Molecular simulation study of the initial stages of droplet coalescence

Coalescence plays a crucial role in nature and industry. In prior continuum analyses, a microscopic bridge joining two drops is assumed to have already formed at the start of coalescence. At these early times, the bridge curvature is exceedingly large, yielding a diverging Laplace pressure at the point of contact between the drops. This singularity generates a flow that leads to drop merging. Continuum analyses identify two regimes for the temporal growth of the bridge. Coalescence, however, is initiated at length and time scales at which the discrete nature of matter is of importance and a continuum description provides no information about the actual formation of the bridge. To provide molecular-level insights, we study the initial stages of coalescence using a hybrid Monte Carlo-Molecular Dynamics (MC-MD) simulation method. We reduce the required computational effort by only simulating those parts of the drops directly facing each other. Particle reservoirs, along with grand canonical MC steps (GCMC), are used to maintain the bulk densities of the drops at specified values. These GCMC steps also enable the drops to be both thermally and chemically equilibrated prior to coalescence eliminating any potential evaporative phenomena that may confound the analysis.