Fabrication of Novel Superhydrophobic Surfaces and Droplet Bouncing Behavior — Part 2: Water Droplet Impact Experiment on Superhydrophobic Surfaces Constructed Using ZnO Nanoparticles

Abstract

When a liquid droplet impacts a solid surface, it spreads up to a point and the kinetic energy is dissipated by viscosity, collision and surface energy during the process. The droplet can retract if the energy dissipation during the impact process which is only partly governed by surface properties is not too large. Otherwise, the droplet would stick to the surface or break into smaller droplets. In this second part, we introduced contact angle hysteresis (CAH) and studied the impact behavior between a water droplet and a superhydrophobic surface both theoretically and experimentally. On our superhydrophobic surface, the contact angle is about 155° , so the kinetic energy of the droplet can be largely transferred to surface energy. Thus, under certain conditions, the droplet can fully bounce. The impact behavior of normal impact was analyzed theoretically. The critical falling heights for rebound (CFHR) were investigated on constructed ZnO–PDMS superhydrophobic surface in both normal and oblique impact conditions, and CFHR was found to increase with the increase of tilt angle. This shows that the normal Weber number (We n ) is the major factor governing the rebound, while the tangential Weber number (We t ) also has effect on the phenomenon. Compared to the energy dissipated by collision and viscosity, the influence of surface properties is relatively small. The adhesion number (N a ) is the parameter determining the energy dissipated by surface tension and N a has direct relation with contact angle (CA) and CAH.