A Nanotechnology Approach

Editor: Walid A. Daoud

Self-Cleaning MAteriAls ANd surfAces

Self-Cleaning Materialsand Surfaces

A Nanotechnology Approach

Self-Cleaning Materialsand Surfaces

A Nanotechnology Approach

Edited by

WALID A. DAOUD

School of Energy and Environment, City University of Hong Kong,Hong Kong

This edition first published 2013C© 2013 John Wiley & Sons, Ltd

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Library of Congress Cataloging-in-Publication Data

Self-cleaning materials and surfaces : a nanotechnology approach / edited by Walid A. Daoud.pages cm

Includes bibliographical references and index.ISBN 978-1-119-99177-9 (cloth)

1. Coatings. 2. Surface active agents. 3. Materials–Cleaning. 4. Nanostructured materials. I. Daoud, Walid A.TA418.9.C57S45 2013667′.9–dc23

2013016955

A catalogue record for this book is available from the British Library.

ISBN: 9781119991779

Set in 10/12pt Times by Aptara Inc., New Delhi, India.

1 2013

Contents

List of Contributors xiiiPreface xv

PART I CONCEPTS OF SELF-CLEANING SURFACES

1 Superhydrophobicity and Self-Cleaning 3Paul Roach and Neil Shirtcliffe

1.1 Superhydrophobicity 31.1.1 Introducing Superhydrophobicity 31.1.2 Contact Angles and Wetting 41.1.3 Contact Angle Hysteresis 41.1.4 The Effect of Roughness on Contact Angles 61.1.5 Where the Equations Come From 81.1.6 Which State Does a Drop Move Into? 11

1.2 Self-Cleaning on Superhydrophobic Surfaces 121.2.1 Mechanisms of Self-Cleaning on Superhydrophobic Surfaces 121.2.2 Other Factors 151.2.3 Nature’s Answers 171.2.4 Superhydrophilic Self-Cleaning Surfaces 191.2.5 Functional Properties of Superhydrophobic Surfaces 20

1.3 Materials and Fabrication 251.4 Future Perspectives 27References 28

PART II APPLICATIONS OF SELF-CLEANING SURFACES

2 Recent Development on Self-Cleaning Cementitious Coatings 35Daniele Enea

2.1 Introduction 352.2 Atmospheric Pollution: Substances and Laws 36

2.2.1 Nitrogen Oxides 362.2.2 Particulate Matter 372.2.3 Volatile Organic Compounds 37

2.3 Heterogeneous Photocatalysis 38

vi Contents

2.4 Self-Cleaning Surfaces 392.4.1 Mechanisms of Photo-Reduction of Air Pollutants 412.4.2 Some Experimental Evidences 41

2.5 Main Applications 442.6 Test Methods 46

2.6.1 Colour 462.6.2 Photocatalytic Degradation of Nitrogen Oxides 472.6.3 Photocatalytic Degradation of Micro-Pollutants in Air 492.6.4 Photocatalytic Degradation of Rhodamine B 512.6.5 Spectroscopic Techniques 53

2.7 Future Developments 53References 54

3 Recent Progress on Self-Cleaning Glasses and Integration withOther Functions 57Baoshun Liu, Qingnan Zhao and Xiujian Zhao

3.1 Introduction 573.2 Theoretical Fundamentals for Self-Cleaning Glasses 58

3.2.1 Wettability 583.2.2 Photoinduced Hydrophilicity 593.2.3 Heterogeneous Photocatalysis 62

3.3 Self-Cleaning Glasses Based on Photocatalysis and PhotoinducedHydrophilicity 633.3.1 Self-Cleaning Glasses with Pores 633.3.2 Doping to Realize Visible-Light-Induced Self-Cleaning

Glasses 653.3.3 The Use of Hole Transfer to Realize Self-Cleaning 673.3.4 The Effect of Temperature and Atmosphere on the

Photoinduced Hydrophilicity 673.3.5 The Effect of Soda Ions on the Properties of Self-Cleaning

Glasses 693.3.6 The Anti-Bacterial Effect and Anti-Fogging Effect 703.3.7 The Composite SiO2 Films for Self-Cleaning Glasses with

High Antireflection 723.4 Inorganic Hydrophobic Self-Cleaning Glasses 75

3.4.1 Modifying The TiO2 Film by Low-Electronegativity Elements 753.4.2 The Application of ZnO Material in a Superhydrophobic

Material 773.5 Self-Cleaning Glasses Modified by Organic Molecules 793.6 The Functionality of Self-Cleaning Glasses 80References 84

4 Self-Cleaning Surface of Clay Roofing Tiles 89Jonjaua Ranogajec and Miroslava Radeka

4.1 Clay Roofing Tiles and Their Deterioration Phenomena 894.1.1 Raw Material Composition and Firing Process 89

Contents vii

4.1.2 Surface Characteristics of Clay Roofing Tiles 914.1.3 Frost, Chemical and Biocorrosion Deterioration of Clay

Roofing Tiles 964.1.4 Simulation of Weathering of Clay Roofing Tiles in Laboratory

Conditions 974.2 Protective and Self-Cleaning Materials for Clay Roofing Tiles 105

4.2.1 Design of Protective and Self-Cleaning Coatings 1074.2.2 Monitoring the Characteristics of Coated Clay Roofing Tiles 113

References 123

5 Self-Cleaning Fibers and Fabrics 129Wing Sze Tung and Walid A. Daoud

5.1 Introduction 1295.2 Photocatalysis 130

5.2.1 Mechanisms 1315.2.2 Titanium Dioxide Photocatalyst 132

5.3 Photocatalytic Self-Cleaning Surface Functionalization of FibrousMaterials 1345.3.1 Self-Cleaning Cellulosic Fibers 1345.3.2 Self-Cleaning Keratin Fibers 1395.3.3 Self-Cleaning Synthetic Fibers 140

5.4 Application of Photocatalytic Self-Cleaning Fibers 1425.4.1 Protective Clothing 1425.4.2 Household Appliances and Interior Furnishing 143

5.5 Limitations 1445.5.1 Environmental Concerns 1445.5.2 Human Safety Concerns 1445.5.3 Photocatalytic Efficiency and Stability 145

5.6 Future Prospects 1465.6.1 Visible Light Activation 1465.6.2 Remote Photocatalytic Effect 1465.6.3 Process Modification 1465.6.4 Empirical Measurements 147

5.7 Conclusions 147References 147

6 Self-Cleaning Materials for Plastic and Plastic-Containing Substrates 153Houman Yaghoubi

6.1 Introduction 1536.2 TiO2 Thin Films on Polymers: Sol–Gel-Based

Wet Coating Techniques 1556.2.1 Wet Coating Techniques: History and Advantages 1556.2.2 TiO2 Photocatalytic Thin Films on PC and PMMA 1566.2.3 SiO2 Incorporation into TiO2 - SiO2 as an Interfacial

Layer for TiO2 1626.2.4 TiO2 Photocatalytic Thin Films on PET and HDPE 167

viii Contents

6.2.5 TiO2 Photocatalytic Thin Films on PS 1716.2.6 Modified Hybrid TiO2 Sols on Plastics: ABS, Polystyrene,

and PVC 1726.2.7 TiO2 on Paints and Self-Cleaning Paints 1756.2.8 MW Irradiation–Assisted Dip Coating for Low-Temperature

TiO2 Deposition on Polymers 1786.2.9 Nanomechanical Properties of Dipped TiO2 Granular Thin

Films on Polymer Substrates 1796.3 TiO2–Polymer Nanocomposites Review:

Casting (Mixing) Techniques 1816.3.1 Short History and Advantages 1816.3.2 Ag/Polyethylene Glycol (PEG)–Polyurethane (PU)–TiO2

Nanocomposite Films by Solution Casting Techniques 1826.3.3 Antimicrobial Activity of TiO2-Isotactic Polypropylene (iPP)

Composites 1836.3.4 TiO2 Immobilized Biodegradable Polymers 184

6.4 TiO2 Sputter-Coated Films on Polymer Substrates 1876.4.1 DC Reactive Magnetron Sputtering of Photocatalytic TiO2

Films on PC 1876.4.2 Reactive Radio-Frequency [RF] Magnetron Sputtering of

Photocatalytic TiO2 Films on PET 1896.5 TiO2 Thin Films on PET and PMMA by Nanoparticle Deposition

Systems (NPDS) 1906.6 Photo-Responsive Discharging Effect of Static Electricity on

TiO2-Coated Plastic Films 1926.7 Recent Achievements 192

6.7.1 Commercialized Products: Ube-Nitto Kasei Co. and theUniversity of Tokyo 192

6.7.2 Patents: University of Wisconsin 193Acknowledgements 194References 194

PART III ADVANCES IN SELF-CLEANING SURFACES

7 Self-Cleaning Textiles Modified by TiO2 and Bactericide TextilesModified by Ag and Cu 205John Kiwi and Cesar Pulgarin

7.1 Introduction 2057.2 Self-Cleaning Textiles: RF-Plasma Pretreatment to Increase the

Binding of TiO2 2067.3 Self-Cleaning Mechanism for Colorless and Colored Stains

on Textiles 2087.4 Self-Cleaning Textiles: Vacuum-UVC Pretreatment to Increase the

Binding of TiO2 209

Contents ix

7.5 XPS to Follow Stain Discoloration on Cotton Modified with TiO2 andCharacterization of the TiO2 Coating 212

7.6 Bactericide/Ag/Textiles Prepared by Pretreatment with Vacuum-UVC 2147.7 DC-Magnetron Sputtering of Textiles with Ag Inactivating Airborne

Bacteria 2177.8 Inactivation of E. coli by CuO in Suspension in the Dark and Under

Visible Light 2187.9 Inactivation of E. coli by Pretreated Cotton Textiles Modified with

Cu/CuO at the Solid/Air Interface 2207.10 Direct Current Magnetron Sputtering (DC and DCP) of

Nanoparticulate Continuous Cu-Coatings on Cotton Textile InducingBacterial Inactivation in the Dark and Under Light Irradiation 220

7.11 Future Trends 223References 224

8 Liquid Flame Spray as a Means to Achieve Nanoscale Coatings withEasy-to-Clean Properties 229Mikko Aromaa, Joe A. Pimenoff and Jyrki M. Makela

8.1 Gas-Phase Synthesis of Nanoparticles 2298.2 Aerosol Reactors 233

8.2.1 Hot Wall Reactors 2338.2.2 Laser Reactors 2348.2.3 Plasma Reactors 2348.2.4 Flame Reactors 2358.2.5 Spray Pyrolysis 236

8.3 Liquid Flame Spray 2378.3.1 Synthesis of Nanoparticles via LFS 2378.3.2 Multicomponent Nanoparticles 2388.3.3 Synthesis and Deposition of Nanoparticle Coatings 240

8.4 Liquid Flame Spray in Synthesis of Easy-to-CleanAntimicrobial Coatings 2438.4.1 Synthesis of Titanium Dioxide 2438.4.2 Deposition of the Titania Coatings 2448.4.3 Doping of the Coatings 2468.4.4 Performance of the Antimicrobial Easy-to-Clean Coatings 247

8.5 Summary 249References 249

9 Pulsed Laser Deposition of Surfaces with Tunable Wettability 253Evie L. Papadopoulou

9.1 Introduction 2539.2 Basic Theory of Wetting Properties of Surfaces 254

9.2.1 Planar Surfaces 2549.2.2 Rough Surfaces 255

x Contents

9.3 Roughening a Flat Surface 2569.3.1 PLD Technique Overview 2579.3.2 Nanostructures Grown by PLD 257

9.4 Switchable Wettability 2639.4.1 Photoinduced Wettability on PLD Structures 2639.4.2 Electrowetting on PLD Structures 267

9.5 Concluding Remarks 270References 271

10 Fabrication of Antireflective Self-Cleaning Surfaces UsingLayer-by-Layer Assembly Techniques 277Yu-Min Yang

10.1 Introduction 27710.2 Antireflective Coatings 278

10.2.1 Interference Multiple Layers 27810.2.2 Inhomogeneous Layer with Gradient Refractive Index 279

10.3 Solution-Based Layer-by-Layer (LbL) Assembly Techniques 28010.3.1 Electrostatic Assembly 28010.3.2 Langmuir–Blodgett (LB) Assembly 28110.3.3 Self-Assembly 282

10.4 Mechanisms of Self-Cleaning 28310.4.1 Hydrophilic Surfaces 28310.4.2 Hydrophobic Surfaces 284

10.5 Fabrication of Antireflective Self-Cleaning SurfacesUsing Electrostatic Layer-by-Layer (ELbL) Assemblyof Nanoparticles 28510.5.1 Superhydrophilic Self-Cleaning Surfaces with

Antireflective Properties 28510.5.2 Superhydrophobic Self-Cleaning Surfaces with

Antireflective Properties 29110.6 Fabrication of Superhydrophobic Self-Cleaning Surfaces

Using LB Assembly of Micro-/Nanoparticles 29710.7 Characterization of As-Fabricated Surfaces 300

10.7.1 Surface Morphology and Roughness 30010.7.2 Thickness, Porosity, and Refractive Index 30110.7.3 Transmittance 30210.7.4 Photocatalytic Properties 30310.7.5 Contact Angle and Contact Angle Hysteresis 30410.7.6 Mechanical Stability 305

10.8 Challenges and Future Development 30610.9 Conclusion 307References 307

Contents xi

PART IV POTENTIAL HAZARDS AND LIMITATIONSOF SELF-CLEANING SURFACES

11 The Environmental Impact of a Nanoparticle-Based Reduced Needof Cleaning Product and the Limitation Thereof 315L. Reijnders

11.1 Introduction 31511.1.1 Outline 31511.1.2 Nanoparticle-Based Reduced Need of Cleaning Surfaces 316

11.2 Titania and Amorphous Silica Nanoparticles and Carbon NanotubesCan Be Hazardous and May Pose a Risk 31911.2.1 Molecular Mechanisms 32211.2.2 Risk Caused by Nanoparticles 322

11.3 Environmental Impact of a Reduced Need of Cleaning Product 32311.3.1 Direct Environmental Effects of a Nanoparticle-Based

Reduced Need of Cleaning Product 32411.3.2 Net Direct Environmental Benefits 32811.3.3 Indirect Environmental Effects of a Nanoparticle-Based

Reduced Need of Cleaning Product 32911.4 Limiting the Direct Environmental Impact of a Nanoparticle-Based

Reduced Need of Cleaning Product, Including Limitation of RisksFollowing from Exposure to Nanoparticles 33011.4.1 Limiting the Direct Environmental Impact 33011.4.2 Limitation of Risks Following from Exposure to Nanoparticles 330

11.5 Conclusion 331References 331

Index 347

List of Contributors

Mikko Aromaa, Aerosol Physics Laboratory, Department of Physics, Tampere Universityof Technology, Finland

Walid A. Daoud, School of Energy and Environment, City University of Hong Kong,Hong Kong

Daniele Enea, Department of Architecture, University of Palermo, Italy

John Kiwi, Institute of Chemical Sciences and Engineering, Swiss Federal Institute ofTechnology Lausanne (EPFL), Switzerland

Baoshun Liu, State Key Laboratory of Silicate Materials for Architectures, Wuhan Uni-versity of Technology, PR China and School of Material Science and Engineering, WuhanUniversity of Technology, PR China

Jyrki M. Makela, Aerosol Physics Laboratory, Department of Physics, Tampere Universityof Technology, Finland

Evie L. Papadopoulou, Institute of Electronic Structures and Lasers, Foundation forResearch and Technology-Hellas, Greece. Current address: Istituto Italiano di Tecnologia,Genova, Italy

Joe A. Pimenoff, Beneq Oy, Vantaa, Finland

Cesar Pulgarin, Institute of Chemical Sciences and Engineering, Swiss Federal Instituteof Technology Lausanne (EPFL), Switzerland

Miroslava Radeka, Faculty of Technical Sciences, University of Novi Sad, Serbia

Jonjaua Ranogajec, Faculty of Technology, University of Novi Sad, Serbia

L. Reijnders, Institute for Biodiversity and Ecosystem Dynamics, University of Amster-dam, The Netherlands

Paul Roach, Institute for Science and Technology in Medicine, Guy Hilton ResearchCentre, Keele University, UK

xiv List of Contributors

Neil Shirtcliffe, Faculty of Technology and Bionics, Hochschule Rhein-Waal, Germany

Wing Sze Tung, School of Applied Sciences and Engineering, Monash University,Australia

Houman Yaghoubi, Department of Mechanical Engineering/Department of ElectricalEngineering, University of South Florida, USA

Yu-Min Yang, Department of Chemical Engineering, National Cheng Kung University,Taiwan

Qingnan Zhao, State Key Laboratory of Silicate Materials for Architectures, WuhanUniversity of Technology, PR China

Xiujian Zhao, State Key Laboratory of Silicate Materials for Architectures, Wuhan Uni-versity of Technology, PR China

Preface

With increasing demand for hygienic, self-disinfecting, and contamination-free surfaces,interest in developing efficient self-cleaning, protective surfaces and materials has grown.Due to rising population density, the spreading of antibiotic-resistant pathogens remains agrowing global concern. The ability of microorganisms to survive on environmental surfacesmakes infection transmission a critical issue, and studies have shown that some infectiousbacteria can survive on the surface of various polymeric and textile materials for morethan 90 days. Self-cleaning surfaces not only provide protection against infectious diseasesbut also against odor, staining, deterioration and allergies. Advances in nanotechnologiescould make dirt-free (or no-wash) surfaces a reality. This would improve the environmentthrough reduced use of water, energy and petroleum-derived detergents.

Having been an active researcher in self-cleaning nanotechnology since 2002, witnessinga rapidly growing interest in the field of self-cleaning coatings, surfaces and materials fromthe media, industry, and academia, I felt a compelling need for a book that describes therecent developments and provides a timely account of this topic.

Following an invitation from Wiley, I have approached fellow researchers from acrossthe globe, renowned experts in the field, to contribute to this book with their fascinatingachievements covering all areas from the basic and fundamental knowledge of the concepts,potential applications, and recent and future development of self-cleaning nanotechnolo-gies, to their potential hazards and environmental impact.

The book is divided into four parts, starting with the general concepts of self-cleaningmechanisms covering both hydrophobic and hydrophilic surfaces. This is followed byspecific applications of self-cleaning surfaces and coatings, such as cementitious materials,glasses, clay roof tiles, textiles and plastics. The third part describes recent achievementsin self-cleaning surfaces, using advanced materials and technologies, such as liquid flamespray, pulsed laser deposition, and layer-by-layer assembly. In the last part, the potentialhazards, environmental impact, and limitations of self-cleaning surfaces are discussedtoward further development.

Many aspects of this book can be used for general public education, further research anddevelopment, as well as in the curriculum development of courses in the areas of materialsscience and engineering, nanotechnology, and textile finishing.

I would like to take this opportunity to express my sincere gratitude to all the authors, myPhD student, Dr Wing Sze Tung, and my research assistant, Ms Stephanie Kung. Specialthanks are also due to Wiley editorial staff, Ms Emma Strickland, Ms Sarah Tilley, and theediting team.

Walid A. Daoud

Part IConcepts of Self-Cleaning Surfaces

1Superhydrophobicity and

Self-Cleaning

Paul Roach1 and Neil Shirtcliffe2

1Institute for Science and Technology in Medicine, Guy Hilton Research Centre,Keele University, UK

2Faculty of Technology and Bionics, Hochschule Rhein-Waal, Germany

One of the ways that surfaces can be self-cleaning is by repelling water so effectively thatwater-borne contaminants cannot attach – by being superhydrophobic. This is demonstratedparticularly well by the Indian Lotus, Nelumbo nucifera, which has leaves that remain cleanin muddy water. The leaves can be cleaned of most things by drops of water, an effect thathas been patented and used in technical systems [1].

1.1 Superhydrophobicity

1.1.1 Introducing Superhydrophobicity

Superhydrophobicity is where a surface repels water more effectively than any flat surface,including one of PTFE (Teflon R©). This is possible if the surface of a hydrophobic solidis roughened; the liquid/solid interfacial area is increased and the surface energy costincreases. If the roughness is made very large, water drops bounce off the surface and it canbecome self-cleaning when it is periodically wetted. To understand more about this type ofself-cleaning it is necessary to consider how normal surfaces become wetted and becomedirty. The effect has been a focus of much recent research and has been reviewed recently [2–7]. A good mathematical explanation can be found in a recent book chapter by Extrand [8].

Self-Cleaning Materials and Surfaces: A Nanotechnology Approach, First Edition. Edited by Walid A. Daoud.© 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

4 Self-Cleaning Materials and Surfaces: A Nanotechnology Approach

Figure 1.1 Cross-section of a drop on a flat surface with the contact angle θ . Contact anglesalso form at the edge of larger pools of water, in tubes, at bubbles on underwater surfaces andany other configuration where a liquid interface meets a solid.

1.1.2 Contact Angles and Wetting

When a liquid rests on a surface the “contact angle” is measured through the dropletbetween the solid/liquid and liquid/air interfaces. The equilibrium angle that forms isknown as Young’s angle after a theory proposed by Young, but not actually formulated inhis work [9]. Young’s equation can be considered as a force balance of lateral forces on acontact line. In a perfect system the contact line cannot sustain any lateral force, so willalways move to a position where the forces balance. This is achieved mathematically bytaking the components of each force in the plane of the surface, at right angles to the contactline, as shown in Figure 1.1.

γSG = γSL + γLG cos θ (1.1)

where γ is the interfacial tension and the subscripts refer to solid, liquid and gas, forexample, γ SL is the interfacial tension between solid and liquid.

Young’s equation can also be derived from the surface and interfacial energies and theirchanges. The contact angle is an important measure of the interaction between the threephases, one solid, a liquid and another fluid, which may be a liquid or a gas. For small dropson a flat surface the drops form spherical caps, spheres intersecting the surface. Externalfactors, such as electric fields, may also influence the drop shape, with gravity playing arole in distorting larger droplets. At the contact line the angle tends to the Young angleexcept when the contact line is moving relatively rapidly. In most systems there is a certainuncertainty in contact angle known as contact angle hysteresis.

1.1.3 Contact Angle Hysteresis

In practice the equilibrium angle is often difficult to measure because there are a smallrange of angles on every surface that are stable. These are often described as local energyminima close to the global energy minimum. In practice the contact line therefore oftenbehaves as though it were fixed over a small range of angles close to the equilibrium angle[10]. Traditionally, the equilibrium contact angle was approached by vibrating the surfaceto supply the energy for the drop to escape the local minima. Although the static angle can

Superhydrophobicity and Self-Cleaning 5

Figure 1.2 A drop on a vertical surface sliding slowly with advancing angle at the front andreceding angle at the back, in practice geometrical factors and speed of movement will changethe angles away from the actual advancing and receding angles.

vary, the contact line begins to move at a certain angle when the liquid front is advanced andat a different angle when it recedes. These values are simpler to measure so it is often thegreatest stable angle and the lowest stable angle that are measured, known as the advancingand receding angles. The angles commonly quoted are those measured at a very low speedas the measured angles are affected by the speed of motion of the contact line. This isusually carried out by injecting liquid slowly into a drop and removing it again. Oftenthe advancing and receding angles are of more practical use than the equilibrium angle,although the equilibrium value can be used to derive surface energies. It is sometimespossible to determine the equilibrium angle if both advancing and receding angles aremeasured. This still assumes that hysteresis is not very large and the surface is reasonablyflat [11].

The difference between the advancing and receding angles, or rather the differencebetween the cosines of the angles governs whether liquids will stick to a surface or slideor fall off. A drop on a vertical sheet can have the advancing angle at the bottom and thereceding angle at the top without moving (Figure 1.2). Surfaces with low hysteresis allowdrops to slide over them whatever the equilibrium contact angle. The energy required for adrop to move can be calculated as [12],

F = 1

2πrγLG(cos θrec − cos θadv) (1.2)

6 Self-Cleaning Materials and Surfaces: A Nanotechnology Approach

where r is the base radius of the drop. The contact angle itself enters the equation in twoways: first the cosine function enhances differences near 90◦; secondly the value of thecontact radius r, for a given volume depends upon the contact angle.

Furmidge calculated the angle of tilt, α, required in order for a drop to slide [13],mg sin α

w= γLG (cos θrec − cos θadv) (1.3)

where w is the width of the drop.Measurement of the force required to remove drops from surfaces and tilting angles

shows the general trend is correct but some differences can be measured, particularly forsofter surfaces. Going back to Young’s equation, if the force balance approach is used, thesurface tension components in the plane of the solid are balanced to give the contact angle,but this leaves a vertical force on the surface, depending upon contact angle. Theories by deGennes and Shanahan [14] and experiments on soft materials suggest that this force distortsthe surface, generating a ring like an atoll around the base of the drop and increasing theforce restraining the drop from sliding on the surface. Of course the drop profile is alsofar from a circle if hysteresis is significant, particularly for large drops (for example thatshown in Figure 1.2).

The receding angle (and liquid properties) controls whether a drop falls off an invertedsurface, the advancing angle is not involved as it is never reached in this case.

The work needed to pull a liquid from a surface has been reported to be determinedby [15, 16].

W = γLG (1 + cos θR) (1.4)

1.1.4 The Effect of Roughness on Contact Angles

1.1.4.1 Fully Wet Surfaces; Wenzel’s Equation

As the roughness is increased the water initially wets the entire surface, as shown inFigure 1.3b, the increasing surface area of the interface means that the advancing contactangle on a surface with a flat contact angle of greater than 90◦ increases, whereas that ofone below 90◦ decreases. A surface with exactly 90◦ contact angle would show no effect ofroughness. This type of wetting can, therefore, be considered to be an amplification of theproperties of the surface by the roughness. The contact angle of a rough surface of this typecan be calculated using Wenzel’s equation [17], which modifies the cosine of the angle bythe specific surface area, r , the amount of times the surface is larger than a flat surface of thesame size. The subscript e has been used to highlight that usually the equilibrium contactangle is considered as opposed to the receding or advancing contact angles introduced inSection 1.1.3.

cos θrough = r cos θe (1.5)

The amplification of both hydrophilicity and hydrophobicity arises from the change insign of cosθ at 90◦.

1.1.4.2 Bridging the Roughness; Cassie and Baxter’s Equation

If the surface is roughened it eventually becomes energetically favourable for the liquid tosit on the top of the roughness and reduce the area of the interface, as shown in Figure 1.3c.

Superhydrophobicity and Self-Cleaning 7

Figure 1.3 Wetting on flat and rough surfaces: (a) flat, (b) rough, Wenzel case; (c) Cassieand Baxter case.

In this case the state approaches that of a liquid on a flat surface with domains of differentcontact angles but where one of the materials is the second fluid (in this example air).

The simplest expression for the contact angle on a surface of this type was formulatedin 1944 by Cassie and Baxter [18]. This considers the cosine of the angle to be the mean ofthe cosines of both contributing surfaces weighted by their relative areas, denoted by f, thefraction of the interface that is solid.

cos θrough = f cos θe + ( f − 1) (1.6)

This equation considers both the solid/liquid and the liquid/gas interfaces to be planar,which is only the case if the surface consists of equal height flat-topped pillars. Theoriginal Cassie and Baxter paper allowed for deviations from this by effectively usingWenzel’s equation for the wetted part and allowing changes in the effective roughness withpenetration. The main problem with this approach is that it is often difficult to determinewhere the liquid/solid interface lies.

8 Self-Cleaning Materials and Surfaces: A Nanotechnology Approach

1.1.5 Where the Equations Come From

Both Wenzel and Cassie–Baxter equations can be derived from forces at the contact line orfrom interfacial areas. Using the interfacial areas effectively considers a minimisation ofthe surface energy of the system. A force balance argument is equivalent, but considers thesurface energy from the forces it generates and creates conceptual difficulties when sharpcorners are considered [19].

Because of the increased interfacial area in the Wenzel case and the decrease in interfacialarea in the Cassie–Baxter case the hysteresis observed increases in the Wenzel state anddecreases in the Cassie–Baxter state, giving rise to low water adhesion in the Cassie–Baxterstate [20].

1.1.5.1 Flat Surfaces

Consider a liquid on a surface with a contact line at a contact angle; if we allow this line tomove by an infinitesimal amount and assume that it will move in this manner until it reachesan energy minimum the energy minimum can be defined as the position where moving thecontact line by a small amount does not change the interfacial energy. This does assumethat there is a single minimum in the energy profile – a reasonable assumption for a flatsurface.

The energy change for moving forward a small amount is illustrated in Figure 1.4. Thearea of the liquid/fluid interface changes by �A cos θ , the solid liquid interface changesarea by �A and replaces or is replaced by the same amount of solid surface (depending onthe direction of motion). The total change in surface free energy, �F , accompanying anadvance of the contact line is therefore,

�F = (γSL − γSG) �A + �AγLG cos θ (1.7)

If we set the change in free energy to zero we will find the minimum or maximum ofthe expression, in this case because we are starting close to the global minimum we willapproach that. The result can be rearranged to form Young’s equation.

Figure 1.4 Contact angle and surface free energy.

Superhydrophobicity and Self-Cleaning 9

Figure 1.5 Wenzel wetting.

On a flat surface this treatment is equivalent to a force balance, but on rough surfaces thissurface free energy treatment averages over a period of the roughness or a representativearea. Unlike a force balance there are no difficulties when the contact line meets the cornerof a feature and the intrinsic assumption that the contact line is always on a representativeproportion of the surface is slightly more obvious. In cases where this is not true, for patternsthat are large compared with the size of the drop, when the contact line can sit on one partof the pattern or when the pattern is anisotropic (e.g., parallel grooves) the approach cannotbe applied without some modification.

1.1.5.2 Wenzel Case

For a rough surface where the liquid wets into the rough features (Figure 1.5), the treatment isthe same as the flat surface but the surface areas of both the solid/liquid and the solid/vapourinterfaces associated with the advance of the contact line are increased by a factor, r, thespecific surface area of the rough surface at the contact line. In other words the number oftimes larger the area is than if it were flat. The roughness factor compares the rough surfaceto a two-dimensional surface of the same size and is, therefore, better served by this surfaceenergy treatment. When the new values of the surface energies are treated in the same wayas before the following expressions result,

�F = (γSL − γSG) r�A + �AγLG cos θ (1.8)

If �F = 0 then cos θ = (γSL − γSG)r�A

�AγLG(1.9)

This can be substituted into Young’s equation to give Wenzel’s equation.

1.1.5.3 Cassie–Baxter Case

To consider only bridging wetting we can imagine flat-topped pillars with water bridg-ing the gaps between with horizontal menisci, as shown in Figure 1.6. In this particular

10 Self-Cleaning Materials and Surfaces: A Nanotechnology Approach

Figure 1.6 The Cassie and Baxter case.

configuration the surface area of the base of the water is the same as it would be on a flatsurface.

Again the air/liquid interface at the top of the drop is unaffected by the roughness, thelower part advances over a combination of fluid (air) and solid, the interfacial area, A, canbe divided into two components and these assigned to the solid or the fluid interface. Theproportions of these two components are determined by the shape of the surface, in thiscase the relative areas of the tops of the pillars to the gaps.

The surface free energy can be minimised as before giving:

�F = (γSL − γSG) f �A + (1 − f ) �AγLG + γLG�A cos θ (1.10)

and, again, with substitution into Young’s equation it becomes reduced to the form of Cassieand Baxter’s equation;

cos θrough = f cos θe + ( f − 1) (1.11)

It can be seen that the observed contact angle on this type of surface is intermediatebetween the liquid/solid contact angle and the liquid/fluid contact angle. If the second fluidis air or another gas the contact angle will always increase, even if the surface is hydrophilic.The reverse situation can be imagined where the pores at the surface are pre-filled with thesame liquid as the drop, in this case the contact angle will decrease, even if the surfaceis repellent to the liquid. On hydrophilic surfaces this situation can arise when a film ofliquid spreads through the roughness of the surface before the macroscopic drop spreads.The same equation can be used for flat surfaces with areas of different contact angle aslong as they are distributed well. As mentioned above the original Cassie and Baxter paperconsidered the combined effect of these two situations.

Superhydrophobicity and Self-Cleaning 11

1.1.5.4 Important Considerations

There has been some criticism of these equations, but these can also be interpreted ascriticism of their misuse [21–24]. Both equations require a set of assumptions to be true(or at least locally true) for them to apply.

First, there is a requirement that the pattern of roughness or chemistry is arranged so thatthe contact line is always on the average of all parts of the structures. This is implicit in thetreatments above where always an entire cycle of roughness (or chemical pattern) is taken.This requirement is broken if the pattern allows the contact line to arrange itself so that itis mostly on one type of the surface. This is particularly evident in grooved surfaces wherethe contact angles parallel and perpendicular to the grooves are different. Perpendicular tothe grooves the expected angles form, whilst parallel to the groove direction a cyclic changeis observed as the contact line moves over the peaks and troughs. Similar problems arisefrom other pattern geometries. Another way this requirement can be broken is if the size ofthe patterned features becomes large enough such that the contact line bends to reduce theinterfacial energy of the liquid.

κ−1 =√

γLG

ρg(1.12)

The capillary length (Eq. 1.12) describes the general size where gravity will have a largereffect than surface tension on a drop of liquid. As can be seen the quantities comparedare the surface tension (γLG) and the effect of gravity on the liquid through density(ρ) andgravity (g). For water on our planet this critical length is 2.73 mm; drops of radius muchsmaller than this, typically a tenth of this size, are almost spherical. In the same way themeniscus bridging two features will be distorted by gravity and this can be considered tobecome significant as the gaps reach a tenth of the capillary length. Structures larger thanthis can distort the contact line as they influence it via interfacial tension.

Secondly, as the thought experiment that generates the equations considers small move-ments from the equilibrium position the state of a liquid is only determined by the surfacenear the contact line. This is a long-winded way of stating that a drop on a hydrophobicsurface will not spontaneously jump to a hydrophilic surface unless the contact line inter-sects both surfaces. It means that the solid/liquid interfacial area under the drop but awayfrom the contact line is largely irrelevant when determining the contact angle, but if thereare differences these will be revealed if the contact line moves over the surface – if a dropslides over the surface for example.

1.1.6 Which State Does a Drop Move Into?

As the Wenzel type of wetting is very different from Cassie and Baxter bridging wetting itis important to know which surface will end up in which state. Initial attempts to predictwhich state a surface would go into from Cassie–Baxter and Wenzel’s equations met withmixed success. Even when the surface allows this type of comparison it is only possiblefrom the equations above to find which of the states has the lowest energy minimum.Some theoretical treatments of the transition do exist and have shown success predictingexperiments. In the simplest the energy levels of both states and those of intermediatestates are calculated to determine which states are lowest in energy. More complicatedones attempt to discover when a water drop on the surface can become trapped in one

12 Self-Cleaning Materials and Surfaces: A Nanotechnology Approach

state or the other. In many experimental cases the small-scale roughness of the surface isdifficult to measure, preventing this type of detailed calculation [25–27]. If a drop is placedonto the surface it is likely to start in the Cassie–Baxer state and may become trappedthere even if the Wenzel state has a lower energy. Conversely, if water condenses onto asuperhydrophobic surface it initially wets inside the roughness so generally starts in theWenzel state and almost always becomes trapped there [28].

1.2 Self-Cleaning on Superhydrophobic Surfaces

1.2.1 Mechanisms of Self-Cleaning on Superhydrophobic Surfaces

Self-cleaning superhydrophobic surfaces first received attention when a paper was publishedon the Lotus leaf [29]. Lotus leaves remain clean in muddy water because of the way theirsurfaces are structured and water repellent. The leaves are strongly superhydrophobic and,although they collect particles of dust, they are fully cleaned by rain.

One of the mechanisms for self-cleaning, and that initially suggested for the lotus leaf,depends on how the water drop moves. A drop on a surface with high contact angle and lowcontact angle hysteresis, usually a bridging super-hydrophobic surface, can roll instead ofsliding. This type of motion allows the drop to collect more of the particles at the surfaceof the solid compared to the usual sliding mechanism.

The question that then arises is why a rolling drop should collect hydrophobic particlesfrom a superhydrophobic surface.

Particles, even hydrophobic ones, are strongly attached to a liquid/gas interface. If theparticle is modelled as a sphere its lowest energy configuration is when it is located in theinterface; partially immersed so that the local contact angle can be the equilibrium contactangle. The energy of attachment of a particle on a liquid interface can be calculated bycomparing the surface energies of three possibilities, the particle away from the liquid, theparticle at its equilibrium position in the interface and the particle inside the liquid. For ahydrophobic particle and water the third case will not be the lowest in energy so we canconsider the energy change from the particle resting in air to being held at the interface.

When a spherical particle of radius R and contact angle θe attaches to a liquid interfacethe angle between the surfaces is the contact angle (Figure 1.7). The area of the sphere thatbecomes wetted can be described by Eq. (1.13), this is both the solid/gas interface that islost and the solid/liquid interface gained. The liquid also loses some interface, the circular

Figure 1.7 A hydrophobic, spherical particle moving from the air to a position in a waterinterface where it has its contact angle with the liquid.