Coalescence-induced droplet jumping on nanodimpled surfaces: A molecular dynamics study

Coalescence-induced droplet jumping on structured surfaces is strongly influenced by surface morphology and has been shown to enhance jumping velocities for droplets within millimeter to micrometer scales. However, at the nanoscale, increased viscous dissipation would hinder jumping, and the critical droplet size and radius ratios of coalescence-induced droplet jumping remain controversial. This work investigates the coalescence of water droplets located on adjacent nanodimples of the same surfaces via molecular dynamics (MD) simulations. The results indicate that increasing the surface hydrophilicity enhances both adhesion energy and viscous dissipation, resulting in three typical post-coalescence outcomes: coalescence-induced jumping, departure from the nanodimple of the surface via wetting transition, and formation of larger Wenzel droplets adhering to nanodimpled surfaces. For coalescence-induced droplets jumping, the liquid bridge radius with coalesced time satisfies Rb ∼ tc1/2, consistent with an inertial-limited-viscous mechanism. Increasing droplet size leads to a mismatch with nanodimpled geometry, causing disordered internal flow and elevated viscous losses, which significantly reduce the jumping kinetic energy. For example, when the droplet size increases from 3 to 9 nm, the energy conversion efficiency decreases markedly from 6.1% to 0.6%. Furthermore, decreasing droplet radius ratios would make the smaller droplet retract earlier under the action of the surface tension, and coalescence-induced jumping occurs only when the droplet radius ratio ranges between 0.65 and 1.