  Using an electrical method and high-speed imaging we probe drop coalescence
down to 10 ns after the drops touch. By varying the liquid viscosity over two
decades, we conclude that at sufficiently low approach velocity where
deformation is not present, the drops coalesce with an unexpectedly late
crossover time between a regime dominated by viscous and one dominated by
inertial effects. We argue that the late crossover, not accounted for in the
theory, can be explained by an appropriate choice of length-scales present in
the flow geometry.
