Rotational separation after temporary coalescence in binary droplet collisions

Collision between two droplets plays a critical role in a wide range of processes, including raindrop formation, nuclear reactions, atomization and spraying in combustors, and various cooling, coating, and painting techniques. It is known that when two droplets collide nearly head-on, they may coalesce temporarily and then separate when the impact energy is so large that the rebounding motions of internal flows tend to stretch out and break the merged droplets. If the impact is sufficiently off-center, however, two distinct mechanisms have been argued to cause breakup exclusively. That is, separation has been reported to occur above a threshold of increasing impact angle (as characterized by an impact parameter, B), due to either stretching or rotational dynamics, whereas only one of them is supposed to cause the transition from permanent coalescence to separation. Therefore, which one renders the sole mechanism leading to off-center separation is not clear in the literature. This has been a discrepancy in the past decades, considering the fact that both mechanisms have been used, respectively, in different studies to interpret and analyze the transition criteria. To resolve this ambiguity, here we demonstrate experimentally a new regime, named rotational separation, which is governed by the coupling of outer rotating flow and center rebounding flow in the tentatively united drops. This regime occurs at an intermediate B, in contrast to that dominated by stretching kinetics created at a slightly larger B. It thus indicates simultaneous existence of the two regimes but in different range on a phase diagram. Along with numerical simulations and physical models, we elucidate the two scenarios of off-center separations comprehensively and solve the long-standing puzzle.