Characterizing the wettability/non-wettability transition during drop impact

Recent experiments have shown a new bouncing mechanism on smooth surfaces where the drop is supported on a thin cushion of gas beneath it. Using incompressible high-resolution computations, we study drop impact dynamics at moderate speeds of impact. We restrict our attention to parameters where drop undergoes complete rebound and show that five distinct regimes can be identified in a Reynolds-Weber number phase diagram. In a broad sense, a stable gas cushion can be formed for low Reynolds number but for a wide range of Weber numbers. With increasing Reynolds/Stokes number, the gas film undergoes rupture either in an annular ring near the drop periphery or near the center of the drop. Each of the five regimes differs in the shape of the gas film near the impact surface. The exact transition boundary between wettability-independent and wettability-dependent bouncing is difficult to determine, but the simulations are found to be in good agreement with known scaling laws for initial deformation height, maximum spreading radius and minimum thickness of the gas film attained. Access to velocity and surface energy data allows us to characterize the energy exchange during the impact process. The obtained coefficient of restitution is again found to be in good agreement with experiments.