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 URand 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 efor 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 eon UI. In the limit of low UI, energy loss is dominated by contact-line pinning and eis sensitive to the surface wetting properties, in particular to contact angle hysteresis Δ cos θ of the surface. In contrast, eis dominated by inertial-capillary effects and does not depend on Δ cos θ in the limit of high UI.