Evaporation characteristics of droplets on a gradient microhole-patterned surface

Abstract

The evaporation characteristics of deionized water droplets are investigated on two copper surfaces, a smooth surface and a microhole-patterned surface with wettability gradient. It is observed that at room temperature, the apparent contact angle on the smooth surface linearly decreased with time, while the three-phase contact line is pinned during the evaporation process. However, when a droplet is gently deposited on the gradient microhole-patterned surface, as the side length of the microholes increases, the apparent contact angle goes through three stages: the increasing stage, the stable stage and the re-increasing stage, and the lifetime of evaporating droplets undergoes these three stages too. The solid–liquid contact area plays a key role on the droplet evaporation. In a certain area of the microhole-patterned surface, obvious mesoscopic films are formed. As the droplet evaporates, the mesoscopic films are ruptured; simultaneously, the three-phase contact line shrinks inwards and the apparent contact angle increases. The rupture of the mesoscopic film is analyzed theoretically in this paper.