Preface Surface properties have critical roles in determination of the overall performance and applications of materials in many diverse fields. Some of these properties include friction, scratch resistance, ultraviolet/oxidative stability, soil/water repellence, biocompatibility, and fouling resistance. As a result of very rapid advancements in almost all technological areas, including microelectronics, medicine, communications, transportation, housing and energy, the need for materials with smart, multifunctional surface properties has been increasing steadily. The wetting behaviour of material surfaces with water is one of the most interesting and critical properties because it plays a fundamental part in a wide range of applications, including protective coatings, membranes, sensors, microfluidics, biomedical devices, textiles, solar panels, and windshields. It is well known that nature and natural processes are adaptable to changing environmental conditions, and so they are smart and multifunctional. With this in mind, scientists tried to understand the biological principles behind natural structures and use them as a guide to design and produce superior synthetic materials. This approach is termed as biomimetics. Studies using scanning electron microscopy on many types of plant leaves carried out two decades ago led to clear understanding of the close relationship between the topography ( roughness ) and wetting behaviour of natural surfaces. Such studies resulted in a dramatic increase in the research and development on preparation, characterisation and potential applications of organic, inorganic and hybrid materials with superhydrophobic surfaces. In accordance with iii
Superhydrophobic Polymer Surfaces: Preparation, Properties and Applications those research efforts, the number of scientific publications, patents and review articles in the field has also shown dramatic growth. The main aim of this book is to provide a comprehensive overview on new developments in the preparation, characterisation and potential applications of superhydrophobic polymer surfaces. In addition, characteristics of naturally superhydrophilic, superoleophobic and surfaces with tunable wettability are discussed. Special emphasis is given to discussion of the theoretical foundations and models proposed in description of the wetting behaviour of rough surfaces. Several methods used in preparation of robust and durable superhydrophobic polymer surfaces, as well as their advantages and disadvantages, are provided. The close relationship between polymer structure and composition, hierarchical micro/nanosurface topography, average surface roughness and superhydrophobic behaviour is discussed. We thank Smithers Rapra for providing the opportunity and support for publishing this book. We hope that this book will provide fundamental understanding of the theoretical background, critical issues and practical information to researchers and application engineers working in this exciting and fast-moving field. Çağla Koşak Söz Emel Yılgör İskender Yılgör Koç University, İstanbul, Turkey September 2016 iv
Contents 1 Polymer Surfaces... 1 1.1 Surface Properties and Surface Energies of Polymers... 1 1.2 Wettability, Hydrophilic Surfaces and Hydrophobic Surfaces... 7 1.3 Balancing the Bulk and Surface Properties of Polymers... 11 1.4 Superhydrophobic Surfaces... 13 1.5 Scope and Aim of this Book... 15 2 Theoretical Models of Surface Wetting... 21 2.1 Liquid Droplets on Ideal Smooth Solid Surfaces: Young s Equation... 21 2.2 Liquid Droplets on Rough Surfaces: Wenzel and Cassie Baxter Models... 26 2.3 Contact Angle Hysteresis... 34 2.4 Experimental Methods for Determination of Contact Angle Hysteresis... 35 2.5 Applicability of Wenzel and Cassie Baxter Models... 36 3 Wetting Behaviour of Natural Surfaces: Superhydrophobicity, Superhydrophilicity and Superoleophobicity... 45 3.1 Superhydrophobic Surfaces... 45 3.2 Self-Cleaning Surfaces: The Lotus Effect... 50
Superhydrophobic Polymer Surfaces: Preparation, Properties and Applications 3.3 Superhydrophilicity and Superoleophobicity in Nature... 64 4 Methods and Processes used for the Preparation of Superhydrophobic Polymer Surfaces... 75 4.1 Plasma Etching and Plasma Polymerisation... 78 4.2 Electrospinning... 82 4.3 Layer-by-Layer Deposition and Self-Assembly... 86 4.4 Microphase Separation... 89 4.5 Sol Gel Methods... 92 4.6 Spin-Coating... 94 4.7 Dip-Coating... 102 4.8 Doctor-Blade Coating... 103 4.9 Spray-Coating... 106 4.10 Templating... 114 4.11 Etching and Lithography... 116 4.12 Chemical and Physical Vapour Deposition... 119 5 Characterisation Methods of Superhydrophobic Surfaces... 131 5.1 Scanning Electron Microscopy... 131 5.2 Atomic Force Microscopy... 134 5.3 White Light Interferometry... 137 5.4 Contact Angle Measurements Goniometry... 140 6 Stimuli-Responsive Surfaces with Tunable Wettability... 145 6.1 Electro-Responsive Surfaces... 146 6.2 Ultraviolet-Responsive Surfaces... 148 6.3 Ion-Responsive and ph-responsive Surfaces... 153 6.4 Temperature-Responsive Surfaces... 158 6.5 Other Approaches to Produce Polymer Surfaces with Tunable Wettability... 161 vi
Contents 7 Current and Potential Applications of Polymers with Superhydrophobic Surfaces... 171 7.1 Self-Cleaning Surfaces... 171 7.2 Anti-Fogging Coatings... 175 7.3 Anti-Fouling and Foul-Release Coatings... 177 7.4 Drag Reduction... 178 7.5 Anti-Icing and Ice-Release Coatings... 181 7.6 Anti-Reflective Coatings with Improved Solar Cell Efficiency... 185 7.7 Corrosion-Resistant Coatings... 188 7.8 Biocompatible Surfaces... 189 7.9 Superhydrophobic Coatings for Oil Water Separation... 191 7.10 Microfluidic Applications... 192 7.11 Water-Harvesting Surfaces... 194 7.12 Concluding Remarks... 196 Abbreviations... 207 Index... 213 vii
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