Bouncing droplets on micro-grooved non-wetting surfaces

Abstract

Water droplets impinging on micro-grooved polydimethylsiloxane surfaces were studied. Depending on the impact velocity and surface roughness, different phenomena such as no bouncing, complete rebound (CR), bouncing occurring with droplet breakup (BDB), partial rebound, and sticky state were observed. The lower limit of impact velocity for bouncing droplets can be determined by balancing the kinetic energy of the droplet with energy barrier due to contact angle hysteresis. To predict the upper limit of impact velocity for bouncing droplets, a high-speed camera was used to record droplet impact at an ultrahigh speed and it was found that the transition from CR to BDB was attributed to a local wetting transition from the Cassie–Baxter state to the Wenzel state. Based on the experimental observation, a theoretical model was developed to predict the upper limit of impact velocity taking into account the penetration of the liquid into the micro-grooves. In addition, there was a shorter contact time of bouncing droplets with the decrease in the Weber number and surface roughness has a small influence on the contact time in our experiments.