Molecular Dynamics Simulation of Water Nanodroplet Bounce Back from Flat and Nanopillared Surface.

Molecular dynamics simulations of impinging nanodroplets were performed to study the bounce-back condition for flat and nanopillared surfaces. We found that the bounce-back condition can be closely related to the degree of droplet deformation upon collision with the solid surface. When the droplets have little or small deformation, the bounce-back condition solely depends on the hydrophobicity of the surface. On the other hand, when the droplet deformation is large, the impinging velocity dependence of the bounce-back condition becomes stronger due to the increase of the liquid-vapor interfacial area of colliding droplet, which is proportional to the liquid-vapor surface energy. The impinging droplet simulations with nanopillared hydrophobic surfaces were also performed. The contribution of droplet deformation in this case is relatively small because the surface hydrophobicity is enhanced due to the existence of pillars. Finally, we find that the maximum spreading diameter of the impinging droplets exhibits a consistent trend, in terms of the Weber number dependence, as the experimental measurements with macrodroplets.