Molecular simulation of the initial stages of drop coalescence

Drop coalescence plays a crucial role in nature and industry. In continuum theory, after the two drops are taken to touch at a point at the onset of coalescence, two scaling regimes for the temporal growth of the bridge connecting the drops have been identified. Coalescence, however, is initiated at length and timescales at which the discrete nature of matter is of importance. Therefore, a continuum description provides no information about the mechanistic details of the actual initiation of bridge formation. To provide molecular-level insights into bridge formation and growth, 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, thereby eliminating any potential evaporative phenomena that may confound the analysis. We identify three distinct regimes of droplet coalescence. In the first regime, the bridge expands linearly with time, t. The new simulations show that this regime is initiated by "molecular jumps" into the bridge, in agreement with the results of prior MD simulations of others, and where the density in the bridge is seen to quickly increase until it reaches nearly the bulk value. Afterwards, the bridge dynamics eventually transition to a second, viscous regime in which the bridge radius appears to scale with time as -tlnt, as also predicted in continuum analyses but which heretofore had not been observed in molecular simulations. Finally, a third, inertial regime is observed at longer times, during which the bridge radius follows a scaling law akin to the t^{1/2} scaling also predicted in continuum analyses.