Motion of a merged drop upon impacting a liquid pool

Drop impact on liquid films is relevant to many natural and industrial processes. Examples include the impact of sprayed fuel droplets on the fuel films formed from previously deposited droplets on the walls and piston head of internal combustion engines, and the impact of rain drops on ocean and wetted soil surfaces. Since the impacted drop can contain liquids with different temperature and chemical species than the impacted film, it is the motion of the merged drop and the associated mixing that initiates many post-merging thermal and chemical processes. In this paper we report an experimental study on the morphological evolution of an impacting drop inside the liquid pool after coalescence, using high speed laser induced fluorescence technique. We shall show that the drop motion can be divided into three stages, where inertia, capillary and viscous effects alternatingly play dominant roles to characteristically modulate the drop penetration. Furthermore, we shall also evaluate the relevant time and length scales of these three stages, and propose a unified description of the drop motion in the impacted liquid. Finally, we will use the experimental data from a wide range of drop diameter and impact speed to validate the proposed scaling.