Morpho-dynamic evolution due to inertia-mediated impact of a compound drop on a deep liquid pool

A compound drop impacting on a liquid pool exhibits intriguing coalescence patterns that are primarily attributable to the complicated interplay of inertia with other physical parameters such as radius ratio of core to shell drop and density and viscosity contrasts of the two fluids. By executing comprehensive numerical investigations, here, we identify three different regimes based on the radius ratio of compound drop, viz., secondary drop pinch-off without bubble bursting, secondary drop pinch-off after bubble bursting, and compound breakage. Our findings also depict a transition in the shape of a secondary droplet from prolate to oblate or vice versa, a phenomenon non-trivially culminated by secondary drop pinch-off timing, neck radius, and amplitude and wavelength of capillary wave propagation. Our results bring out the fact that higher wavelength and amplitude of capillary waves are responsible for secondary drop pinch-off without bubble bursting. Furthermore, in the case of highly viscous core drop and surrounding fluid, we observe both complete and partial coalescence phenomena, which are critically dictated by the confluence of inertia and radius ratio of a compound drop leading to three different regimes, viz., complete coalescence without bubble bursting, complete coalescence with bubble bursting, and partial coalescence with bubble bursting, distinctively different from the observations for single droplet based investigations reported in earlier studies. These implications are likely to be beneficial in illustrating the physical functionalities accompanying the targeted release of encapsulated biological or pathological entities when they are transmitted under the action of an inertial force into another fluidic medium, a paradigm that has hitherto remained unexplored.