Hydrodynamics of Drop Coalescence

Hydrodynamics of drop coalescence has been studied theoretically and numerically by solving Navier Stokes equation considering a single fluid after the first touch. Many experiments documented bridge growth with the use of high speed videography and electrical method. However, internal fluid motion during coalescence has not been extensively studied, in part due to the spherical shape of the drop. Here we observed overall fluid motion (except at the site of early coalescence) using particle image velocimetry for 2D coalescence. We observed that fluid motion inside the bulk drops are inertial and governing fluid flow in the bridge region is one dimensional. At the merging interface, incoming liquids join and coflow in the perpendicular direction. We extended our observation to 3D coalescence and formulated a new scaling law encompassing all viscous liquids. We validated scaling argument at early stage of coalescence through experimentation. Finally, we conclude that while flow in the bulk drops is inertial, the dominant resistance comes through viscous effect in the merging interface region and at the lesser extent in the bridge region. Early dynamics of drop coalescence was dominated by viscous flow and possible crossover from viscous to inertial depends on the Ohnesorge number.