Coalescence of a compound drop with a simple drop

In this study, the coalescence dynamics of a compound drop and a simple drop of similar liquids is numerically investigated by the coupled level set and volume-of-fluid method. Computations are performed at a fixed Bond number for a wide range of Ohnesorge numbers (Oh). Various pinch-off regimes are identified based on the non-dimensional parameters. The results reveal that for small-sized compound drops, the coalescence phenomenon is somewhat analogous to that of a simple drop. However, with the increase in size of the bubble beyond a critical value, the bubble hovers in the neck region, restricting the pinch-off process. The influence of surface tension force on the coalescing drops reduces with an increase in Oh; pinch-off process is observed up to Oh=0.0125. Furthermore, six distinct full and partial coalescence mechanisms with and without bubble burst are observed as depicted in a regime map. The interface dynamics of these processes illustrate that the bubble ruptures due to the combined effect of buoyancy, viscous drag, and coalescence-induced push from the parent drops, particularly at low values of Oh.