Energy Loss for Droplets Bouncing Off Superhydrophobic Surfaces

A water droplet can bounce off superhydrophobic surfaces multiple times before coming to a stop. The energy loss for such droplet rebounds can be quantified by the ratio of the rebound speed UR and the initial impact speed UI; i.e., its restitution coefficient e = UR/UI. Despite much work in this area, a mechanistic explanation for the energy loss for rebounding droplets is still lacking. Here, we measured e for submillimeter-and millimeter-sized droplets impacting two different superhydrophobic surfaces over a wide range of UI (4- 700 cm s-1). We proposed simple scaling laws to explain the observed nonmonotonic dependence of e on UI. In the limit of low UI, energy loss is dominated by contact-line pinning and e is sensitive to the surface wetting properties, in particular to contact angle hysteresis Delta cos 0 of the surface. In contrast, e is dominated by inertial-capillary effects and does not depend on Delta cos 0 in the limit of high UI.

Automated summary

A water droplet can rebound multiple times on superhydrophobic surfaces, and the energy loss during such rebounds depends on the ratio of rebound speed to initial impact speed. We conducted experiments on different superhydrophobic surfaces with droplets of various sizes and proposed scaling laws to explain the nonmonotonic relationship between the energy loss and impact speed. At low impact speeds, the energy loss is influenced by contact-line pinning and the surface wetting properties, while at high impact speeds, it is dominated by inertial-capillary effects.