Non-coalescence and in-plane momentum generation in sessile droplet clusters

Abstract

Intuitively, droplets in proximity merge when brought into contact. However, under certain conditions, they may not coalesce due to the entrapment of an interstitial gas film. Non-coalescence between water droplets has so far been observed during collisions of droplets moving with relative centroidal velocity, or in the presence of specific enabling effects such as high intervening gas pressures, surfactants, or large droplet sizes (diameter $\gtrsim 1~\mathrm{mm}$). Here, we report non-coalescence between water droplets over a much wider range of droplet diameters, from millimeters to as small as 100 microns, without the need for any of the above factors. Such non-coalescence occurs in sessile droplet clusters on water-repellent surfaces. When any two droplets in a cluster coalesce, the evolving interface of the coalescing droplets comes in apparent contact with other neighbouring droplets in the cluster, but does not necessarily trigger further coalescence. In fact, such apparent contact can manifest as a bouncing interaction, and depending on the initial geometric arrangement of droplets, it can result in significant lateral momentum generation, consequently leading to spontaneous in-plane self-propulsion of the participating droplets. The energy conversion efficiency of this process reaches as high as 9\% for closely packed clusters of three sessile droplets and increases further with an increase in the number of participating droplets. The resulting self-propulsion of such small droplets reveals a new pathway for passive droplet removal and surface renewal during dropwise condensation on superhydrophobic surfaces, critical in multiple applications.