Free Energy and Dynamics of Water Droplet Coalescence

Droplet coalescence is a critical issue in atmospheric sciences. In this work, the coalescence of water nanodroplets was studied by performing equilibrium and nonequilibrium molecular dynamics simulations. To understand the intrinsic nature of the process, we obtained the free energy change as a function of droplet size and droplet droplet distance. We decomposed the free energy change Delta F into energetic Delta U and entropic T Delta S contributions to understand the molecular details. Delta U was dominated by the change in Coulomb interactions, which strongly correlated with the change in the number of hydrogen bonds. We found a strong positive correlation between the mobility of water molecules and T Delta S. To analyze the dynamics, two colliding water droplets of the same size were given different initial speeds, impact parameters, and collision angles. We found when the collision is head-on, the time for thorough mixing between interfacial and bulk molecules decreases when the initial speed increases, whereas when the collision is off-center, the induced torque significantly increases the mixing time, which can last up to hundreds of picoseconds. The initial impact of a collision can push the interfacial molecules away from the center of mass and provide an evaporation mechanism of the interfacial/adsorbed molecules on the droplet.