Coalescence of immersed droplets on a substrate

The dynamics of droplets on substrates has a strong impact on microfluidic systems ranging from commercially available lab-on-chip systems to state of the art developments in open microfluidics. Coalescence of micro and nano droplets on a substrate has been studied extensively, but in previous studies the focus has been on the interface movement. Here, we use computer simulations to investigate coalescence of droplets immersed in another liquid, in an inertia-dominated regime and also investigate the droplet's internal flow field. It is found that qualitatively the dynamics is similar to coalescence in air, with the same self-similar growth laws. We here point out the ambiguity in the scaling argument for droplets of 90 degrees, that shows itself in the velocities. We show that droplets with a contact angle below 90 degrees exhibit a self-similar velocity field, and the corresponding scaling laws are identified. For drops of 90 degrees, however, it is shown that the velocity field has a more intricate structure that is beyond the usual scaling arguments invoked for coalescence.