On Droplet Coalescence in Quasi-Two-Dimensional Fluids.

Coalescence of droplets plays a crucial role in nature and modern technology. Various experimental and theoretical studies explored droplet dynamics in three-dimensional (3D) and on 2D solid or liquid substrates. In this paper, we demonstrate the complete coalescence of isotropic droplets in thin quasi-2D liquids-overheated smectic films. We observe the merging of micrometer-sized flat droplets using high-speed imaging and analyze the shape transformations of the droplets on the timescale of milliseconds. Our studies reveal the scaling laws of the coalescence time, which exhibits a different dependence on the droplet geometry from that in the case of droplets on a solid substrate. A theoretical model is proposed to explain the difference in behavior.