Contact Time of a Bouncing Nanodroplet

We study the bouncing dynamics of nanodroplets on superhydrophobic surfaces. We show that there are three velocity regimes with different scaling laws of the contact time, tau. Although tau remains constant over a wide velocity range, as seen for macroscale bouncing, we demonstrate that viscosity plays an essential role in nanodroplet bouncing even for low-viscosity fluids. We propose a new scaling tau similar to (p mu R-0(4)/gamma(2))(1/3) = (R-0/v(0))We(2/3)Re(-1/3) to characterize the viscosity effect, which agrees well with the simulated results for water and argon nanodroplets with various radii and hydrophobicities. We also find pancake bouncing of nanodroplets, which is responsible for an abruptly reduced tau in a high-velocity regime.