关键词: 超亲水, 多孔介质, 液滴, 动力学, 渗透, 小液滴发射

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