The effect of confined microstructures on the coalescence-induced droplet jumping with low surface tension

doi:10.11949/0438-1157.20201250

Abstract

Abstract:

In this work, the hierarchically superhydrophobic surface was fabricated by the combination of CuO nanostructures and regular microgrooves. The deionized water and aqueous ethanol with 8% and 16% mass fractions were utilized as test liquids. The characteristics of confined growth of single droplet in microgroove and the coalescence-induced jumping between deformed droplet in microgroove and undeformed droplet outside microgroove were studied by the microdroplet deposition method to explore the role played by confined microstructure on the coalescence-induced droplet jumping with low surface tension. The results showed that the confinement of microgroove caused the deformation of growing droplet in microgroove. The self-jumping of deformed water droplet in microgroove occurred by the driving of Laplace pressure difference generating in droplet. However, the deformed droplets of aqueous ethanol with 8% and 16% mass fractions spontaneously moved upward and suspended on the microgroove instead of self-jumping, due to the increased surface adhesion caused by the decreased liquid surface tension. The increase of surface adhesion resulted in the increase of Laplace pressure difference for deformed droplet detachment from valley of microgroove, in turn causing larger degree of droplet deformation. Thus, the size of spherical droplet originated from deformed droplet recovering increased. The confined microstructures enabled the enhancement in coalescence-induced droplet jumping by regulating the morphologies of droplet and the interaction between coalesced droplet and surface. However, the enhancement reduced with the decreased liquid surface tension. Compared with the deionized water, the coalescence-induced jumping velocities between deformed and undeformed droplets of aqueous ethanol with 8% and 16% mass fractions respectively decreased by 26.7% and 75.9%, and the energy conversion efficiency has been reduced by 17.8% and 90%, respectively.

Key words: superhydrophobic, coalescence-induced droplet jumping, interfacial tension, hydrodynamics, nanostructure, confined growth, aqueous ethanol

摘要：

设计并制备了CuO纳米结构和矩形微槽相结合的层级微槽超疏水表面，采用去离子水、质量分数为8%和16%的乙醇溶液为工质，研究了三种表面张力的单个液滴在微槽内的受限生长特征以及槽内变形液滴与槽外正常液滴的合并弹跳行为，探讨了受限微结构对较低表面张力液滴合并弹跳的影响。结果表明，在Laplace压力差的驱动下，微槽内受限变形的水滴发生自弹跳行为，随着溶液中乙醇浓度的提升，液体表面张力减小，表面对液滴的吸附增强，乙醇质量分数为8%和16%的槽内变形液滴不发生自弹跳，而是爬升并悬浮于微槽上方。受限微结构对液滴合并弹跳的强化作用随液体表面张力的减小而减弱，与去离子水相比，乙醇质量分数为8%和16%的受限变形液滴与槽外正常液滴的合并弹跳速度分别降低了26.7%和75.9%，能量转化效率分别降低了17.8%和90%。

关键词: 超疏水, 液滴合并弹跳, 界面张力, 流体动力学, 纳米结构, 受限生长, 乙醇溶液

CLC Number:

TK 124

PENG Qi, JIA Li, DING Yi, ZHANG Yongxin, DANG Chao, YIN Liaofei. The effect of confined microstructures on the coalescence-induced droplet jumping with low surface tension[J]. CIESC Journal, 2021, 72(4): 1920-1929.

彭启, 贾力, 丁艺, 张永欣, 党超, 银了飞. 受限微结构对低表面张力液滴合并弹跳的影响[J]. 化工学报, 2021, 72(4): 1920-1929.

Figures/Tables 8

Fig.1 The morphology of hierarchically microgrooved superhydrophobic surface

Table 1 The properties of working fluids at room temperature

乙醇质量分数/%	密度/(kg/m³)	黏度/(mPa·s)	表面张力/(mN/m)
0	998.2	1.005	72
8	978.9	1.402	50.8
16	957.8	1.948	40.9

Fig.2 The experimental apparatus for coalescence-induced droplet jumping

Fig.3 Dynamic behaviors of confined growth of droplets in microgrooves with different surface tension

Fig.4 The assumed model of deformed droplet residing in microgroove

Fig.5 The characteristics of deformed droplets with different surface tension in confined microgroove

Fig.6 The coalescence-induced jumping between deformed and undeformed droplets with different surface tensions

Fig.7 The coalescence-induced jumping performance between deformed and undeformed droplets with different surface tensions

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