Self-Propelled Dropwise Condensate on Superhydrophobic Surfaces

Boreyko, Jonathan B.; Chen, Chuan‐Hua

doi:10.1103/physrevlett.103.184501

Cited by 1,134 publications

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“…Boreyko and Chen [55] of a self-jumping droplet by assuming that the surface energy (E s ), released via the coalescence of condensate microdrops, is fully converted into kinetic energy (E k ). However, the experimental velocities of the self-jumped microdrops are apparently smaller than their theoretical values.…”

Section: Cmdsp Mechanismmentioning

confidence: 99%

“…Different from traditional superhydrophobic surfaces, which are characterized by the bouncing or rolling off of deposited millimeter-size large drops, [32,33] CMDSP surfaces support the self-removal capability of smallscale condensate microdrops. It has been reported that classical superhydrophobic lotus leaves (Figure 1a), [34][35][36][37] as well as artificial surfaces consisting of hierarchical micro-and nanostructures, [38] one-tier microstructures, [39][40][41][42][43] or nanostructures [44,45] with larger characteristic interspaces, present a low-adhesivity property to the deposited water macrodrops, but become highly adhesive to condensed microdrops (Figure 1b). This is because moisture easily penetrates the microscale valleys or cavities.…”

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Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

2017

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interest has focused on utilizing the lowadhesivity nature of superhydrophobic surfaces for the rapid removal of condensate microdrops, as this has significance in fundamental research and technological innovation. Example applications are the enhancement of condensation heat transfer for high-efficiency thermal management and energy utilization, [20][21][22][23][24][25] energy-effective antifreezing for airconditioner heat exchangers and aircraft wings, [26][27][28] and electrostatic energy harvesting. [29] In general, condensate drops on typical flat hydrophobic surfaces are only shed off under gravity when their sizes are comparable to the capillary length (≈2.7 mm for water), [30] which creates undesirable effects such as large thermal resistance [20][21][22][23][24][25] and the freezing of subcooled drops. [26][27][28] Clearly, a great challenge is the timely removal of condensate drops at the microscale where the gravitational effect does not work. The latest research has indicated that these small-scale condensate microdrops may self-remove via mutual coalescence as long as the coalescence-released excess surface energy is larger than the interface adhesion-induced energy dissipation. [31] Much effort has been devoted to exploring the utilization of knowledge of bionic low-adhesivity superhydrophobicity to develop innovative condensate microdrop self-propelling (CMDSP) surfaces. Different from traditional superhydrophobic surfaces, which are characterized by the bouncing or rolling off of deposited millimeter-size large drops, [32,33] CMDSP surfaces support the self-removal capability of smallscale condensate microdrops. It has been reported that classical superhydrophobic lotus leaves (Figure 1a), [34][35][36][37] as well as artificial surfaces consisting of hierarchical micro-and nanostructures, [38] one-tier microstructures, [39][40][41][42][43] or nanostructures [44,45] with larger characteristic interspaces, present a low-adhesivity property to the deposited water macrodrops, but become highly adhesive to condensed microdrops (Figure 1b). This is because moisture easily penetrates the microscale valleys or cavities. Clearly, superhydrophobic surfaces with irrationally designed surface structures can enhance the solid-liquid interfacial adhesion instead of promoting the rapid removal of condensed drops. Accordingly, it is still a challenge to design and fabricate very effective low-adhesivity superhydrophobic surfaces with the self-removal ability of condensate microdrops. Recently, there has been a large breakthrough in understanding the mechanism and construction rules of bionic CMDSP surfaces, leading to the exploration of fabrication methods for the surface functionalization of widely used metal materials and the Bionic condensate microdrop self-propelling (CMDSP) surfaces are attracting increased attention as novel, low-adhesivity superhydrophobic surfaces due to their value in fundamental research and technological innovation, e.g., for enhancing heat transfer, energy-effective antifreezing, and...

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Section: Cmdsp Mechanismmentioning

confidence: 99%

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confidence: 99%

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

2017

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“…Coalescence-induced jumping phenomena occur on superhydrophobic surfaces but within a small range of initial droplet radii. Recent interest in these phenomena has led to the influence of the droplets radii on the resulting jumping velocity to be explored [3,[8][9][10][11][12]. However, previous analysis on coalescence-induced jumping droplets focused only a limited range of contact angles and droplet radius [10][11][12].…”

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Dropwise condensation heat transfer process optimisation on superhydrophobic surfaces using a multi-disciplinary approach

Khatir

Kubiak

Jimack

et al. 2016

Applied Thermal Engineering

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Dropwise condensation has superior heat transfer efficiency than filmwise condensation; however condensate evacuation from the surface still remains a significant technological challenge. The process of droplets jumping, against adhesive forces, from a solid surface upon coalescence has been studied using both experimental and Computational Fluid Dynamics (CFD) analysis. Both Lattice Boltzmann (LBM) and Volume of Fluid (VOF) methods have been used to evaluate different kinematic conditions of coalescence inducing a jump velocity. In this paper, an optimisation framework for superhydrophobic surface designs is presented which uses experimentally verified high fidelity CFD analyses to identify optimal combinations of design features which maximise desirable characteristics such as the vertical velocity of the merged jumping droplet from the surface and energy efficiency. A Radial Basis Function (RBF)-based surrogate modelling approach using design of experiment (DOE) technique was used to establish near-optimal initial process parameters around which to focus the study. This multidisciplinary approach allows us to evaluate the jumping phenomenon for superhydrophobic surfaces for which several input parameters may be varied, so as to improve the heat transfer exchange rate on the surface during condensation. Reliable conditions were found to occur for droplets within initial radius range of r=20-40 m and static contact angle s~160º. Moreover, the jumping phenomenon was observed for droplets with initial radius of up to 500 m. Lastly, our study also reveals that a critical contact angle for droplets to jump upon coalescence is c~1 40º. KeywordsCondensation heat transfer, Super-hydrophobic surface, Jumping droplets velocity, Multi-disciplinary optimisation. IntroductionDrop-wise condensation processes, where condensation occurs through small droplets on a solid surface, has been demonstrated to significantly improve heat transfer rates in comparison to filmwise condensation (where a whole surface is covered by a thin film of liquid) [1,2]. Drop-wise condensation usually takes place on hydrophobic or super-hydrophobic surfaces as demonstrated by Boreyko and Chen [3]. One of the main technological challenges is to create such a surface, so as to allow condensation and evacuation of the droplets to take place in a continuous manner. Droplet coalescence is a complex physical phenomenon and optimisation of kinematic conditions leading to surface dewetting and jumping of droplets is of paramount importance for processes like heat transfer, de-icing, atmospheric water harvesting or dehumidification [4][5][6]. Dropwise condensation heat transfer performance can be enhanced by allowing condensed droplets to be removed rapidly from the surface to minimize the thermal barrier [7]. Recently, researchers showed that superhydrophobic surfaces provide a higher mobility of condensates, which may enhance the heat transfer performance [1][2][3][4][5][6][7]. Coalescence-induced jumping phenomena occur on superhydrophobic surfa...

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“…[36] This departure size is in close agreement with previous experimental observations and theoretical predictions. [37][38][39] From the set of hierarchical surfaces investigated ( Figure S3), droplets departed most frequently from the surface with an edge-to-edge pillar spacing of ~20 m.…”

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confidence: 99%

Exploiting Microscale Roughness on Hierarchical Superhydrophobic Copper Surfaces for Enhanced Dropwise Condensation

Chen

Weibel

Garimella

2015

Adv Materials Inter

111

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Condensation of water vapor is of great interest in thermal management [1] and power generation [2] owing to the improved heat transfer by phase change, and in desalination [3] and dew/fog harvesting [4,5] systems for its ability to extract vapor from carrier gases. Enhancement of the condensation heat and mass transfer processes in these systems could lead to considerable performance gains, economic return, and energy savings. Heterogeneous condensation is dramatically influenced by the physical structure and chemical properties of a surface. Depending on the surface wettability, water vapor can condense on a surface either as a continuous liquid film (filmwise) or as individual droplets (dropwise). It is reported that dropwise condensation can produce heat transfer coefficients that are an order of magnitude higher than in filmwise condensation. [6] To attain this performance, condensed droplets must be rapidly removed from the surface and fresh spaces on the substrate exposed for nucleation; otherwise, accumulated large droplets will act as a thermal barrier and inhibit the heat/mass transfer rate. [7] To promote removal of condensate droplets, the droplet adhesion to the substrate must be minimized. Structured superhydrophobic surfaces [8][9][10][11][12] could offer an ideal

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Self-Propelled Dropwise Condensate on Superhydrophobic Surfaces

Cited by 1,134 publications

References 34 publications

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

Dropwise condensation heat transfer process optimisation on superhydrophobic surfaces using a multi-disciplinary approach

Exploiting Microscale Roughness on Hierarchical Superhydrophobic Copper Surfaces for Enhanced Dropwise Condensation

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