Electrowetting

Starting from the basic principles of wetting, electrowetting and fluid dynamics all the way up to those engineering aspects relevant for the development of specific devices, this is a comprehensive introduction and overview of the theoretical and practical aspects. Written by two of the most knowledgeable experts in the field, the text covers both current as well as possible future applications, providing basic working principles of lab-on-a-chip devices and such optofluidic devices as adaptive lenses and optical switches. Furthermore, novel e-paper display technology, energy harvesting and supercapacitors as well as electrowetting in the nano-world are discussed. Finally, the book contains a series of exercises and questions for use in courses on microfluidics or electrowetting. With its all-encompassing scope, this book will equally serve the growing community of students and academic and industrial researchers as both an introduction and a standard reference.

Autorentext
Frieder Mugele is the head of the Physics of Complex Fluids group at the University of Twente in Enschede, The Netherlands. Having obtained his academic degrees in physics at the University of Konstanz, Germany, he spent several years at the University of California in Berkeley, USA, and the University of Ulm, Germany, before his present appointment in Twente. Professor Mugele's research focuses on various aspects of solid-liquid interfaces and the properties of liquids on the micro- and nanoscale. He has been active in electrowetting since the late 1990s contributing in particular to the theoretical understanding and to fundamental concepts of electrowetting-driven microfluidics.

Jason Heikenfeld is a Professor and Assistant Vice President for Commercialization at the Univ. of Cincinnati. He directs the Novel Devices Laboratory which has established highly-focused international leadership roles in an emergent technological paradigms including electrowetting, electronic paper, and most recently sweat biosensing technology. Prof. Heikenfeld's research approach centers on discovering and addressing the hidden challenges that can hinder the transition of innovative science into commercial application. Professor Heikenfeld is also a prolific inventor and serial entrepreneur, and during his teaching years was the highest-rated STEM educator at the University of Cincinnati.

Inhalt

Preface xi

1 Introduction to Capillarity and Wetting Phenomena 1

1.1 Surface Tension and Surface Free Energy 2

1.1.1 The Microscopic Origin of Surface Energies 2

1.1.2 Macroscopic Definition of Surface Energy and Surface Tension 5

1.2 YoungLaplace Equation: The Basic Law of Capillarity 7

1.2.1 Laplace's Equation and the Pressure Jump Across Liquid Surfaces 7

1.2.2 Applications of the YoungLaplace Equation: The RayleighPlateau Instability 11

1.3 YoungDupré Equation: The Basic Law of Wetting 13

1.3.1 To Spread or Not to Spread: From Solid Surface Tension to Liquid Spreading 13

1.3.2 Partial Wetting: The Young Equation 16

1.4 Wetting in the Presence of Gravity 19

1.4.1 Bond Number and Capillary Length 21

1.4.2 Case Studies 22

1.4.2.1 The Shape of a Liquid Puddle 22

1.4.2.2 The Pendant Drop Method: Measuring Surface Tension by Balancing Capillary and Gravity Forces 24

1.4.2.3 Capillary Rise 25

1.5 Variational Derivation of the YoungLaplace and the YoungDupré Equation 26

1.6 Wetting at the Nanoscale 29

1.6.1 The Effective Interface Potential 30

1.6.2 Case Studies 32

1.6.2.1 The Effective Interface Potential for van der Waals Interaction 32

1.6.2.2 Equilibrium Surface Profile Near the Three-Phase Contact Line 34

1.7 Wetting of Heterogeneous Surfaces 35

1.7.1 YoungLaplace and YoungDupré Equation for Heterogeneous Surfaces 35

1.7.2 Gibbs Criterion for Contact Line Pinning at Domain Boundaries 37

1.7.3 From Discrete Morphology Transitions to Contact Angle Hysteresis 38

1.7.4 Optimum Contact Angle on Heterogeneous Surfaces: The Laws of Wenzel and Cassie 43

1.7.5 Superhydrophobic Surfaces 45

1.7.6 Wetting of Heterogeneous Surfaces in Three Dimensions 48

1.7.7 Wetting of Complex Surfaces in Three Dimensions: Morphology Transitions, Instabilities, and Symmetry Breaking 50

1.A Mechanical Equilibrium and Stress Tensor 55

Problems 56

References 58

2 Electrostatics 61

2.1 Fundamental Laws of Electrostatics 61

2.1.1 Electric Fields and the Electrostatic Potential 61

2.1.2 Specific Examples 64

2.2 Materials in Electric Fields 66

2.2.1 Conductors 66

2.2.2 Dielectrics 68

2.2.3 Dielectric Liquids and Leaky Dielectrics 73

2.3 Electrostatic Energy 76

2.3.1 Energy of Charges, Conductors, and Electric Fields 76

2.3.2 Capacitance Coefficients and Capacitance 78

2.3.3 Thermodynamic Energy of Charged Systems: Constant Charge Versus Constant Potential 80

2.4 Electrostatic Stresses and Forces 82

2.4.1 Global Forces Acting on Rigid Bodies 82

2.4.2 Local Forces: The Maxwell Stress Tensor 83

2.4.3 Stress Boundary Condition at Interfaces 85

2.5 Two Generic Case Studies 87

2.5.1 Parallel Plate Capacitor 87

2.5.2 Charge and Energy Distribution for Two Capacitors in Series 90

Problems 92

References 93

3 Adsorption at Interfaces 95

3.1 Adsorption Equilibrium 96

3.1.1 General Principles 96

3.1.2 Langmuir Adsorption 96

3.1.3 Reduction of Surface Tension 99

3.2 Adsorption Kinetics 101

3.3 Surface-Active Solutes: From Surfactants to Polymers, Proteins, and Particles 105

3.A A StatisticalMechanics Model of Interfacial Adsorption 107

Problems 110

References 110

4 From Electric Double Layer Theory to Lippmann's Electrocapillary Equation 113

4.1 Electrocapillarity: the Historic Origins 113

4.2 The Electric Double Layer at SolidElectrolyte Interfaces 115

4.2.1 PoissonBoltzmann Theory and GouyChapman Model of the EDL 116

4.2.2 Total Charge and Capacitance of the Diffuse Layer 120

4.2.3 Voltage Dependence of the Free Energy: Electrowetting 122 4.3 Shortcomings o...