How solid surface free energy determines coalescence-induced nanodroplet jumping: A molecular dynamics investigation

Molecular dynamics simulation is performed to investigate how solid surface free energy determines the coalescence-induced jumping of nanodroplet on superhydrophobic surfaces. The nanodroplet-jumping is found highly sensitive to the solid surface free energy represented by the fluid-solid bonding strength parameter β. The coalesced-nanodroplet fails to jump off the surface when β is 0.15 (contact angle being 145°) while succeeds to jump off the surface when β is 0.05 (contact angle being 175°). We find that a small proportion (ca. 2%-4%) of the surface free energy released in both cases is eventually converted to kinetic energy in the jumping direction, which is in the same order as the conversion efficiency previously predicted for microdroplets. A lower solid surface free energy decreases viscous and interfacial dissipation and hence increases the kinetic energy converted and eventually leads to sufficient kinetic energy in the jumping direction for nanodroplet to jump up. Our results also address the importance of the liquid-solid interaction in the coalescence-induced jumping of nanodroplets and the determination of the minimum size of jumping nanodroplets.