Coalescence mechanisms of nanodroplets on interfaces with different hydrophobicity: A dynamic density functional study

Abstract

The coalescence of nanodroplets on interfaces is an important subject in many fields but the underlying mechanisms remain unsettled. In this work, we introduce a dynamic density functional theory (DDFT) to examine this process, focusing on surface hydrophobicity. We found that coalescence time monotonically correlates with the contact angle (CA) of the droplets and there are three typical coalescence modes: vapor bridging, surface bridging and evaporation, depending on the hydrophobicity and the size difference of the droplet. Hydrophilic surfaces consistently induce surface bridging. On hydrophobic surfaces, vapor bridging occurs when droplet sizes are similar; conversely, when one droplet is large enough to encompass the center point, coalescence will perform in the evaporation mode. The evolution of density profile, local chemical potential, flux and free energy have been examined, which provide an insight into the interfacial coalescence of nanodroplet.