Topologies of Nanoscale Droplets upon Head-On Collision from Large Molecular Dynamics Simulations

The binary collision of nanoscale droplets is studied with molecular dynamics simulation for droplets consisting of up to 2 × 10⁷ molecules interacting via a truncated and shifted form of the Lennard-Jones potential. Considering head-on collisions of droplets with a temperature near the triple point that occur in a saturated vapor of the same fluid, this work explores a range of collision topologies. Four droplet sizes, with a radius ranging from 30 to 120 molecule diameters, are simulated with a varying initial relative collision velocity, covering 36 cases in total. Due to the relatively large size of the droplets, this study aims to resolve the differences in the collision behavior between droplets on the micro- and on the macroscale. By analyzing various metrics of the impact, four distinct collision regimes are found: coalescence, stable collision, holes and shattering. Coalescence, observed at low Weber and Reynolds numbers, is the formation of a stable droplet without significant deformations of the merging objects. Stable collisions, characterized by the formation of one stable droplet with notable deformations during collision, occur within a Weber number range between 10 and 505. The holes regime is only observed for droplet radii greater than 30 molecule diameters and a Weber number between 505 to 750, while collision cases surpassing this Weber number fall into the shattering regime, resulting in the breakup into satellite structures.