When two liquid drops coalesce into one, the overall surface area is reduced and the released surface energy can be converted into kinetic energy, leading to self-propelled motion of the merged drop. To generate translational motion, the symmetry of the drop coalescence process must be broken. In this talk, we will first discuss two strategies to introduce asymmetry. One strategy is to place an impermeable solid surface on one side of the coalescing drops, e.g. a superhydrophobic substrate with minimal adhesion or a hydrophobic fiber with minimal contact with the drops. The other strategy is to arrange the coalescing drops asymmetrically, an example being the ballistospore launching system in which a spherical drop merges with a flattened drop. We will then elucidate the physical mechanisms through high-speed imaging and interfacial flow simulations. The two strategies to introduce asymmetry are illustrated with applications in both engineering and biology, including self-jumping drops to cool electronic hotspots and self-catapulting systems to discharge fungal spores.

Self-propelled jumping and catapulting upon drop coalescence

254th National Meeting (2017)

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