Evaporation-driven coalescence of two droplets undergoing freezing

TL;DR

This study uses numerical simulations to explore how evaporation-induced capillary flows influence the coalescence of freezing droplets, revealing the roles of humidity and temperature in this process.

Contribution

It introduces a detailed numerical model of evaporation-driven droplet coalescence during freezing, highlighting the effects of environmental conditions on the process.

Findings

Higher humidity delays coalescence by reducing evaporation flux.

Lower substrate temperatures accelerate freezing, restricting droplet coalescence.

Evaporation asymmetry induces capillary flows that drive droplets together.

Abstract

We examine the evaporation-induced coalescence of two droplets undergoing freezing by conducting numerical simulations employing the lubrication approximation. When two sessile drops undergo freezing in close vicinity over a substrate, they interact with each other through the gaseous phase and the simultaneous presence of evaporation/condensation. In an unsaturated environment, the evaporation flux over the two volatile sessile drops is asymmetric, with lower evaporation in the region between the two drops. This asymmetry in the evaporation flux generates an asymmetric curvature in each drop, which results in a capillary flow that drives the drops closer to each other, eventually leading to their coalescence. This capillary flow, driven by evaporation, competes with the upward movement of the freezing front, depending on the relative humidity in the surrounding environment. We found that higher relative humidity reduces the evaporative flux, delaying capillary flow and impeding coalescence by restricting contact line motion. For a constant relative humidity, the substrate temperature governs the coalescence phenomenon, and resulting condensation can accelerate this process. Interestingly, lower substrate temperatures are observed to facilitate faster propagation of the freezing front, which, in turn, restricts coalescence.