt jump in the sebum diameter on the FD3 at 300 s

Sebum Drop Diameter (mm)

5

4

FD 1 FD 2 FD 3

3

0

100

200

300 400 Time (sec)

500

600

700

Figure 10.10 Time-dependent contact diameter D of sebum droplets on the three foundations.

Advances in Contact Angle, Wettability and Adhesion Contact angle of sebum at 180sec (degree)

212

40 35 FD1 30 25

FD2 FD3

20 15 10 5 0 13.55

13.51

11.93

Surface Roughness Ra (μm)

Figure 10.11 Effect of surface roughness on the sebum contact angles on the three foundation substrates at 180 s.

indicated that there was interaction of sebum with the surface through radial penetration into the substrate as shown in Figure 10.10. It was reported that oleic acid had some interaction with polar groups such as OH and TiO2 that could result in lower contact angle values [65]. Therefore, the spreading and wetting of sebum on FD2 and FD3 in this region might be influenced from the interaction of sebum containing oleic acid with chemical component such as silanol groups or OH groups on the substrate or with fillers such as TiO2 in the films through adsorption, while FD1 substrate had no interaction with sebum during this time period. To understand the quick spreading phenomenon of sebum on FD3 compared to FD1 and FD2 substrates in the first region, we used Nikon stereomicroscope to capture the spreading of a 10 μL sebum droplets on these substrates. It is interesting to see that the sebum droplet with a wicking front spread so quickly on the foundation FD3 with a grooved surface as shown in Figure 10.12. This phenomenon is due to the sebum drop acts as a reservoir and the sebum flows under capillary force into the groove network. The observed wicking flow of sebum on the foundation deposited on the bio skin plate was similar to the wicking flow from paraffin oil on the artificial skins with various groove patterns [48] or from ester oils

Evaluation of Sebum Resistance for Make-Up 213 (a)

(b)

(c)

Figure 10.12 Wicking of sebum droplet on the foundation FD3 substrate as function of time at a) 10 s, b) 60 s, and c) 180 s.

on the forearm skin surface with networked micro-channels [66]. Figure 10.13 shows the wicking front of sebum droplets on FD1 and FD3 at 180 s at which the sebum droplet on FD1 had an elongation shape and much smaller size than of the radial sebum droplet on FD3. The difference in the wicking drop shapes might come from the size of the grooves and surface roughness on these two foundation surfaces that were observed from stereomicroscope. Therefore, the spreading of sebum on FD1 was slower than on FD3 because the wicking flow velocity was reported to be dependent on the groove shape, depth, and density [66]. In order to evaluate the sebum resistance by the foundation product, we used the CA results of artificial sebum on the foundation substrate as an indicator for our present study. The foundation could be considered as sebum resistant when the sebum equilibrium contact angle θ on the film was greater than 20° with medium spreading. The foundation with CA less than 20° could be considered as having no sebum resistance as shown in Table 10.4. From this criterion, we could conclude that the wetting of FD1 with a CA of 32° had less sebum interaction with the foundation film than FD2 Time: 0:03:00.000

Time: 0:03:00.031 End Length = 5690.79 μm

Start Length = 5671.56 μm

(a)

1000 μm

100 μm Start Length = 3739.39 μm 1000 μm

(b)

Figure 10.13 Wicking of sebum droplets on the foundation FD1 (a) and FD3 (b) substrates at 180 s.

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Table 10.4 Criterion for the sebum resistance of the foundation based on the contact angle and spreading of sebum on the surface. Contact angle θ of sebum on foundation

Spreading

Sebum resistance of foundation film

High (θ > 90°)

Low

Very high resistance to sebum

Medium (20° < θ < 90°)

Medium

High & medium resistance to sebum

Low (θ < 20°)

High

Low or no resistance to sebum

and FD3 with CAs of 22° and 17° respectively in the time frame of 10 min. However, the spreading of sebum on these foundations was observed to be influenced by surface chemical composition, especially oil absorbing fillers and surface roughness as discussed above. Therefore, it is difficult to have a clear conclusion for sebum resistance to foundation by using the values of CA alone.

10.6 Effect of Sebum on Color Transfer and Film Integrity Each foundation was deposited on the bio skin using a 3mm drawdown bar. After drying overnight, wear assay was conducted by dropping a sebum droplet on the foundation film for 5 minutes and then rubbing with a cotton pad at a constant force for 10 times. The color transfer resistance on the cotton pad from the rubbing and the integrity of the film were evaluated by visual observation. Figure 10.14a shows the color transfer and film integrity after rubbing with sebum for the three foundation samples. For FD1, a clear visual circular spot with a diameter equal to the initial sebum drop was observed after rubbing. This phenomenon may be from the diffusion of sebum through the foundation film surface and plasticizing the film during the drop resided on the surface for 5 min. The FD2 had a larger spot than FD1, due to sebum spreading over 5 min, while no circular spot was observed for the FD3 except a large area of the foundation was removed after rubbing as shown in Figure 10.14a. These results could be explained by the fast sebum spreading on the substrates that could minimize the solubility of the film locally. The visual evaluation of color transferred from these foundation surfaces to the cotton pads did not show any

Evaluation of Sebum Resistance for Make-Up 215 FD 1

FD 2

FD 3

(a) FD 1

FD 2

FD 3

(b)

Figure 10.14 (a) Surface damage to the foundation films on bio skin after rubbing the surface with sebum being on the foundation for 5 minutes. (b) Surface damage to dry foundation films on bio skin after stretching.

significant difference. The film integrity after rubbing with sebum was in a good agreement with the results of the wetting and spreading of sebum on these surfaces. In addition, a stretching test was also performed on the foundation deposited on bio skin plate. Each sample was stretched 5 times horizontally and 5 times vertically, then the cohesiveness of the deposit film was visually evaluated (Figure 10.14b). The results showed that the film integrity of FD1 and FD2 was much better than of FD3 that exhibited a large cracking/ flaking due to its thinner film and higher surface roughness.

10.7 Summary and Prospects In this study, we utilized contact angle measurements to characterize the wetting of three leading long-wear foundations by sebum and the spreading behavior of artificial sebum on their substrates. The results of this study showed that: 1. The wetting of the selected foundations by artificial sebum was found to be dependent on the surface free energy (SFE),

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2.

3.

4.

5.

6.

7.

surface roughness, film thickness and composition of the product. The surface free energy of foundation films was dependent on the surface composition containing silicone molecules oriented at solid/air interface. The differences in the SFE among these foundations were due to the silicone concentration and type of silicone film-former/resins used in the formulations. From the wetting and spreading of both water and sebum on the foundation substrates, it could be revealed that the FD2 and FD3 contained the same type of silicone resin, but were different in the silanol and hydroxyl densities at the surface. The results from SFE and water contact angle (CA) indicated that FD1 and FD2 surfaces were hydrophobic and FD3 surface was more hydrophilic. The wetting and spreading of sebum droplets on the foundation substrates were observed to be dependent on the surface roughness, and the groove network formation on the surface. The wetting of artificial sebum on these foundation substrates was observed to go through 2 regimes of fast and slow spreading, and the sebum CAs were found to decrease with increase of surface roughness. In the fast spreading regime, the contact diameter of sebum droplet on the FD3 increased at much faster rate compared to FD2 and FD1 due to the sebum flowing through the grooves with wicking phenomenon. The use of the sebum CA on the foundation substrate alone was not a good indicator for sebum-resistant foundation in our present study, due to complex behaviors originating from surface structures such as surface roughness, groove pattern and geometry. The degree of color transfer and integrity of the foundation films under rubbing with sebum were observed to be dependent on the wetting and spreading of sebum on the film surface and the film thickness. Due to the affinity of sebum to the foundation substrate, low spreading of sebum caused local dissolution and plasticization of the film as observed in the case of FD1, while with a high spreading of sebum, the film could be removed under rubbing as observed for FD2 and FD3. Therefore, the resistance to sebum can be achieved

Evaluation of Sebum Resistance for Make-Up 217 by controlling the levels of sebum absorbing fillers in the formulation. 8. A systematic study on the interaction of surface treated pigments/fillers with sebum is important for designing the best long-wear and sebum-resistant foundation formulation.

Acknowledgements The authors would like to thank William Peabody from Keyence Corporation of America for measuring the surface roughness during the demo of the VK-X250 at L’OREAL Clark NJ. Also, thanks to Dr. Ronni Weinkauf, Dr. Stephan Habif and the L’Oreal Research and Innovation Department for supporting this work.

References 1. Guichard, S. and Roulier, V., Facial foundation, in: Cosmetic Dermatology: Products and Procedures, Z.D. Draelos (Ed.), pp. 167–175, Wiley-Blackwell Publishing, West Sussex, UK, 2010. 2. Dow Corning Corp, Silicones in Industrial Applications. https://www.dowcor ning.com/content/publishedlit/silicones-in-industrial-applications.pdf, 2009. 3. V. Ferrari, B. Bouarfa, G. Brun, Cosmetic composition for making up the skin. US Patent 9649264, assigned to L’OREAL, 2017. 4. M. Kobayashi, W. Zavadoski, W. Kalriess, I. Smith, S. Kishida, Pigments and extender pigments with enhanced skin adhesion for cosmetic preparations. US Patent 6887494, assigned to US Cosmetics, 2005. 5. M. Kitagawa, K. Nishimoto, T. Tanaka, Cosmetic pigments, their production method, and cosmetics containing the cosmetic pigments. US Patent 9181436, assigned to Toyobo CO, LTD and Daito Kasei Kogyo CO, JP, 2015. 6. Chibowski, E., Holysz, L., Szczes, A., Wettability of powders, in: Adhesion in Pharmaceutical, Biomedical and Dental Fields, K.L. Mittal and F.M. Etzler (Eds.), pp. 23–49, Wiley-Scrivener, Beverly, MA, 2017. 7. S. Tomomasa, H. Takada, Y. Soyama, Make-up cosmetic composition. US Patent 5948393, assigned to Shiseido, 1999. 8. Zhang, F., Vrckovnik, R., O’Lenick, T., MQ resins in personal care applications, Personal Care Europe Magazine, 7, 6, 61–66, 2014. 9. Baney, R.H., Itch, M., Sakakibara, A., Suzuki, T., Silsesquioxanes. Chem. Rev., 95, 1409–1430, 1995. 10. G.V. Gordan, R.G. Schmidt, L.A. Stark-Kasley, G.M. Wieber, MQ and T-propyl siloxane resins compositions. US Patent 7803358, assigned to Dow Corning, 2010.

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11. Dow Corning 670 Fluid, Product Information: Personal Care, Ref. No. 27/1158-01, 2004. 12. Dow Corning 680 ID Fluid, Product Information: Personal Care, Ref. No. 27/1397B-01, 2012. 13. H.S. Bui, S. Halpern, M. Kanji, Comfortable transfer-resistant colored cosmetic compositions containing a silsesquioxane wax. US Patent Application 20080305061A1, assigned to L’OREAL, 2008. 14. H.S. Bui, S. Halpern, M. Kanji, Comfortable transfer-resistant colored cosmetic compositions containing a silsesquioxane wax. US Patent 9089503, assigned to L’OREAL, 2015. 15. Dow Corning FA 4001 CM Silicone Acrylate. Product Information: Personal Care, Ref. No. 27-1225B-012011, 2011. 16. Dow Corning FA 4002 ID Silicone Acrylate. Product Information: Personal Care, Ref. No. 27-1226D-01, 2014. 17. P. Arnaud and A. Collette, Cosmetic composition comprising at least one vinyl polymer and at least one olefin copolymer. US Patent 8828372, assigned to L’OREAL, 2014. 18. Bui, H.S. and Coleman-Nally, D., Film-forming technology and skin adhesion in long-wear cosmetics, in: Adhesion in Pharmaceutical, Biomedical and Dental Fields, K.L. Mittal and F.M. Etzler (Eds.), pp. 114–166, WileyScrivener, Beverly, MA, 2017. 19. Bio Skin Plate. www. BeauLax.com.JP. 20. Keyence Wide-Area 3D Laser Scanning Confocal Microscope VK-X250/ X150/X120 catalog, 2015. 21. Pal, R. and Rhodes, E., Viscosity/concentration relationships for emulsions. J. Rheology, 33, 1021–1045, 1989. 22. Foudazi, R., Masalova, I., Malkin, A.Y., The rheology of binary mixtures of highly concentrated emulsions: Effect of droplet size ratio. J. Rheology, 56, 1299–1314, 2012. 23. Komatsu, H., Takahashi, M., Fukushima, S., Viscoelastic properties of waterin-oil cream, Trans. Soc. Rheol., 21, 219–236, 1977. 24. Sandoval-Rodríguez, L.S., Cañas-Marín, W.A., Martínez-Rey, R., Rheological behavior of water-in-oil emulsions of heavy and extra-heavy live oils: Experimental evaluation. CT&F - Ciencia, Tecnología y Futuro, 5, 4, 5–24, 2014. 25. Wang, L. and Fang, J., Rheological properties and water-in-oil structural stability of emulsion matrixes. Central Eur. J. Energetic Mater., 10, 1, 87–102, 2013. 26. Ariffina, T.S.T., Yahyaa, E., Husin, H., The rheology of light crude oil and water-in-oil-emulsion. Procedia Eng., 148, 1149–1155, 2016. 27. de Oliveira, C.B.Z., Souza, W.J., Santana, C.F., Santana, C.C., Dariva, C., Franceschi, E., Guarnieri, R.A., Fortuny, M., Santos, A.F., Rheological properties of water-in-Brazilian crude oil emulsions: Effect of water content, salinity, and pH. Energy Fuels, 32, 8880–8890, 2018.

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11 Contact Angle Hysteresis of Pressure-Sensitive Adhesives due to Adhesion Tension Relaxation Naoto Shiomura1, Takashi Sekine1 and Dehua Yang2* 1

Kyowa Interface Science Co., Ltd., Niiza, Saitama, Japan 2 Ebatco, Eden Prairie, MN, USA

Abstract In this paper, several acrylic pressure-sensitive adhesives (PSAs) were studied through adhesion tension relaxation (ATR) technique introduced by Kasemura and Takahashi. These acrylic PSA samples were also analyzed through static contact angle, surface free energy, dynamic contact angle hysteresis, and peel force measurements. The study has shown that the acrylic PSAs are multicomponent polymeric systems which reorient their surface segments so as to minimize interfacial tension in response to environmental changes. Therefore, it is important to consider the mobility of the surface segments of PSAs in understanding their contact angle hysteresis. Further, the ATR technique has proven to be useful in estimating such mobility. Keywords: Adhesion tension relaxation (ATR), surface free energy, contact angle hysteresis, peel force

11.1 Introduction Intermolecular forces between a pressure-sensitive adhesive (PSA) and an adherend greatly contribute to the adhesiveness of PSA in spite of the fact that the absolute intermolecular forces are very small in comparison to the forces of dynamic responses (elasticity, viscosity, viscoelasticity) accompanied by deformation of PSA and the backing when they are detached *Corresponding author: [email protected] K.L. Mittal (ed.) Advances in Contact Angle, Wettability and Adhesion: Volume 4, (223–238) © 2020 Scrivener Publishing LLC

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[1]. This can be demonstrated by the fact that the adhesiveness of a PSA varies depending on the material of adherend. It is generally said that the polarities of PSA and the adherend greatly contribute to the adhesiveness and it can be assumed that the differences in polarities between PSA and adherend cause the differences in molecular interactions at the interface [2]. Further, according to the Young’s Equation [3], the intermolecular forces at the liquid - solid interface can be described by the material surface properties, i.e., liquid surface tension (or surface free energy), solid surface tension (or surface free energy) and contact angle. Thus, it is easy to reckon that these surface properties may be relevant to the adhesiveness of PSAs. To further understand and study the interrelationship between the surface properties and the adhesiveness of PSAs, in this study, conventional static contact angle, surface free energy (SFE) and adhesiveness of PSAs were first investigated. However, the correlation between the static measurement results and the adhesiveness was not successful. With the understanding that the PSAs are multicomponent polymeric systems and that the surface segments of the PSAs reorient in order to minimize interfacial tension with environmental changes [4–7], the mobility of surface segments of the PSAs was investigated. The potential of evaluating PSAs using dynamic contact angle (DCA) technique and adhesion tension relaxation (ATR) technique [4–6] was explored. The contact angle hysteresis of PSAs due to ATR is explained.

11.2 Theoretical Background Multicomponent polymeric systems selectively reorient their segments in order to minimize interfacial tension in response to environmental changes. Figure 11.1 is a schematic representation of mobility of surface molecules in response to environment [3, 5]. When the surface is exposed to vapor, hydrophobic segments orient on the surface and the system is stable with low interfacial tension between polymer surface and vapor. However, once the surface is immersed in water, the hydrophilic segments are oriented to the interface so as to minimize the interfacial tension. When the surface is pulled out of water and is exposed to vapor again, the hydrophobic segments are oriented back to the interface. For study of surface characteristics, dynamic contact angle and the hysteresis are very useful, and the Wilhelmy plate method is one of the techniques to determine dynamic contact angle. Figure 11.2 depicts the theory of the Wilhelmy plate method [7, 8]. The method is performed by first fixing a sample substrate on the holder connected to an electronic balance.

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Hydrophobic segment

t0

t3

t2

Hydrophilic segment t1

Vapor Water

Advancing process

Receding process

Figure 11.1 Schematic representation of mobility of surface molecules in response to environment: From t0 to t1 it shows the advancing process where the hydrophilic segments reorient on the surface. t0 specifies the timing just after the surface is immersed in water and t1 specifies the equilibration of reorientation. From t2 to t3 it shows the receding process where the hydrophobic segments reorient on the surface. t2 specifies the timing just after the surface is exposed to vapor and t3 specifies the equilibration of reorientation.

F

F=LγLcosθ - Shρg Where: γL: liquid surface tension L: perimeter of substrate θ: contact angle (advancing or receding depending on direction of movement) Shρg: buoyancy of substrate S: cross-sectional area of substrate h: height of substrate immersed ρ: liquid density g: gravitational acceleration

AIR

Test solution LγLcosθ

Figure 11.2 Theory of the Wilhelmy plate method.

Then the sample substrate is immersed in a sample liquid prepared in the lab dish on the stage. Finally the contact angle can be determined with the relations of forces acting on the sample substrate by the equation: F=LγLcosθ - Shρg. Contact angle θ can be measured in both directions of stage travel. Figure 11.3 shows the change of force acting on the sample substrate as a function of stage movement in both directions. Advancing angle θA is obtained while immersing the substrate (wetting process) and receding

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Force acting on the substrate (mN)

8 6

withdrawing (Receding contact angle)

4

γLcosθR

2 0

γLcosθA

–2 –4 –6

immersing (Advancing contact angle)

–8

0

10

20 30 40 Stage position (mm)

50

Figure 11.3 Change of force acting on the sample substrate as a function of stage movement in both directions.

angle θR is measured while withdrawing the substrate from the sample liquid (dewetting process). Contact angle hysteresis is the difference between θA and θR as Δθ = θA-θR. The contact angle hysteresis is attributed to surface heterogeneity, roughness, reorientation of molecular segments, adsorption/desorption, deformation, contamination, etc. [7, 9, 10]. The time range of the measurement processes is generally in minutes or less. Slow mobility of surface segments is not reflected in the hysteresis measurement results. Kasemura [4, 5] and Takahashi [6] introduced the concept of ATR. In the Wilhelmy plate method, if the buoyancy of substrate is cancelled under the condition that the sample substrate is immersed in a sample liquid at a certain depth, equation (11.1) below is derived and it can be modified to equation (11.2). Through the Young equation γS = γLcosθ +γSL. (where: γS solid surface free energy, γSL solid/liquid interfacial free energy), equation (11.3) is derived and the obtained quantity F/L is called Adhesion Tension.

F = LγLcosθ

(11.1)

F/L = γLcosθ

(11.2)

F/L = γLcosθ = γS-γSL

(11.3)

A minor modification of the process for contact angle measurement using the Wilhelmy plate method allows obtaining ATR. The relaxation

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here represents the mobility of surface segments toward equilibration in response to a given environment [6]. Figure 11.4 is a model chart of ATR for acrylic PSA. The stage’s vertical movement is stopped when the sample substrate is immersed in water at a certain depth during the advancing contact angle measurement, and is kept stationary for 20 to 30 minutes. This arrangement measures advancing adhesion tension FA(t). When the stage is reversed for process of receding contact angle measurement and is kept stationary for 20 to 30 minutes before being pulled out of the water, receding adhesion tension FR(t) is measured. The adhesion tensions FA(t) and FR(t) specified here are derived as equations (11.4) and (11.5) from equations (11.1) and (11.3):

FA (t ) L L cos L(

S.A

(t )

FR (t ) L L cos L(

S .R

A SL . A

(t ))

(11.4)

SL . R

(t ))

(11.5)

R

(t )

Adhesion tension FA(t)/L & FR(t)/L [mN/m]

FA(t) should increase with time because the interfacial tension (γSL. A) between the acrylic PSA and DI water is lowered due to reorientation of hydrophilic segments on PSA. On the other hand, FR(t) should decrease with time because the PSA surface hydrated in DI water is exposed to the air and the surface tension (γS. R) is lowered due to reorientation of

9 8 7 6 Advancing adhesion tension FA(t)/L

5 4

Receding adhesion tension FR(t)/L

3 2 1 0

0

500

1500 1000 Elapsed time [s]

Figure 11.4 Model chart of ATR of acrylic PSA.

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2500

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hydrophobic segments on PSA. These changes represent the mobility of surface segments toward equilibration and are called adhesion tension relaxation (ATR).

11.3 Experimental 11.3.1

Preparation of Samples and Experimental Conditions

Seven kinds of acrylic PSAs with different alkyl chain lengths (C4, C6, C6+2, C8, C12, C14, and C16) were prepared as the test samples. The alkyl chains of these PSAs were cross-linked. C6+2 is a 2-ethylhexyl acrylate with 2-ethyl bonded to hexyl acrylate (C6). They were diluted with 20% ethylacetate/ hexane mixture and were dip-coated on the microscopic glass slides (size 26mm x 1mm). For finishing, they were dried for 3 minutes in 100°C air. The dip-coating process was done on a KYOWA Surface Tensiometer DY-700, which is an instrument configured with a built-in electronic balance and a vertically movable stage on the main chassis. The surface tensiometer can be used to measure surface tension, interfacial tension, and contact angle through the Wilhelmy plate method. Dip-coating function is also available on this instrument.

11.3.2

Static Contact Angle Measurement

The sessile drop method was used for measuring static contact angles using KYOWA contact angle meter DMo-501 and FAMAS software. This contact angle meter is a video based contact angle measurement system configured with an automatic liquid dispenser and a 150mm square X-Y movable stage for sample positioning. The sample glass slides coated with acrylic PSA were placed horizontally flat on the square stage and two probe liquids (DI water and methylene iodide) were used as test liquids. The volume of the test liquid was set at 2μL. The surrounding temperature was controlled between 25 and 27°C. The timing of determination of contact angle was set at 1 second after depositing the droplet on the coated sample surface.

11.3.3

Surface Free Energy (SFE) Analysis

Static contact angle data of DI water and methylene iodide measured on the PSA samples using the process described in Section 11.3.2 were used for SFE analysis. Among the many models/approaches to determine surface free

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energy of solids using contact angle measurements, the theory of Kaelble-Uy [11, 12] was adopted here, which simply determined the SFE by dividing it into polar (γp) and dispersion (γd) components. From their molecular structures, it is considered that the PSA samples have the functional groups and the molecular polarities to generate different interfacial interactions between them and the adherend [2]. Further it was considered that the theory of Kaelble-Uy was appropriate to characterize the acrylic PSA samples.

11.3.4

Dynamic Contact Angle as a Function of Time

Using the DMo-501 contact angle meter, the sessile drop method was performed to determine contact angle as a function of time. The sample glass slides coated with acrylic PSA were placed horizontally flat on the square stage, and DI water was used as the test liquid. The liquid droplet volume was set at 2μL and each sessile droplet was measured for 60 seconds during the contact angle measurement. The temperature of the surrounding environment for the contact angle tests was controlled between 25 and 27°C.

11.3.5

Dynamic Contact Angle Hysteresis with the Wilhelmy Plate Method

Using the DY-700 tensiometer, the Wilhelmy plate method was adopted to determine advancing angle, receding angle, and contact angle hysteresis. First, DI water with temperature controlled between 22 and 26°C was prepared in a 150mL lab dish. The sample glass slide coated with acrylic PSA was fixed on the holder connected to the electronic balance. Then the stage moved up and down under constant speed at 12mm/min so that the glass slide was immersed into DI water 25mm deep and then withdrawn to return to its initial position. In the immersion process of the sample glass slide, the advancing angle θA was obtained, while during the withdrawal process of the sample glass slide the receding angle θR was measured. As a result, contact angle hysteresis Δθ=θA-θR was obtained through simple arithmetic calculation.

11.3.6

Adhesion Tension Relaxation (ATR)

As for the measurement of dynamic contact angle hysteresis, the DY-700 tensiometer was also used here, the Wilhelmy plate method was adopted, DI water with temperature controlled between 22 and 26°C was prepared in a 150mL lab dish, and the sample glass slide coated with acrylic PSA was fixed on the holder connected to the electronic balance. At first, the stage

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moved up at a higher speed of 50mm/s to immerse the sample glass slide and stopped when the sample slide was immersed 20mm deep. The stage was kept stationary for 20 minutes in order for the instrument to record the change in force due to reorientation of PSA segments at the interface between the acrylic PSA coated sample glass slide and DI water. This is the process that measures advancing adhesion tension FA(t). Afterwards, the stage moved down by 10mm to withdraw the sample glass slide at a speed of 50mm/s and stopped. The stage was kept at this position for 20 minutes in order for the instrument to measure the change in force again. This is the process that measures receding adhesion tension FR(t).

11.3.7

Peel Force Measurement

Peel forces of the acrylic PSA samples were measured in order to rank the adhesiveness of the PSAs through peel force experiment. KYOWA Versatile Peel Analyzer VPA-2 was used for the measurement. VPA-2 is a unique, yet versatile peel force measurement apparatus with multiple load-cells (0.1N, 1N, 5N, 10N, 50N, 100N) for excellent measurement accuracy and broad load range. It holds PSA tapes with an adherend holder which can travel at a variety of travel speeds during the peel test. It also comes with an adjustable peel angle from 0 to 180 degree. The sample acrylic PSAs were deposited on Mylar film (25mm x 50μm) using the spiral bar coater No.9 (wet coating thickness 20μm) and dried for 3 minutes in 100°C air. A stainless steel SUS304 plate finished with bright annealed (BA) process was used as a sample adherend, and the sample Mylar film coated with acrylic PSA was rolled onto the sample adherend using a 2kg pressure roller with two rolling cycles. The test surrounding temperature was kept between 25 and 27°C. The measurement conditions were as follows: i. ii.

Peel angle was fixed at 90 degree and 4 different peel speeds, i.e., 300, 1000, 3000 and 9000 mm/min were tested. Peel speed was fixed at 300mm/min and 5 different peel angles, i.e., 30, 60, 90, 120 and 180 degree were tested.

11.4 Results and Discussion 11.4.1

Static Contact Angles and SFE Analysis

Figure 11.5 shows the results of static contact angles measured with DI water and methylene iodide (M.I.). It can be seen that the water contact

Contact angles from the sessil drop method (˚)

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160 140 Water

120 100 80 60

Methylene iodide

40 20 0 4

6 6+2 14 8 12 Number of carbons in side chain

16

Figure 11.5 Results of static contact angles.

angles on the PSAs ranged from 100° to 140°, showing dependence on the length of alkyl chain (number of carbons). However, they are in the range of contact angle for a typical hydrophobic surface, which exemplifies the surface as poor wetting and poor adhesion. Table 11.1 shows the results of SFE analysis. γd specifies the dispersion component and Υp specifies the polar component of the SFE. γtotal specifies the sum of γd and γp. As can be seen from Table 11.1, the total SFE values for all samples are as low as that of Teflon (γtotal 21.6 mJ/m2 [11] and a typical hydrophobic surface). It can also be seen that the polar component which

Table 11.1 Results for different PSAs. Contact angle (°)

SFE and its components (mJ/m2)

No. of carbons

Water

M.I.

γd

γp

γtotal

C4

103.8

65.4

25.4

0.3

25.7

C6

114.2

71.3

22.6

0

22.6

C6+2

112.0

73.7

21.5

0

21.5

C8

116.8

67.4

23.1

0

23.1

C12

117.3

71.0

21.6

0

21.6

C14

133.6

108.5

6.1

0

6.1

C16

109.4

72.3

21.9

0.1

22.0

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contributes to the adhesiveness is almost zero in every sample while those samples have the functional group. Based on these measurement results it was considered that measurements of static contact angle and analysis of SFE do not present significant data to discuss the strength of adhesiveness of these acrylic PSA samples.

11.4.2

Dynamic Contact Angle as a Function of Time

Figure 11.6 shows the contact angle variation as a function of time. Although the equilibrated contact angle values for all samples are still higher than 90°, which is exemplified as the surface of poor wettability and poor adhesion, the lowering trends of contact angles with time indicate the surface reorientation mechanism of these acrylic PSA samples.

11.4.3

Dynamic Contact Angle Hysteresis

Figure 11.7 shows the advancing angle θA, receding angle θR and contact angle hysteresis Δθ for each acrylic PSA. The graph shows that higher hysteresis values were found for the PSAs with chain length from C6 to C14, and lower hysteresis values for the PSAs with C4 and C16 chain lengths. It is known that advancing angle depends on hydrophobic segments and receding angle depends on hydrophilic segments [7]. Contact angle hysteresis reflects the heterogeneity of the surface. From this point of view it is speculated that the acrylic PSA samples with chain length from C6 to C14 would have greater adhesiveness among the samples tested.

Contact angle of sessile drop (º)

140 4

130

6

120

6+2

110

8

100

12

90

14

80

16

70 60 0

20

60 40 Elapsed time [s]

Figure 11.6 Dynamic contact angles as a function of time.

80

Dynamic contact angle & Hysteresis (º)

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160.0 140.0 120.0 100.0 80.0

θA

60.0

θR

40.0

θ

20.0 0.0 4

6 6+2 8 12 14 16 Number of carbon in side chain

Figure 11.7 Dynamic contact angle hysteresis.

It must be noted that this dynamic contact angle measurement took only about 4 minutes for the entire process. If the reorientation of the PSA segments is expected but the mobility is slow, this measured contact angle hysteresis may not represent the mobility of molecules well. Therefore, more attention was paid to ATR measurement.

11.4.4

Adhesion Tension Relaxation (ATR)

Figure 11.8 is a chart for the measured ATR in advancing adhesion tension for 20 minutes after the sample substrate was immersed in DI water about 20mm deep in the advancing process of dynamic contact angle

Adhesion tension FA(t)/L (mN/m)

80 70 60

C4 C6

50

C6+2

40

C8 C12

30

C14

20

C16

10 0 0

200

400

600 800 1000 1200 1400 Elapsed time [s]

Figure 11.8 Results of ATR in advancing adhesion tension.

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measurement. From Figure 11.8, it can be seen that the adhesion tensions of the samples exhibit remarkable differences in three important aspects: initial values, slopes of relaxation curves, and equilibrium values. Discussion on these aspects is provided in the following paragraphs. Comparing the initial adhesion tension values, it can be found that C4 indicates far higher value than the others and C14 indicates lower value than the others. These results are similar to the trends for the results of SFE as shown in Table 11.1 although SFE was not considered a good indicator initially. The relaxation curves are classified into three trends. First, the changes are quick in the initial stage but slow down later such as for C4. Second, the changes showed curves similar to parabolas in the cases for C6 ~ C12. The slopes of the changes are different but they are not considered significant enough to be characterized as such. Third, the changes are insignificant such as for C14 and C16. It is our assumption that the short chain of C4 contributes to the quick mobility of molecules. For further study, faster ATR data acquisition may be useful for sample C4. As for samples C14 and C16, the mobility of molecules was very slow because of long alkyl chain. It is our belief that the equilibrated values of advancing adhesion tension FA(t)/L could reflect the absolute wettability of the PSA samples to DI water and this could contribute to enhance the adhesiveness. Figure 11.9 is a chart for the ATR obtained in the receding adhesion tension for 20 minutes after measuring the advancing adhesion tension and withdrawing the sample substrate. This process is dewetting with

Adhesion tension FR(t)/L (mN/m)

90 80 70 C4

60 50 40

C6 C6+2 C8 C12

30

C14

20

C16

10 0 1200 1400 1600 1800 2000 2200 2400 2600 Elapsed time [s]

Figure 11.9 Results of ATR in receding adhesion tension.

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reorientation of hydrophobic segments and the inverted parabolic curves are drawn. The results of C14 and C16 show only little changes in adhesion tension consistently and this reflects very slow or little mobility of molecules. C4 changes in the initial stage quickly and equilibrates earlier than the other samples as shown in its advancing adhesion tension. C6 ~ C12 also show trends similar to their advancing adhesion tension, and high mobility of segments can be assumed. The equilibrated values of all samples except C14 show the same order as the advancing adhesion tension. We do not understand the reason for the exception of C14 at this time.

11.4.5

Peel Force

Both Figure 11.10 and Figure 11.11 show the measurement results of peel force for each acrylic PSA. Figure 11.10 shows the results of peel force variation with peel angle from 30, to 60, 90, 120 and 150 degree while the peel speed was fixed at 300mm/min. Figure 11.11 shows the results of peel force variation with peel speed from 300, to 1000, 3000, and 9000mm/min while the peel angle was fixed at 90 degree. The samples with chain length C14 and C16 did not adhere to the SUS304 adherend and the peel force data for these two samples could not be obtained. It is obvious that these two samples are weaker in adhesiveness than the other five samples. The results of ATR measurements have shown that C14 and C16 had slow mobility of segments than the other five samples. This explains why these two samples failed to stick to the SUS304 adherend. Both Figure 11.10 and Figure 11.11 indicate the general trend that the shorter the alkyl chain length of PSA is, the stronger is the peel force.

Peel force (N/25mm)

2.5 2 C4

1.5

C6 C6+2

1

C8 C12

0.5 0 30o

60o 90o 120o Peel angle (degree)

180o

Figure 11.10 Peel force as a function of peel angle. Peel speed was fixed at 300mm/min. (C14 and C16 did not adhere to the adherend.)

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Peel force (N/25mm)

0.06 0.05

C4 C6

0.04

C6+2

0.03

C8

0.02

C12

0.01 0 300

3000 1000 9000 Peel speed (mm/min)

Figure 11.11 Peel force as a function of peel speed. Peel angle was fixed at 90 degree. (C14 and C16 did not adhere to the adherend.)

11.5 Conclusion For the study of the characteristics of acrylic PSAs, SFE analysis was not adequate for evaluation of adhesion. Static contact angle measurements did not produce significant results. Contact angle hysteresis represents several characteristics of a surface but if the reorientation of PSA segments is expected and the mobility is slow, the contact angle hysteresis may not represent the mobility of molecules. In general, contact angle hysteresis is determined by measuring advancing angle and receding angle while immersing and withdrawing a solid sample from a liquid at a certain speed. If the mobility of molecules on the solid sample is very low the advancing and receding angle measurements will not be able to reflect the effect of slow motion of the molecules. For acrylic PSAs, characterizing the mobility of surface segments is also important for study of adhesiveness, and adhesion tension relaxation (ATR) can provide significant information on the mobility of segments in response to environmental changes and thus is very helpful for evaluation of adhesiveness of PSAs. The measured results of ATR correlated well with the results of peel force measurements. For future work, a few important aspects related to ATR measurement technique can be conducted, such as the time scale for equilibration, immersion speed, more careful preparation of samples, etc.

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References 1. Yamazaki, Y., Morphology formation and dynamics of adhesive in deformation. J. Japan Soc. Colour Mater., 81, 361–369, 2008. 2. Yamazaki, Y., Considering “Physics of Adhesion” from the action of peeling PSA tape. J. Physical Soc. Japan, 71, 318–322, 2016. 3. Good, R.J., Contact angle, wettability and adhesion: A critical review, in: Contact Angle, Wetting and Adhesion, K.L. Mittal (Ed.), pp. 3–36, VSP, Zeist, The Netherlands, 1993. 4. Kasemura, T., Dynamic contact angle and adhesion tension relaxation for multi-component polymeric systems. J. Membrane Soc. Japan, 21, 197–204, 1996. 5. Kasemura, T., Selective adsorption of segment of multicomponent polymeric systems to interface (II). J. Adhes. Soc. Japan, 29, 273–279, 383–391, 1993. 6. Takahashi, S., Polymer surface dynamics in terms of dynamic wettability analysis. J. Adhes. Soc. Japan, 46, 372–378, 2010. 7. Vegelati, C., Perwuelz, A., Vovelle, L., Poly(ethylene terephthalate) surface dynamics in air and water studied by tensiometry and molecular modelling. Polymer, 35, 262–270, 1994. 8. Johnson, R.E., Jr. and Dettre, R.H., Contact angle hysteresis. III. Study of an idealized heterogeneous surface. J. Phys. Chem., 68, 1744–1750, 1964. 9. Schultz, J., Tentative correlation between contact angle hysteresis and adhesive performance. J. Colloid Interface Sci., 201, 247–249, 1998. 10. Extrand, C.W. and Kumagai, Y., An experimental study of contact angle hysteresis. J. Colloid Interface Sci., 191, 378–383, 1997. 11. Kaelble, D.H. and Uy, K.C., A reinterpretation of organic liquid-polytetrafluoroethylene surface interactions. J. Adhesion, 2, 50–60, 1970. 12. Kaelble, D.H., Dispersion-polar surface tension properties of organic solids. J. Adhesion, 2, 66–81, 1970.

12 The Potential of Surface Nano-Engineering and Superhydrophobic Surfaces in Drag Reduction Ali Shahsavari, Amir Nejat and Seyed Farshid Chini* School of Mechanical Engineering, College of Engineering, University of Tehran, Iran

Abstract Recently superhydrophobic surfaces have been used for passive drag reduction, as fluids slip on them. The present work reviews the application of superhydrophobic surfaces in drag reduction. The concept of slip length is introduced as a key factor affecting the drag reduction performance of superhydrophobic surfaces. Different techniques for measuring the slip length are explained. Parameters that may affect the slip length (e.g. surface wettability, Reynolds and turbulent structures) are discussed. It is shown that although superhydrophobic surfaces decrease the friction drag, they do not necessarily decrease the pressure drag. Therefore, for general applications it is beneficial to use partial superhydrophobicity. A procedure is introduced to find the optimum location of superhydrophobic patches to meet the desired goal. To explain the procedure, we used the following example. We divided the surface of a 2D NACA 0012 hydrofoil into 10 sections, where each section may be either superhydrophobic or uncoated. So, 1024 chromosomes, where each has 10 genomes, may represent all the cases. Using the classic genetic algorithm, along with numerical simulations, the drag forces are found. Parents of the next generations are chosen using the Roulette-wheel approach. After 7 generations, we found the chromosome which minimized the drag, and 21% drag reduction was achieved. Keywords: Superhydrophobic, drag reduction, foil, slip length, genetic algorithm, lift to drag ratio

*Corresponding author: [email protected] K.L. Mittal (ed.) Advances in Contact Angle, Wettability and Adhesion: Volume 4, (239–266) © 2020 Scrivener Publishing LLC

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Nomenclature b Ca d GF H h k Ls P p Q R Re T USP uτ V W X x y

Distance between two adjacent surface patterns (m) Capillary number Thickness of surface patterns (m) Gas fraction Height of channel (m) Height of roughness (m) Roughness average size (m) Slip length (m) Pressure (Pa) Roughness periodicity (m) Volumetric flow rate (m3/s) Radius (m) Reynolds number Torque (N.m) Underwater surface property Shear velocity (m/s) Velocity (m/s) Width of channel (m) Distance along upper or lower surface of hydrofoil from leading edge to trailing edge (m) Horizontal coordinate component (m) Vertical coordinate component (m)

Greek Letters α β γ δ δv θ θhys μ ν ρ τw φ Ω

Cone angle (degree) Angle representing coated area for a cylinder (degree) Surface tension (N/m) Boundary layer thickness (m) Viscous length scale (m) Contact angle (degree) Contact angle hysteresis (degree) Dynamic viscosity (Pa.s) Kinematic viscosity (m2/s) Density (kg/m3) Shear stress on wall (N/m2) Roughness wedge angle (degree) Rotational speed (rad/s)

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Subscripts i s

Index of surface roughness Slip

Superscript +

Normalized with viscous length scale

12.1 Introduction The force exerted on moving objects in fluids is typically broken into two components: along the streamline (drag) and normal to the streamline (lift). The origin of these hydrodynamics forces is either non-uniform distribution of pressure around the object, or shear stress. It is mostly desirable to increase the lift and decrease the drag. The early works on drag reduction were on optimizing the geometry. Streamlined objects (e.g. foils) minimize the pressure drag. To further decrease the drag, active and passive approaches have been developed. Active techniques (e.g. gas or bubble injection, polymer additives, fluid blowing or suction, vibrant and flexible wall) usually require application of external energy. But passive drag reduction techniques operate without external energy input. There are some techniques which use advantages of both active and passive approaches simultaneously [1, 2], but still complexity and cost cannot be ignored. Riblets are one of passive approaches that reduce drag force by two mechanisms. In addition to preventing translation of streamwise vortices and modifying outer layer turbulent, riblets can reduce the surface exposed to high shear flow velocity by moving up the vortices [3]. Riblets should be small enough so that the vortices do not penetrate into the troughs (see Figure 12.1). If vortices penetrate into the riblets, drag increases. Size of riblets is in the order of hundred micrometers to millimeters based on application. Riblets are employed in both laminar and turbulent regimes for both internal and external flow fields [4, 5]. Recently, the use of superhydrophobic surfaces is suggested as one of passive drag reduction methods. The superhydrophobicity, in addition to lowering the fabrication cost and being more compatible with different applications, offers higher drag reduction performance in comparison to riblets [3]. Water drops bead up and roll off easily on superhydrophobic

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Figure 12.1 The vortex on top of riblets helps liquid or air move easier on surface (the left-hand side case). The vortex between gaps of riblets (the right-hand side case) leads to more drag due to larger surface exposed to high shear flow.

surfaces. To quantify superhydrophobicity, contact angle is typically used. Contact angle is the angle between tangent to the water drop and solid surface at the three-phase line, see Figure 12.2. If the contact angle is larger than 90° the surface is hydrophobic and if smaller than 90° the surface is hydrophilic. For superhydrophobic surfaces, contact angle is larger than 150°. To attain high contact angle, surface free energy should be lowered. The maximum contact angle that one may attain on smooth surfaces (with lowering the surface free energy) is 120o for a surface covered with CF3 groups. Teflon consists of CF2 groups and its contact angle is 108°. For heterogeneous surfaces as shown by Cassie-Baxter relation, the apparent contact angle is the weighted average of the contact angles of patches. As the contact angle of water on air is 180o, by roughening the surface, we may have contact angles larger than 120o. On rough or heterogeneous surfaces, the apparent contact angle is not unique. The apparent contact angle changes between a minimum and a maximum. To find the maximum and minimum contact angles, different methods have evolved. The sessile drop method is one of the most consistent methods. In this method, by adding/subtracting water to/from a drop on a surface, advancing and receding (maximum and minimum) contact angles are calculated [6]. Another approach is the inclined surface method.

Contact Angle

Figure 12.2 Contact angle of a sessile drop on a solid substrate.

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The difference between advancing and receding contact angles is contact angle hysteresis which qualitatively shows the adhesion between drop and solid surfaces [7]. On superhydrophobic surfaces, contact angle hysteresis is less than 10o. The superhydrophobic roughness in micro or nano-scale is fabricated by different methods [8], e.g. chemical etching or sol-gel. Many different shapes can be obtained for surface features as shown in Figure 12.3, but hierarchical structure, which is mostly found in nature, is the most efficient in drag reduction. One of the goals in surface nano-engineering is to increase the energy barrier between Wenzel and Cassie-Baxter states by designing optimum surface features. Therefore, one may prevent the transition from CassieBaxter to Wenzel (see Figure 12.4). Apparent surface area, periodicity, gas fraction, pattern ratio, and wedge angle are parameters used for characterizing a rough surface (see Figure 12.5). Periodicity (p) is defined as:

p = bi + di

(12.1)

(a)

(b)

Figure 12.3 Different features for superhydrophobic surfaces. (a) micro-scale, (b) microand nano-scale (hierarchical) roughness.

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(a)

(b)

Figure 12.4 Two equilibrium states of water drops on rough surfaces: (a) Wenzel and (b) Cassie-Baxter state.

where bi and di are shown in Figure 12.5. Gas fraction (GF) is the ratio of surface in contact with air to apparent surface area which is defined as:

GF

bi bi di

(12.2)

Roughness ratio (r) is another parameter for superhydrophobic features representing the ratio of true surface to apparent surface. Wedge angle is the angle at the tip angle of features which is represented by “φ” in Figure 12.5.

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Apparent area True area di h

p

φ

bi

Figure 12.5 Geometric parameters for a simplified pattern of a rough surface.

Due to the existence of trapped air in features of superhydrophobic surfaces, liquid-solid interface is substituted by liquid-air interface in some portion of interface [9–11]. No-slip boundary condition is still valid on the solid-liquid interface; however, liquid slips on air cavities. The averaged stick-slip velocity in macroscale is usually modeled by a slip velocity (see Figure 12.6). To model the slip velocity, Navier’s slip model is usually adopted for superhydrophobic surfaces. It should be noted that Navier’s slip model was previously used for gases at high Knudsen number where continuity does not necessarily hold. Figure 12.6 and Eq. 12.3 represent the

Water

No-Slip

Free

Air

Shear -

Shear -

Shear No-Slip

Free

Air

No-Slip

Free Vs Air

Ls

y

x

Figure 12.6 Liquid slips on air-liquid interface (shear-free) and on solid -liquid interface no-slip boundary condition is valid. On average and from macroscopic point of view, liquid slips on superhydrophobic surfaces.

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concept of slip length (Ls) which is the ratio of slip velocity (Vs) to shear dVs rate on surface as: dy

Ls

Vs dVs dy

(12.3)

There are some other approaches to model macroscopic slip on superhydrophobic surfaces such as volume of fluid (VOF) [9, 12–14], alternative full-slip and no-slip boundary conditions [15–21] and deformable airwater interface [22]. However, there is no significant difference in results of flow field by employing these complicated methods, and slip length is the most optimum in both cost of computation and predicting drag reduction induced by superhydrophobic surfaces.

12.2 Parameters Affecting the Slip Length As discussed earlier, slip length is introduced as a characteristic of superhydrophobic surfaces for evaluating drag reduction performance. Higher slip length leads to lower friction. In order to have an efficient superhydrophobic surface with high amount of drag reduction, it is essential to investigate the effective parameters on slip length. It is observed that in addition to surface topography and wettability [20, 23–25], some other factors such as direction of features, pressure and shear rate may affect the slip length [26, 27]. Regarding the surface topography, it is observed that slip length is proportional to periodicity and gas fraction of features with different shapes [20, 23–25, 28–34]. Gas fraction increases the shear-free region and apparent slip velocity [35]. There are some simplified equations describing the relation between slip length and gas fraction or periodicity. Some are presented in Table 12.1. It should be considered that slip length, periodicity and channel height are all normalized by viscous length scale as shown in equation (12.4).

v v

w/

(12.4)

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where ν, ρ and τw are respectively fluid kinematic viscosity, fluid density and wall shear stress at no-slip wall. There is a maximum micro-ridge spacing above which significant drag reduction cannot be achieved [43], as surfaces may easily transit from Cassie-Baxter to Wenzel state [32, 44]. Maximum allowable pressure (Pmax) for the transition for a liquid droplet (with surface tension γ) on a surface with contact angle θ is [45]: Pmax

2 (cos ) (1 GF ) p GF

(12.5)

Table 12.1 Semi-analytical equations for slip length as a function of gas fraction and periodicity. Slip length equation Ls

p

Ls H

ln cos

5 1 (GF ) 2 2

Description

Reference

Developed for alternative slip and no-slip boundary, Ls Ls / v and p+ = p/δv

[36, 37]

H+ = H/δv

[38]

[39]

GF )

Developed for mesh-like superhydrophobic surfaces Developed for patterned surfaces of posts

[40]

0 44

Developed for patterned surfaces of posts on a lattice

[41]

An implicit scaling law for square post with GF >0.88

[42]

GF 2

0 02

Ls

p ln 3

Ls

p

Ls

p

p

Ls 0 328 Ls 1 GF 0 535

2 (1

0 325 1 GF 3 16 1 GF

3 ln 1 2

2

3

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The slip length relations in Table 12.1 are based on some assumptions (e.g. flat air-liquid interface) and do not consider the shape, distribution and orientation of features [2, 19, 24, 46]. For a wide range of gas fractions and low Reynolds numbers (Re < 500) the effective slip length of stream-wise grooves is always more than that of square posts, holes and span-wise grooves [19,28]. However, at high Reynolds numbers, posts have higher slip velocity in comparison to stream-wise grooves [24, 47–49]. Distribution of features also affects slip length. Random, square and staggered distributions of features are shown in Figure 12.7. The slip velocity of random (Figure 12.7c) is higher than that of square and staggered (Figure 12.7b) posts [44]. The findings suggest that among all different shapes of surface features hierarchical structures represent the highest and the most stable slip length in comparison to others [45, 50]. In addition to geometric parameters of surface features, wettability also affects the slip length. Increase in contact angle and reduction in contact angle hysteresis result in large slip and drag reduction [51, 52]. Although, in some literature slip length is shown to be a function of surface topography [40], there are some others that believe slip depends on the flow field [26, 27, 53]. A shear-dependent slip length is likely to occur especially in turbulent regime [16, 27]. For capillary number (equation (12.6), ratio of viscous to surface tension forces) in order of 10-10 to 10-4, the bubbles trapped inside the surface asperities will not deform significantly. For higher shear rates, with the capillary number in order of 10-2, the bubbles will become elongated in the direction of the flow which leads to higher gas fractions. This causes the slip length to increase with shear [23].

V

Ca

(12.6)

where μ, V and γ are dynamic viscosity, velocity and surface tension, respectively.

(a)

(b)

(c)

Figure 12.7 Different distributions of surface features. (a) Square, (b) Staggered, and (c) Random.

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For laminar internal flow, shape of features can also affect the dependency of slip length on Reynolds number. For instance, for random, square posts, holes, and transverse grooves slip length decreases by increasing the Reynolds numbers [28, 54], while for longitudinal grooves slip length remains constant [28].

12.3 Slip Length Measurement on Superhydrophobic Surfaces As discussed before, there are different parameters affecting the slip length. In general, it is not possible to estimate slip length of different superhydrophobic surfaces due to a variety of surface heterogeneities, chemistries and working conditions. It seems that experimental techniques are the most efficient approaches to measure slip length. PIV (particle image velocimetry) is one of the techniques used by many researchers to measure slip length on superhydrophobic surfaces [29, 55–58]. The μ-PIV technique is also employed to measure slip velocity especially in micro-fluidic devices [40, 43, 52, 59]. The images can be used to measure velocity distribution in order to find near-wall shear rate and slip length [60]. Pressure drop measurement is another approach to measure the slip length, especially in micro-channels. In order to relate slip length to pressure drop, internal flow is restricted to laminar regime [30, 61]. Equation (12.7) represents the slip length for laminar channel flow as a function of dP , liquid viscosity (μ), volumetric flow rate (Q), height pressure drop dx (H) and width of channel (W).

Ls

23 Q 6WH 2

1

dP dx

H 3

(12.7)

Rheometers are also employed for slip length measurement [54, 62, 63]. Similar to pressure drop experiments, a rheometer should work in laminar Couette flow in order to use theoretical relations relating the measured torque and slip length. For a rheometer shown in Figure 12.8, slip length can be determined by equation (12.8). This method is relatively accurate with less than 10% error in measuring the slip length [64].

Ls

R 4

1

T

8 R

3

13 3

(12.8)

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R α

No-Slip

Figure 12.8 A cone-plate rheometer with superhydrophobic cone surface used to measure slip length.

where R and α are radius and cone angle of rheometer (see Figure 12.8), Ω  is angular velocity and T is the measured torque.

12.4 Drag Reduction of Superhydrophobic Surfaces Drag reduction of superhydrophobic surfaces is due to entrapped air in surface asperities [38, 65]. In addition to lower skin friction, superhydrophobic surfaces can delay transition to turbulent regime [66]. Superhydrophobic surfaces cause more drag reduction in turbulent flow regime which is mostly attributed to modified turbulent structures [16, 67]. Due to strong dependency of drag reduction on slip length, it seems that parameters affecting the slip length can also be important in drag reduction. For instance, direction and size of features, wettability, shear rate and Reynolds number all may affect the drag reduction.

12.4.1

Wettability Parameters

Some experiments show that drag reduction has no clear relationship with contact angle. The underwater surface property (USP) is introduced in order to evaluate the drag reduction of superhydrophobic surfaces with different wettability and texture (shown in Figure 12.5), equation (12.9).

USP

Contact Angle Hysteresis bi Contact Angle h

2

(12.9)

It has been proven that drag reduction and USP are correlated for both turbulent and laminar flows [68].

The Potential of Surface Nano-Engineering

12.4.2

251

Reynolds Number and Shear Rate

In laminar flow, similar to uncoated surfaces, for superhydrophobic surfaces at lower Poiseuille number (the product of friction factor and Reynolds number) the friction factor is inversely proportional to Reynolds number [51]. According to Moody diagram, in turbulent regime and for uncoated surfaces, by increasing the Reynolds number and decreasing the roughness, friction factor decreases. For superhydrophobic surfaces there is a controversy [9]. As shown in [53] superhydrophobic surfaces behave similarly to smooth uncoated pipes. However, at very high Reynolds number values, not only no drag reduction is observed but also the drag force increases to values higher than that for a smooth sample [70, 71]. The drag increase at very high Reynolds number values is due to the instability of lubricating air layer and disappearance of air plastron [2, 13, 71, 72] over the superhydrophobic surface [73]. In addition to depletion of entrapped air, higher momentum causes mixing of air bubbles with flowing water. In other words, the air bubbles sticking to the surface create more roughness [74]. For deeper insight into drag reduction mechanism of superhydrophobic surfaces, turbulent structures induced on such kind of surfaces should be investigated more precisely.

12.4.2.1

Turbulent Structure

Depending on the size of features in a turbulent flow, a superhydrophobic surface may increase or decrease the drag. Turbulence parameters (e.g. Reynolds stress [56, 75, 76], turbulence intensity [77–79], production of turbulence [76, 77], vortex structures and secondary flow [76, 78]) can lead to different velocity profiles in the inner layer [56, 75, 76, 78, 80]. Viscous sublayer is the first near-wall layer of turbulent boundary layer with y+