纳米结构表面上部分润湿液滴合并诱导弹跳的理论研究

A theoretical study on coalescence-induced jumping of partially wetted condensed droplets on nano-textured surfaces

部分润湿液滴是适宜纳米结构表面上滴状冷凝传热的主要载体,研究纳米结构参数与部分润湿液滴合并弹跳之间的关系有重要意义,本文依据冷凝液滴生长过程中能量增加最小的原理来判断其是否为部分润湿状态,并根据液滴合并前后的体积和界面自由能守恒,确定了合并液滴的初始形状,进而对合并液滴变形过程的动力学方程进行了求解,结果表明:部分润湿冷凝液滴仅在纳米柱具有一定高度、直径间距比较大的表面上形成,而当纳米柱高度过低、直径间距比小于0.1时则形成完全润湿的冷凝液滴;液滴合并后能否弹跳与纳米结构参数紧密相关,仅在纳米柱较高、直径间距比适宜的表面上,部分润湿液滴合并后才能诱发弹跳;液滴尺度及待合并液滴间的尺度比对合并弹跳也有重要影响;多个部分润湿液滴合并后由于具有更多的过剩界面自由能而比两个液滴合并更容易诱发弹跳,本模型对纳米结构表面上冷凝液滴是否合并诱发弹跳的计算结果与绝大部分实测结果相一致,准确率达到95%。

Partially wetted （PW） droplets specially exist on textured surfaces with proper nano-structural parameters. Such tiny drops can depart from surfaces by coalescence-induced jumping, and become the main medium for dropwise conden-sation heat transfer. Therefore, it is of great importance to study the relationship between nano-structural parameters and PW drop post-merging jumping. In this study, the principle of minimum energy increasing during condensed droplets growth was used to judge if a condensed drop is in PW state. The initial shape of a coalesced droplet was determined based on the conservation of PW drop interface free energy and viscous dissipation energy before and after two or more PW condensed droplets merge. The dynamic equation describing the shape conversion of the post-coalescence droplet was then solved. Whether jumping or not of a merged drop was determined by whether the base radius of the droplet can reduce to 0 and if existing a up moving speed of drop gravity center at this moment. The calculation results show that PW droplets can form only on the textured-surfaces with certain nano-pillar height and relatively larger ratio between pillar diameter and pitch, dn/s, while completely wetted droplets easily form on the surfaces with low pillar height and dn/s less than 0.1. Meanwhile, post-coalescence jumping of PW droplets closely relates to nano-structural parameters. Not all PW drops can jump after merging. Instead, self-propelled jumping of PW drops takes place only on the surfaces with relatively higher nano-pillar height and suitable dn/s. Moreover, PW drop size and the scale ratio between two PW droplets to merge also have significant effect on the coalescence-induced jumping. It is difficult for a merged drop to jump spontaneously if the size of PW drops is too large or too small, or the scale ratio of two PW drops is too small. Fi-nally, post-coalescence jumping of multi-droplets is easier than that of two drops since more surplus interface free energy exists in the former case. The calculation results of this model are well consistent with the experimental observations in literatures for whether the post-coalescence condensed drops jump on nano-textured surfaces, with accuracy of 95%. In conclusion, coalescence-induced jumping takes place only when PW droplets with suitable size on the textured surfaces with proper nano-structural parameters.