Satellite formation during the coalescence of unequal sized drops

Contrary to intuitive expectations, the coalescence of two unequal sized drops does not necessarily result in their complete convergence into a single larger mass in an effort to minimize the surface energy. Rather, the process often pinches off a smaller daughter drop, also known as satellite drop. We have performed comprehensive numerical investigations using Coupled Level Set and Volume of Fluid (CLSVOF) method to unveil the mechanism of partial coalescence. The capillary waves are generated owing to the sharp curvature produced near the contact region. We reveal that the deformation of the bottom drop because of the downward pull generated by the capillary waves propagating along its interface affect the pinch-off dynamics. Local curvature of the neck plays a crucial role in the pinch-off dynamics. A sharper axial curvature of the neck increases the local capillary pressure and restricts the pinch-off. The critical diameter ratio above which a satellite pinches off during the coalescence of two free falling drops increases with increasing relative strength of the viscous force and gravity force. The critical diameter ratio for satellite generation can be as low as 1.2 at lower strength of gravity and viscous forces.