  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.
