Evaporation-driven coalescence of two droplets undergoing freezing

We examine the evaporation-induced coalescence of two droplets undergoing freezing through numerical simulations using the lubrication approximation. When two sessile droplets freeze in close proximity on a substrate, they interact through the gaseous phase, involving simultaneous evaporation and condensation. In an unsaturated environment, the evaporation flux over the two volatile droplets is asymmetric, with lower evaporation in the region between them. This flux asymmetry generates an asymmetric curvature in each droplet, resulting in a capillary flow that drives the droplets closer together, eventually leading to coalescence. The capillary flow driven by evaporation competes with the upward movement of the freezing front, depending on the relative humidity of the surrounding environment. Higher relative humidity reduces the evaporative flux, delaying capillary flow and impeding coalescence by restricting contact line motion. At a constant relative humidity, substrate temperature governs the coalescence phenomenon, with resulting condensation potentially accelerating the process. Interestingly, lower substrate temperatures facilitate faster propagation of the freezing front, thereby restricting coalescence.