How Coalescing Droplets Jump

Enright, Ryan; Miljkovic, Nenad; Sprittles, James E.; Nolan, Kevin; Mitchell, Robert; Wang, Evelyn N.

doi:10.1021/nn503643m

Cited by 314 publications

(413 citation statements)

References 47 publications

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“…Note that this analysis is also applicable to the case of multiple microdrops. [61][62][63] Meanwhile, other hydrodynamic analyses propose that the impact and deformation of instantly formed liquid bridges on a low-adhesivity surface is also key to realizing the ejection of merged microdrops, [64,65] through which the redundant surface energy can be converted into kinetic energy.…”

Section: Cmdsp Mechanismmentioning

confidence: 99%

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%

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

2017

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“…Coalescing droplets on a superhydrophobic substrate, for example, may be affected by their droplet-droplet interactions, as well as inhomogeneities on the substrate. Through studies on zero-adhesion superhydrophobic surfaces, Enright and colleagues 16,17 have underscored the importance of internal fluid dynamics on the surface-to-kinetic energy conversion mechanism. Liu and colleagues have taken a significant step towards a systematic analysis of droplet recoil and leap by studying head-on collision of microdroplets on Leidenfrost surfaces.…”

Section: Introductionmentioning

confidence: 99%

Electrically induced drop detachment and ejection

et al. 2016

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A deformed droplet may leap from a solid substrate, impelled to detach through the conversion of surface energy into kinetic energy that arises as it relaxes to a sphere. Electrowetting provides a means of preparing a droplet on a substrate for lift-off. When a voltage is applied between a water droplet and a dielectric-coated electrode, the wettability of the substrate increases in a controlled way, leading to the spreading of the droplet. Once the voltage is released, the droplet recoils, due to a sudden excess in surface energy, and droplet detachment may follow. The process of drop detachment and lift-off, prevalent in both biology and micro-engineering, has to date been considered primarily in terms of qualitative scaling arguments for idealized superhydrophobic substrates. We here consider the eletrically-induced ejection of droplets from substrates of finite wettability and analyze the process quantitatively. We compare experiments to numerical simulations and analyze how the energy conversion efficiency is affected by the applied voltage and the intrinsic contact angle of the droplet on the substrate. Our results indicate that the finite wettability of the substrate significantly affects the detachment dynamics, and so provide new rationale for the previously reported large critical radius for drop ejection from micro-textured substrates.

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“…[1][2][3][4][5][6][7][8][9][10] More recently, researchers have discovered that when small microdroplets (~10-100 µm) condense and coalesce on a suitably designed superhydrophobic surface, the merged droplet can jump away from the surface irrespective of gravity due to surface-tokinetic energy transfer. [11][12][13][14][15][16] This phenomenon has been termed jumping-droplet condensation and has been shown to further enhance heat transfer by up to 30% when compared to classical dropwise condensation due to a larger population of microdroplets which more efficiently transfer heat to the surface. 17 A number of works have since fabricated superhydrophobic nanostructured surfaces to achieve spontaneous droplet removal [18][19][20][21][22][23][24][25][26][27][28] for a variety of applications including self-cleaning, [29][30][31] thermal diodes, 30,32 anti-icing, [33][34][35][36] vapor chambers, 37 electrostatic energy harvesting, [38][39][40] and condensation heat transfer enhancement.…”

Section: Jumping Droplet Condensation and External Electric Fieldsmentioning

confidence: 99%

“…12 The experimental trajectory was obtained through condensation of water vapor on a superhydrophobic copper sample coated with SPF functionalized CuO nanostructures (for CuO nanostructuring and SPF functionalization details, please see references 17 and 55, respectively). The flat geometry of the sample allowed for simultaneous high-speed imaging of droplet jumping against gravity.…”

Section: Jumping Droplet Modelmentioning

confidence: 99%

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A Comprehensive Model of Electric-Field-Enhanced Jumping-Droplet Condensation on Superhydrophobic Surfaces

Birbarah

Pauls

et al. 2015

Langmuir

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Superhydrophobic micro/nanostructured surfaces for dropwise condensation have recently received significant attention due to their potential to enhance heat transfer performance by shedding positively charged water droplets via coalescence-induced droplet jumping at length scales below the capillary length, and allowing the use of external electric fields to enhance droplet removal and heat transfer, in what has been termed electric-field-enhanced (EFE) jumping-droplet condensation. However, achieving optimal EFE conditions for enhanced heat transfer requires capturing the details of transport processes that is currently lacking. While a comprehensive model has been developed for condensation on micro/nanostructured surfaces, it cannot be applied for EFE condensation due to the dynamic droplet-vapor-electric field interactions. In this work, I developed a comprehensive physical model for EFE condensation on Furthermore, the results suggest that EFE electrodes can be optimized such that the work input is minimized depending on the condensation heat flux. To analyze overall efficiency, I defined an incremental coefficient-of-performance and showed that it is very high (~10 6 ) for EFE iii condensation. I finally proposed mechanisms for condensate collection which would ensure continuous operation of the EFE system, and which can scalably be applied to industrial condensers. This work provides a comprehensive physical model of the EFE condensation process, and offers guidelines for the design of EFE systems to maximize heat transfer.iv ACKNOWLEDGMENTS

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How Coalescing Droplets Jump

Cited by 314 publications

References 47 publications

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

Recent Progress in Bionic Condensate Microdrop Self‐Propelling Surfaces

Electrically induced drop detachment and ejection

A Comprehensive Model of Electric-Field-Enhanced Jumping-Droplet Condensation on Superhydrophobic Surfaces

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