Department of Mechanical Science and Engineering , University of Illinois at Urbana-Champaign , Urbana , Illinois 61801 , United States.
Institute of Nuclear and New Energy Technology , Tsinghua University , Beijing , 100084 , China.
ACS Nano. 2019 Feb 26;13(2):1309-1323. doi: 10.1021/acsnano.8b06677. Epub 2019 Jan 14.
Coalescence-induced droplet jumping has the potential to enhance the efficiency of a plethora of applications. Although binary droplet jumping is quantitatively understood from energy and hydrodynamic perspectives, multiple aspects that affect jumping behavior, including droplet size mismatch, droplet-surface interaction, and condensate thermophysical properties, remain poorly understood. Here, we develop a visualization technique utilizing microdroplet dispensing to study droplet jumping dynamics on nanostructured superhydrophobic, hierarchical superhydrophobic, and hierarchical biphilic surfaces. We show that on the nanostructured superhydrophobic surface the jumping velocity follows inertial-capillary scaling with a dimensionless velocity of 0.26 and a jumping direction perpendicular to the substrate. A droplet mismatch phase diagram was developed showing that jumping is possible for droplet size mismatch up to 70%. On the hierarchical superhydrophobic surface, jumping behavior was dependent on the ratio between the droplet radius R and surface structure length scale L. For small droplets ( R ≤ 5 L), the jumping velocity was highly scattered, with a deviation of the jumping direction from the substrate normal as high as 80°. Surface structure length scale effects were shown to vanish for large droplets ( R > 5 L). On the hierarchical biphilic surface, similar but more significant scattering of the jumping velocity and direction was observed. Droplet-size-dependent surface adhesion and pinning-mediated droplet rotation were responsible for the reduced jumping velocity and scattered jumping direction. Furthermore, droplet jumping studies of liquids with surface tensions as low as 38 mN/m were performed, further confirming the validity of inertial-capillary scaling for varying condensate fluids. Our work not only demonstrates a powerful platform to study droplet-droplet and droplet-surface interactions but provides insights into the role of fluid-substrate coupling as well as condensate properties during droplet jumping.
聚并诱导的液滴跳跃具有提高众多应用效率的潜力。虽然从能量和流体动力学的角度来看，二元液滴跳跃可以定量理解，但仍有许多影响跳跃行为的方面，包括液滴尺寸不匹配、液滴-表面相互作用以及冷凝物的热物理性质，这些方面仍了解甚少。在这里，我们开发了一种利用微滴分配的可视化技术来研究纳米结构超疏水、分级超疏水和分级双亲表面上的液滴跳跃动力学。我们表明，在纳米结构超疏水表面上，跳跃速度遵循惯性-毛细尺度规律，无量纲速度为 0.26，跳跃方向垂直于基底。开发了液滴不匹配相图，表明对于高达 70%的液滴尺寸不匹配，跳跃是可能的。在分级超疏水表面上，跳跃行为取决于液滴半径 R 和表面结构长度尺度 L 的比值。对于小液滴（R ≤ 5L），跳跃速度高度分散，跳跃方向与基底法线的偏离高达 80°。对于大液滴（R > 5L），表面结构长度尺度的影响消失。在分级双亲表面上，观察到类似但更显著的跳跃速度和方向的分散。液滴尺寸依赖性的表面附着力和钉扎介导的液滴旋转是导致跳跃速度降低和跳跃方向分散的原因。此外，还对表面张力低至 38mN/m 的液体进行了液滴跳跃研究，进一步证实了惯性-毛细缩放对于不同冷凝物流体的有效性。我们的工作不仅展示了一个强大的平台来研究液滴-液滴和液滴-表面相互作用，而且还深入了解了在液滴跳跃过程中流体-基底耦合以及冷凝物性质的作用。
