Experiments on drop impact and coalescence at a liquid/liquid interface

The gravity-driven impact and coalescence of a single drop through a liquid/liquid interface have been investigated experimentally. PIV images were obtained with a high frequency pulsed-laser and a high-speed video camera. Index-matching and a slight camera inclination were employed to eliminate optical distortion. The drop Reynolds numbers based on impact velocity and ambient viscosity were 20 and 68. During the approach towards the interface, viscous stresses generated a vortex ring inside each drop and a wake behind it. The subsequent deformation of the drop and the interface due to impact were observed to be more radical in the higher Re case. Both the wake shearing on the upper drop surface and interfacial deformation were significant in dissipating the vorticity inside the drops. After the vorticity dissipated, an outward flow developed along one radian between the 'resting' drop and the interface below generating a counter-rotating vortex pair. The eventual coalescence was typically off axis. During coalescence, a capillary ring wave propagated radially inward enhancing the collapse of the drop fluid into the bulk liquid below.