Bouncing Dynamics of Oblique Droplet Impact on a Liquid Film

In this study, we have investigated the dynamics of a droplet oblique impact onto a liquid surface with identical liquid properties. Utilizing high-speed photography, we systematically examine the influences of the impact angle (θ), dimensionless film thickness (), and Weber number (We) on the transitions between various impact outcomes. Here, bouncing is observed at lower We values across a wide range of θ. As We increases, in a narrow range of We values, non-bouncing occurs in the intermediate range of θ, showing a nonmonotonic transition from bouncing to non-bouncing to bouncing by decreasing θ from 90. By further increasing We, bouncing only occurs when θ < 20. When increases to deep pool mode, the bouncing regime is substantially promoted and the nonmonotonic transition between bouncing and non-bouncing regimes is substantially suppressed. Regarding the post-impact behavior of the droplet, we have demonstrated the nonmonotonic variation of the deviation angle (θ) by increasing the impact angle. Across all depicted dimensionless film thicknesses, the restitution coefficient (e) consistently exhibits a decreasing trend as the impact angle increases, indicating lesser energy dissipation during more oblique impacts. To better understand the underlying mechanisms of these experimental observations, three-dimensional numerical simulations were performed using the open-source Basilisk solver. The effects of the key parameters on the deformation of the droplet and liquid film during the impact process are characterized. Furthermore, the energy budget, including the kinetic and surface energies and viscous dissipation in the droplet and the film, is calculated, which sheds light on the variations in the angle difference and restitution coefficient with impact angle.