Directional rebounding of a droplet impinging hydrophobic surfaces with roughness gradients

Abstract

Surfaces with a wettability gradient, which require no external energy input, play a significant role in controlling and manipulating the self-transport of liquid drops. This work numerically investigates the directional rebound behavior of water droplets on surfaces with wettability gradients obtained by gradually altering the groove width. The results show that three types of droplet rebound behavior (vertical bouncing, following, or against the roughness gradient) and the rotating motion (counterclockwise), which are dominated by the combined effects of the unbalanced Young's force and the wetting state, are affected by the Weber number (We), groove width, and groove depth. Droplets remain in the Cassie state and rebound following the roughness gradient for a small We, small groove width, and larger groove depth. By contrast, when the Cassie and Wenzel states are both present, droplets are partially arrested by the ridges and grooves and rebound against the roughness gradient. Additionally, vertical rebounding behaviors are observed at extremely small We owing to the low contact-angle hysteresis, and at critical We because the Young's force and capillary emptying process are balanced. Therefore, both the unbalanced Young's force and wetting state should be considered when investigating the bounce behavior of droplets impinging surfaces with a roughness gradient. The results provide important insight into the design of wettability gradient surfaces for controlling droplet transport.