Molecular physics of jumping nanodroplets

Coalescence-induced droplet jumping on non-wetting surfaces is a promising candidate for application in heat transfer and self cleaning processes. In this work, we demonstrate and explain, for the first time, the crucial impact ambient gas rarefaction and interfacial thermal fluctuations have on the jumping characteristics of nanodroplets, by performing unconventionally-large and computationally-expensive molecular dynamics simulations. We show that nanodroplet jumping is governed not just by the Ohnesorge number, as conventionally assumed, but also by two other dimensionless numbers that represent the molecular physics: Knudsen number (Kn) and thermal fluctuation number (Th). By quantifying different dissipation mechanisms present in the system, we explain how the nanodroplet jumping speed depends on Kn. Our results also show that droplets with large Th have significantly skewed and wide statistical distributions of their jumping speeds. Insights from our theoretical analysis and observations can be used to optimize future micro/nanofluidic devices that leverage droplet jumping.