Molecular dynamics of water nanodroplet collisions of equal size: The key role of temperature

Abstract

We investigate the collision dynamics of two equal-sized nanodroplets in a vacuum using molecular dynamics simulations across various droplet sizes. To systematically characterize the outcomes, we construct an off-center distance–Weber number chart based on constant-energy simulations of small nanodroplets. The simulation results — coalescence, shattering, and separation — are mapped onto this chart. For head-on collisions, where the off-center distance is zero, the outcomes are limited to either coalescence or shattering at low and high Weber numbers, respectively. In the coalescence regime, energy conservation allows us to determine the final nanodroplet temperature by considering vapor and liquid densities, surface tension, and the enthalpy of vaporization. We find that the transition between coalescence and shattering occurs when the final temperature reaches a certain value, independent of the nanodroplet size. This insight enables the prediction of the coalescence Weber number for transition based solely on thermodynamic properties. The off-center distance–Weber number chart exhibits a size dependence. However, a simple transformation of both axes collapses the data onto a universal master curve. Furthermore, taking into account rotational energy and imposing angular momentum conservation to the equations provides an estimate of the final temperature within the coalescence region of the chart. Finally, we highlight the excellent agreement between the predictions from a general expression based on macroscopic thermodynamic properties obtained from equilibrium simulations and the results of an out-of-equilibrium collision of nanodroplets.