Experimental analysis of the coalescence process via head-on collisions in a time-dependent flow

The present study experimentally investigates the mechanisms involved in the flow-induced coalescence process for two equal-sized drops (polybutadiene drops suspended in a polydimethylsiloxane matrix), by taking advantage of the capability of the computer-controlled “four-roll mill” to carry out head-on collisions. In this work, head-on collision experiments have been carried out for a time-dependent flow that is designed so that the force along the line of centers mimics the force history due to rotation of the two droplets in a glancing collision. One primary goal of these experiments is to assess the importance of global deformation of the drops in the coalescence process. Specifically, we seek to determine whether global deformation plays a role in the observation that coalescence often occurs during the portion of a glancing collision when the drops are actually being pulled apart by the external flow. By comparison of the results for head-on and glancing collisions, we find that coalescence occurs in an apparently identical fashion in spite of the fact that the overall shape of the drops must be different since the velocity gradient is steady during the glancing collision but time dependent in the head-on collision. Specifically, the (near) axisymmetric film drainage process achieved in a head-on collision is apparently a very good approximation to the same process in a nonaxisymmetric glancing collision, suggesting that the coalescence process is dominated by the time history of the force along the line of centers and is at least approximately independent of the degree of asymmetry in the overall collision process.