Directional Continuous Bouncing Behavior of Water Droplets on a Surface with a Chemical Gradient.

Manipulation of directional bouncing behavior of liquid droplets after impacting solid surfaces is highly significant for biological, agricultural, engineering, and industrial applications. Here, we prepared a surface with a chemical gradient on a Ti-6Al-4V substrate, on which directional multiple bouncing of droplets and long-range movement has been achieved. The wetting gradient of the vapor-deposited surface reached 2.5° mm by finely controlling the distribution of low surface energy functional groups. Droplet adhesion force analysis was carried out to visualize the variation of surface wettability. On this surface with inhomogeneous wettability, the droplet repeated the impacting and rebounding 8 times along the direction of the chemical gradient, displaying an interesting phenomenon of "droplet trampoline". The maximal rebound height and the horizontal jumping distance reached 8.36 and 10.19 mm, respectively. Additionally, the underlying mechanism behind this consecutive bouncing behavior of droplets was thoroughly elucidated from energy and force perspectives. This research is anticipated to advance the understanding of directional continuous bounce behavior of a droplet and provide a promising strategy to delicately manipulate the movement of droplets.