  When two liquid drops touch, a microscopic connecting liquid bridge forms and
rapidly grows as the two drops merge into one. Whereas coalescence has been
thoroughly studied when drops coalesce in vacuum or air, many important
situations involve coalescence in a dense surrounding fluid, such as oil
coalescence in brine. Here we study the merging of gas bubbles and liquid drops
in an external fluid. Our data indicate that the flows occur over much larger
length scales in the outer fluid than inside the drops themselves. Thus we find
that the asymptotic early regime is always dominated by the viscosity of the
drops, independent of the external fluid. A phase diagram showing the
crossovers into the different possible late-time dynamics identifies a
dimensionless number that signifies when the external viscosity can be
important.
