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. Droplet non-coalescence has so far been observed in various scenarios, such as during interactions between droplets moving with relative bulk velocity, droplets with opposite charges and size differences, and Leidenfrost droplets due to continuous vapor generation. Although non-coalescence has been reported for droplets of various fluids and sizes, non-coalescence for water droplets has been reported only for grazing collisions of large droplets (diameter >2 mm) moving with high bulk velocity. Here, we report non-coalescence between water droplets over a wide range of droplet diameters, without the presence of any charges, surfactants, fluid property gradients, any additional vapor generation, or bulk droplet velocities. Such non-coalescence manifests in sessile droplet clusters on water repellant surfaces. When any two droplets in a cluster coalesce, they do not necessarily trigger further coalescence with other neighbouring droplets in the cluster. Further, depending on the initial geometric arrangement of droplets, such non-coalescence results in significant lateral momentum generation and consequently, spontaneous in-plane self-propulsion of participating droplets. The energy conversion efficiency of this process reaches as high as 9% for most closely packed clusters of 3 droplets and increases further with increase in the number of participating droplets. Further, this phenomenon can manifest for droplets as small as 200 {\mu}m in diameter formed during condensation. 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.