EFFECTS OF AIR ON SPLASHING DURING A LARGE DROPLET IMPACT: EXPERIMENTAL AND NUMERICAL INVESTIGATIONS

Recent studies have shown the importance of air in causing the splashing phenomenon (L. Xu, W. Zhang, and S. R. Nagel, Phys. Rev. Lett., vol. 94, 184505, 2005) and the subsequent finger formation for a large-scale liquid droplet impact. The experimental investigation and the relevant computational modeling have been performed to obtain additional insight on the large-scale splashing phenomenon. Previous modeling efforts did not consider the effect of air by starting the simulation at the time of droplet-liquid contact with the substrate. Here we start the simulation using the volume of fluid method at a location one diameter upstream so that the compressed air effect due to a falling droplet is properly taken into account. Both the experiments and simulations demonstrate that the displaced air obtains momentum from a falling droplet and induces a vortex motion right above the contact surface. The splashing (or ejection) occurs when the initial edge of the impacting and spreading liquid is entrained into the displaced and accelerated air. It is also hypothesized that the perturbation generated during the splashing process is radially propagated and is the fundamental instability that eventually forms fingers at the rim of the spreading liquid.