1 Introduction 1
1.1 Research Background and Proposal of Topics 1
1.1.1 Condensation 1
1.1.2 Frosting and Icing 2
1.1.3 Proposal of Topics 3
1.2 Research Status 4
1.2.1 Fabrication of Superhydrophobic Surfaces 5
1.2.2 Condensation and Droplet Behaviors on
Superhydrophobic Surfaces 8
1.2.3 Frost/Ice Melting and Droplet Behaviors
on Superhydrophobic Surfaces 15
1.2.4 Summary of Research Status 17
1.3 Research Contents of Present Work 18
References 19
2 Experimental System and Superhydrophobic Surfaces 27
2.1 Experimental System and Data Processing 27
2.1.1 Overview of Experimental System 27
2.1.2 Data Processing Methods 30
2.2 Fabrication and Characterization of Superhydrophobic Surfaces . 31
2.2.1 Fabrication Methods of Superhydrophobic Surfaces 31
2.2.2 Al-Based Superhydrophobic Surfaces 32
2.2.3 Cu-Based Superhydrophobic Surfaces 36
2.3 Selection of Superhydrophobic Surfaces for Experiments 39
2.4 Summary 40
References . 41 3 Behaviors of Condensed Droplets on Superhydrophobic Surfaces . 43 3.1 Experimental Surfaces and Conditions . 43 3.2 Condensed Droplet Behaviors on Superhydrophobic Surfaces 44 3.2.1 Immobile Droplet Coalescence . 44
XVI Contents
3.2.2 Self-propelled Droplet Jumping 45
3.2.3 Self-propelled Droplet Sweeping 46
3.3 Statistics of Condensed Droplet Behaviors on Superhydrophobic
Surfaces 49
3.4 Critical Conditions for Self-propelled Droplet Behaviors 51
3.4.1 Theoretical Model 52
3.4.2 Minimum Critical Droplet Radius 54
3.4.3 Critical Ratio of Droplet Radius 56
3.4.4 Critical Static Contact Angle 57
3.5 Effect of Self-propelled Droplet Behaviors on Droplet Growth . 58
3.5.1 Droplet Diameter Distribution 58
3.5.2 Average Droplet Diameter 61
3.5.3 Surface Coverage Fractions 62
3.5.4 Effects of Working Conditions 63
3.6 Summary 64
References 65
4 Numerical Simulations of Multi-droplet Coalescence-Induced
Jumping 67
4.1 Simulation Objects and Conditions 68
4.2 Mathematical Model 69
4.2.1 Control Equation 69
4.2.2 Computational Domain， Boundary Conditions，
and Grids 71
4.2.3 Energy Analysis 72
4.3 Model Validation—Two-Droplet Coalescence-Induced Jumping . 73
4.4 Multi-droplet Coalescence-Induced Droplet Jumping 76
4.4.1 Effect of Coalesced Droplet Number 76
4.4.2 Effect of Droplet Position Distribution 81
4.5 Summary 87
References 87
5 Dynamic Melting of Freezing Droplets on Superhydrophobic
Surfaces 89
5.1 Experimental Surfaces and Conditions 89
5.2 Freezing of Condensed Droplets on Superhydrophobic Surfaces . 91
5.3 Self-propelled Behaviors During Melting Process of Freezing
Droplets 94
5.3.1 Melting Droplet Rotating 94
5.3.2 Melting Droplet Jumping 96
5.3.3 Melting Droplet Sliding 97
5.4 Effects of Self-propelled Melting Droplet Behaviors on Surface Coverage Fraction 99
5.5 Summary of This Chapter 101
References 102
Contents XVII
6 Meltwater Evolution During Defrosting on Superhydrophobic
Surfaces 105
6.1 Experimental Surfaces and Conditions 105
6.2 Meltwater Evolution on Superhydrophobic Surfaces 107
6.3 Edge Curling Phenomenon of Meltwater Films 109
6.4 Non-breaking Phenomenon of Chained Droplets 110
6.5 Summary 113
References 114
7 Relation Between Surface Wettability and Droplet Behaviors，
and Hysteresis Number 117
7.1 Morphologies and Behaviors of Condensed Droplets and Melted
Droplets 117
7.1.1 Morphologies and Behaviors of Condensed Droplets 117
7.1.2 Morphologies and Behaviors of Melted Droplets 120
7.2 Relation Between Surface Wettability and Droplet Behaviors 122
7.3 Hysteresis Number 126
7.4 Summary 129
References 130
8 Conclusions and Outlooks 133
8.1 Main Conclusions in the Present Work 133
8.2 Innovations in the Present Work 136
8.3 Outlooks for Future Research 137
內容試閱：
Supervisor’s Foreword
With the development and maturity of nanotechnology and material science， the research and application of superhydrophobic surfaces have received numerous attentions， especially in recent years. Due to the great application potential in the .elds of condensation heat transfer enhancement， ice and frost suppression， power device and chip cooling， dehumidi.cation， mist water collection， self-cleaning， seawater desalination， droplet transport， etc.， the self-propelled droplet movements on superhydrophobic surfaces have become a frontier and hot issue in international scienti.c research. For example， the condensed droplet jumping phenomenon induced by coalescence on superhydrophobic surfaces further enhances the con-densation heat transfer on the shoulders of the hydrophobic surfaces (compared to the .lm condensation on hydrophilic surfaces， the heat transfer coef.cient of dro-plet condensation on hydrophobic surfaces has already been improved by one to two orders of magnitude due to the ef.cient gravity-driven droplet removal mechanism)， as the droplet jumping is able to self-clean microscale droplets， exposes large area of clean surface and achieves continuous condensation. The self-propelled droplet jumping phenomenon on superhydrophobic surfaces also has huge potential in suppressing the condensation frosting. Generally， the condensa-tion frosting undergoes stages including condensation nucleation， droplet growth， droplet coalescence， ice nucleation， droplet freezing and freezing propagation. While the droplet jumping phenomenon makes it possible that the supercooled droplets depart from the surface before ice nucleation， it also increases the distance between adjacent droplets， resulting in a reduced freezing propagation velocity. In addition to the condensed droplets， the melting droplets on superhydrophobic surfaces present various self-propelled movements similarly， such as the droplet rotating， the droplet jumping and the droplet sliding. All these dynamic movements facilitate the removal of meltwater， which improves the comprehensive anti-frosting performance of the superhydrophobic surface.
Under the background of the application of superhydrophobic surfaces in the .elds of condensation enhancement， anti-frosting， hot-spot cooling in electronic chips， dehumidi.cation， water harvesting， self-cleaning， seawater desalination， etc.， the present thesis focuses on the general fundamental problem: the dynamic droplet
Supervisor’s Foreword
behaviors on the superhydrophobic surface and their physical mechanisms. The key point of this problem is actually the correlation between the droplet behavior and the property of the superhydrophobic surface， mainly the micro-nanostructure and the wettability， i.e.， the contact angle and its hysteresis. It is this issue which limits the further development and practical application of superhydrophobic surfaces. The present thesis devotes to solve this problem， in other words， characterizes the dynamic droplet behaviors on superhydrophobic surfaces and reveals their physical mechanisms， and clari.es the relationship between the droplet behavior and the property of the superhydrophobic surface. The present thesis involves multidisci-plinary theory， such as .uid mechanics， thermodynamics， transformation kinetics， surface and interface science， heat and mass transfer rule， nanotechnology and material science. From the point of science， the research results in the present thesis are expected to expand the scienti.c frontier and promote the formation and development of new academic growth points among interdisciplinary subjects. From the aspect of engineering application， the research results in the present thesis could promote the application of superhydrophobic surfaces in multi-industrial .elds， provide guidance for the development of new technologies such as condensation enhancement， anti-frosting， defrosting， anti-corrosion， anti-microbial and .ow drag reduction， and .nally contribute to the sustainable development of the national economy and society.
Prof. Xiaomin Wu Beijing， China September 2020
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