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, there is still incomplete mechanistic explanation for the energy loss for rebounding droplets. Here, we measured e for sub-millimetric and millimetric sized droplets impacting two different superhydrophobic surfaces over a wide range of UI = 4–400 cm s−1. We proposed simple scaling laws to explain the observed non-monotonic 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 Δcos θ of the surface. In contrast, in the limit of high UI , e is dominated by inertial-capillary effects and does not depend on Δcos θ.