Coalescence dynamics of unequal sized drops

Abstract

In this study, coalescence dynamics of two unequal sized drops of the same liquid have been investigated using the coupled level set and volume of fluid method. A broad range of fluid properties is considered with two orders of magnitude variation of Ohnesorge numbers and Atwood number ranging between 0.01 and 0.9976. The pinch-off process and controlling parameters that lead to satellite generation have been investigated. The capillary waves are generated as a result of the sharp curvature produced near the contact region. Here we demonstrate that the capillary waves propagating along the interface of the lower drop can affect the eventual pinch-off of the satellite. The local curvature of the neck plays a crucial role in the pinch-off process. A sharper axial curvature of the neck increases the local capillary pressure which 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 the gravity force. The critical ratio can be as low as 1.2 at a lower relative strength of the viscous and the gravity forces. The coalescence of two unequal sized drops may produce much smaller satellite drops, on account of coalescence in successive steps, with or without intermediate detachment