Coalescence-induced jumping of nanoscale droplets on super-hydrophobic surfaces

Liang, Zhi; Keblinski, Pawel

doi:10.1063/1.4932648

Cited by 63 publications

(99 citation statements)

References 22 publications

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“…5−8 However, experimental and numerical studies have demonstrated that the coalescence of smaller droplets (<10 μm, even down to nanometer size) could cause the droplets to jump away from the surfaces. 4,33−35 Recent studies have provided strong evidence that viscous dissipation is not important at length scales larger than 1 μm. 4,35,36 Instead, finite surface adhesion and unequal momentum flow in the vertical direction govern and limit the droplet jumping.…”

Section: Resultsmentioning

confidence: 99%

Characterization of Coalescence-Induced Droplet Jumping Height on Hierarchical Superhydrophobic Surfaces

Weibel

Garimella

2017

ACS Omega

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Coalescence-induced condensate droplet jumping from superhydrophobic surfaces can be exploited in condensation heat-transfer enhancement, imparting self-cleaning behavior to surfaces, anti-icing coatings, and other industrial uses. An intriguing application would exploit this phenomenon to achieve thermal rectification using a sealed vapor chamber with opposing superhydrophilic and superhydrophobic surfaces. During forward operation, continuous evaporation occurs from the heated superhydrophilic evaporator surface; self-propelled jumping returns the condensate droplets from the cooler superhydrophobic surface to the evaporator, allowing passive recirculation of the working fluid without a reliance on gravity or capillary wicking for fluid return. In reverse operation, when the superhydrophobic surface is heated, the absence of a mechanism for fluid return from the cooler superhydrophilic side to the evaporator restricts heat transport across the (low conductivity) vapor gap. The effectiveness of this jumping-droplet thermal diode can be enhanced by maximizing the distance between the opposing superhydrophobic and superhydrophilic surfaces. It is, therefore, important to investigate the height to which the condensate droplets jump from the superhydrophobic surfaces such that replenishment of liquid to the superhydrophilic surface is maintained. In this study, we systematically investigate the height to which the condensate droplets jump from the hierarchical superhydrophobic surfaces having truncated microcones coated with nanostructures. The condensate droplet jumping height increases with a decrease in microcone pitch. A general theoretical model that accounts for surface adhesion, line tension, and initial wetting states of multiple coalescing droplets of different radii is used to predict and explain the experimentally observed trend of droplet jumping height with surface roughness. The insights provided regarding the effect of surface topography on droplet jumping height are critical to the design of high-performance thermal diode devices.

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Section: Resultsmentioning

confidence: 99%

Characterization of Coalescence-Induced Droplet Jumping Height on Hierarchical Superhydrophobic Surfaces

Weibel

Garimella

2017

ACS Omega

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“…Additionally, the method of distinguishing the phase based on potential energy has a lower computational cost than the density criterion and avoids the undesirable dependency on the size of the subdomain needed for density calculation. It has also been successfully used in study of coalescence of nanoscale droplets on a solid surface [40]. After 2,000 ps, the system reaches a steady state and a spherical-shaped liquid Ar droplet is formed in the center of the simulation box.…”

Section: B the MD Modelmentioning

confidence: 99%

Continuum and molecular-dynamics simulation of nanodroplet collisions

et al. 2016

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The extent to which the continuum treatment holds in binary droplet collisions is examined in the present work by using a continuum-based implicit surface capturing strategy (volume-of-fluid coupled to Navier-Stokes) and a molecular dynamics methodology. The droplet pairs are arranged in a head-on-collision configuration with an initial separation distance of 5.3 nm and a velocity of 3 ms^{-1}. The size of droplets ranges from 10-50 nm. Inspecting the results, the collision process can be described as consisting of two periods: a preimpact phase that ends with the initial contact of both droplets, and a postimpact phase characterized by the merging, deformation, and coalescence of the droplets. The largest difference between the continuum and molecular dynamics (MD) predictions is observed in the preimpact period, where the continuum-based viscous and pressure drag forces significantly overestimate the MD predictions. Due to large value of Knudsen number in the gas (Kn_{gas}=1.972), this behavior is expected. Besides the differences between continuum and MD, it is also observed that the continuum simulations do not converge for the set of grid sizes considered. This is shown to be directly related to the initial velocity profile and the minute size of the nanodroplets. For instance, for micrometer-size droplets, this numerical sensitivity is not an issue. During the postimpact period, both MD and continuum-based simulations are strikingly similar, with only a moderate difference in the peak kinetic energy recorded during the collision process. With values for the Knudsen number in the liquid (Kn_{liquid}=0.01 for D=36nm) much closer to the continuum regime, this behavior is expected. The 50 nm droplet case is sufficiently large to be predicted reasonably well with the continuum treatment. However, for droplets smaller than approximately 36 nm, the departure from continuum behavior becomes noticeably pronounced, and becomes drastically different for the 10 nm droplets.

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“…In recent years, coalescence-induced droplet jumping on a superhydrophobic surface has received more and more attention [1][2][3][4][5][6][7][8][9][10][11], because it has been applied to various fields, including condensation heat transfer [12,13], self-cleaning [14], anti-icing [15,16], anti-dew [17] and so forth. However, the jumping velocity affects the efficiency of these applications.…”

Section: Introductionmentioning

confidence: 99%

Investigation of Coalescence-Induced Droplet Jumping on Mixed-Wettability Superhydrophobic Surfaces

Liao

Duan

2021

Processes

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Coalescence-induced droplet jumping has received more attention recently, because of its potential applications in condensation heat transfer enhancement, anti-icing and self-cleaning, etc. In this paper, the molecular dynamics simulation method is applied to study the coalescence-induced jumping of two nanodroplets with equal size on the surfaces of periodic strip-like wettability patterns. The results show that the strip width, contact angle and relative position of the center of two droplets are all related to the jumping velocity, and the jumping velocity on the mixed-wettability superhydrophobic surfaces can exceed the one on the perfect surface with a 180° contact angle on appropriately designed surfaces. Moreover, the larger both the strip width and the difference of wettability are, the higher the jumping velocity is, and when the width of the hydrophilic strip is fixed, the jumping velocity becomes larger with the increase of the width of the hydrophobic strip, which is contrary to the trend of fixing the width of the hydrophobic strip and altering the other strip width.

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Coalescence-induced jumping of nanoscale droplets on super-hydrophobic surfaces

Cited by 63 publications

References 22 publications

Characterization of Coalescence-Induced Droplet Jumping Height on Hierarchical Superhydrophobic Surfaces

Characterization of Coalescence-Induced Droplet Jumping Height on Hierarchical Superhydrophobic Surfaces

Continuum and molecular-dynamics simulation of nanodroplet collisions

Investigation of Coalescence-Induced Droplet Jumping on Mixed-Wettability Superhydrophobic Surfaces

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