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Dynamics of Non-axisymmetric Droplets: Impact Characteristics on a Solid Surface and Control of Rebound

윤성찬 (Sungchan Yun, 포항공과대학교)

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초록 moremore
The control of drop deposition on a solid surface is an essential issue in numerous industrial and scientific applications, such as inkjet printing, pesticide spraying, spray coating, and spray cooling. In these applications, the efficient deposition of drops with low viscosity and high surface tens...
The control of drop deposition on a solid surface is an essential issue in numerous industrial and scientific applications, such as inkjet printing, pesticide spraying, spray coating, and spray cooling. In these applications, the efficient deposition of drops with low viscosity and high surface tension, such as water drops, on non-wetting surfaces is challenging because the drops retract violently after impact and rebound off the surfaces. Drop rebound may lead to material loss, increased spraying costs, or environmental pollution due to toxic substances. Several methods have been proposed to improve drop deposition and inhibit rebound, including modifying the solid surface and adding surfactant, polymer additives, or particles. However, the controlling ability of these methods might be limited when a combination of solid and drop liquid is given. In this thesis, I report that non-axisymmetric drop shapes can significantly alter impact behavior and successfully suppress rebound. First, I demonstrate that non-axisymmetrically shaped drops strongly affect the impacting behavior and subsequently suppress rebound on a hydrophobic substrate. The drop can be produced by the non-axisymmetrical electric field between a nozzle and a newly designed ring electrode in a typical electrospray device. Experimental study shows that drops non-axisymmetrically spread, retract and then initiate axis switching, thereby leading to rebound suppression, where axis switching denotes alternate expansion and contraction along the principal axes during retraction. The rebound suppression is explained by a kinetic energy (KE) transfer between the horizontal principal axes rather than the vertical axis, which is rationalized by using the numerical simulation. In addition, the effects of the impact velocity and electric charge on impact dynamics are discussed. Second, I focus on ellipsoidal drop impact on a hydrophobic surface and investigate effects of the geometric aspect ratio (AR) and Weber number (We) of drops on the dynamics and outcomes of impacts both experimentally and numerically. The non-axisymmetrical spreading features are characterized by scrutinizing the maximal extensions along the principal axes. The ratio of maximal extensions depends strongly on AR, which agrees with the scaling relation derived in this work. Experimental and numerical studies show that an increasing AR induces a high degree of the axis switching, which can account for rebound suppression. I determine the transition between rebound and deposition (rebound suppression) over AR and We domains and discuss the transition based on a non-axial distribution of KE. In addition, the non-axisymmetrical retraction dynamics and effects of the shape oscillation on impact dynamics are discussed. Third, I present a method to control the bouncing magnitude of Leidenfrost drops on heated surfaces with ellipsoidal shaping. Experimental and numerical studies are used to investigate the effects of AR and We on bouncing dynamics, which shows that maximum bounce heights of ellipsoidal drops can be reduced below spherical cases to nearly 40%. The control of bounce height can be explained in terms of a non-axial KE distribution during retraction. Interestingly, the non-axisymmetric hydrodynamics allows a reduction in contact time below this theoretical limit, which is explored both experimentally and numerically as a function of AR. This finding may provide an understanding of bouncing dynamics on non-wetting surfaces and suggests new avenues for self-cleaning and anti-icing strategies. I believe that non-axisymmetric drop impacts expand the dynamics to three-dimensions and can offer a simple, clean, and convenient pathway for the control of drop deposition without using additives or modifying solid surfaces. The controllable drop shape potentially provides a convenient parameter in various applications, including surface patterning, cleaning, and cooling.
목차 moremore
1. Introduction 1
1.1 Background 1
1.2 Organization of Ph. D. Thesis 5
...
1. Introduction 1
1.1 Background 1
1.2 Organization of Ph. D. Thesis 5
2. Non-axisymmetric drop generation and impact behavior on a solid surface 8
2.1 Introduction 8
2.2 Experimental setup 9
2.3 Results and discussion 10
2.3.1 Shape oscillation 10
2.3.2 Impact dynamics 11
2.4 Conclusion 15
3. Ellipsoidal drop impact on a solid surface and control of rebound 24
3.1 Introduction 24
3.2 Experimental setup 25
3.3 Numerical method 28
3.4 Results and discussion 29
3.4.1 Impact dynamics of ellipsoidal drops 29
3.4.2 Criteria for rebound suppression 35
3.5 Non-axisymmetrical retraction dynamics 39
3.6 Role of the oscillating phase on impact dynamics 40
3.6.1 The dominant effect of AR on impact behavior 40
3.6.2 Contracting drop vs. expanding drop just before impact 41
3.6.3 Oblate ellipsoidal drop vs. prolate ellipsoidal drop just before impact 41
3.7 Non-axial kinetic energy distribution for criteria of rebound suppression 42
3.7.1 Axis-switching time 42
3.7.2 Criteria of rebound suppression 42
3.8 Conclusion 43
4. Control of a bouncing magnitude of a Leidenfrost drop with ellipsoidal shaping 67
4.1 Introduction 67
4.2 Experimental setup and numerical method 69
4.3 Results and discussion 71
4.3.1 Bouncing dynamics of ellipsoidal drops 71
4.3.2 Mechanism of bounce control 74
4.3.3 Reduction in contact time 75
4.4 Conclusion 76
5. Concluding Remarks 86
References 89
Summary (in Korean) 99