Abstract:

Exploring droplet impact onto solid surface is important to explain phenomena occurred both in nature and engineering applications. Compared to non-permeable surfaces, fewer studies have been conducted specifically on droplet impact onto permeable surfaces. Four different types of superhydrophilic micro/nano porous structures were prepared and used to investigate ejection behavior and kinetics of droplets at low impact velocity under the influence of structural parameters, including dimensional characteristics, porosity, and surface roughness of porous media. An inertial regime was identified during early stage spreading of droplets on porous surface, and the normalized spreading diameter obeyed the power-law function. The porosity change of nanoporous media had no significant effect on the power law, but the porosity increase of microporous media led to a decrease in values of both C and α. The increase in C value was also observed with increasing surface roughness. Two novel models of droplet ejection were discovered as the first-stage pinch-off and the second-stage pinch-off. Being a competitive process between the horizontal and vertical rates of collapse, the horizontal collapse could induce pinch-off when the vertical collapse was sufficiently delayed by the capillary waves. Fast inertial spreading velocity and long inertial time resulted in the first-stage pinch-off occurred in nanoporous media.The increased vertical collapse rate of droplets and the decreased inertial time by improvement of permeability were responsible for the second-stage pinch-off occurred in microporous media.

Key words: superhydrophilic, porous media, droplet, kinetics, permeation, droplet ejection

摘要：

构建了4类不同的超亲水微/纳多孔结构，通过低速液滴撞击实验研究了多孔介质的结构参数如微/纳尺度特征、孔隙率以及表面粗糙度等对液滴行为和动力学特性的影响。结果表明：多孔表面液滴的早期扩散符合惯性扩散的幂函数规律，并且纳米级结构孔隙率的变化对幂函数关系无显著影响，微米级结构C和α的值随孔隙率增大而降低，粗糙度的提高能够导致C值增大。发现了两种新颖的小液滴发射模式，分别称为第1阶段断裂和第2阶段断裂。纳米级多孔结构发生第1阶段断裂的原因在于较快的惯性扩散速度和较长的惯性时间；第2阶段断裂发生于微米级多孔结构，原因在于渗透的强化导致液滴高度的快速下降以及惯性时间的缩短。

关键词: 超亲水, 多孔介质, 液滴, 动力学, 渗透, 小液滴发射