Bouncing droplets on an elastic, superhydrophobic cantilever beam

Abstract

The impact dynamics of a water droplet on a flexible substrate is useful for designing pesticide sprays and understanding insects flying in rainfall. We experimentally analyze the impact dynamics of a microliter water droplet on a superhydrophobic cantilever beam for Weber number in the range of 30–76. A thin copper sheet was coated with a commercial coating to render it superhydrophobic and high-speed imaging was used for visualization. During the impact, the spreading droplet converts its inertial energy into surface energy and elastic energy of the substrate. While retraction of the contact line, the latter energies convert to the kinetic energy of the droplet, and the droplet could bounce off the deforming cantilever beam. The characteristics timescales of droplet and cantilever beams are varied by changing the droplet diameter and impact velocity, and beam length, respectively. We show that the overall system dynamics, i.e., bouncing of the droplet and oscillations of the cantilever, is dependent on the interplay of these two timescales. A spring-mass system has been used to model this coupling and to explain the experimental observations. These findings can help to design systems to achieve desirable contact time, droplet rebound kinetic energy, energy transfer to the cantilever beam, and the droplet spreading diameter.