Capillary-Wave-Driven Jumping Droplets on Superhydrophobic Colloidal Rafts

Known droplet jumping phenomena include coalescence-induced jumping, single-droplet jumping of partially constrained droplets due to a mismatch in Laplace pressure, and evaporation-induced trampolining. In this study, we introduce a novel droplet jumping phenomenon, in which multiple microdroplets jump nearly simultaneously from superhydrophobic colloidal rafts. This phenomenon is triggered by a coalescence of a microdroplet with the underlying water, which generates a radially propagating capillary wave. As the capillary wave propagates, it perturbs the rafts on which microdroplets are sitting, which leads to the launch of these droplets. Therefore, the mechanism for droplet launching is the transfer of energy from the initial coalescence site through a capillary wave to other droplets resting on the perturbed interface. A simple model is derived to estimate the size of the region on the surface in which launching of the microdroplets takes place. These findings reveal that the capillary-inertial symmetry breaking that occurs for coalescing droplets can impart useful work to flexible or floating substrates to effectively propel neighboring droplets.