Coalescence Dynamics of a Hollow Drop*

When a drop (Fluid 1) falls through another fluid (Fluid 2) to eventually hit a pool of the same liquid (Fluid 1), after coalescence, the drop may produce a daughter drop by draining some liquid into the pool. This phenomenon of forming daughter drops of smaller volumes is called partial coalescence. Although satellite drop formation has been widely studied for the case of a drop coalescing in a liquid pool, the satellite formation during the coalescence of a hollow drop has remained unexplored. We have performed comprehensive numerical investigations using Coupled Level Set and Volume of Fluid (CLSVOF) method to unveil the mechanism of partial coalescence of a hollow drop on a liquid pool. The coalescence phenomenon depends on the volume of the bubble inside the drop which is analyzed in terms of the inner diameter to the outer diameter ratio of the drop. There exists a critical diameter ratio above which partial coalescence is inhibited. Moreover, the break-up of the drop is also observed depending on its diameter ratio. We have explored the critical values of appropriate non-dimensional parameters for which transition from partial to complete coalescence occurs.