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 U
R and the initial impact speed U
I; i.e., its restitution coefficient e = U
R/U
I. 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 U
I (4–700 cm s–1). We proposed simple
scaling laws to explain the observed nonmonotonic dependence of e on U
I. In the limit of low U
I, energy loss is dominated by contact-line
pinning and e is sensitive to the surface wetting
properties, in particular to contact angle hysteresis Δ cos
θ of the surface. In contrast, e is dominated
by inertial-capillary effects and does not depend on Δ cos θ
in the limit of high U
I.
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.
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