  The coalescence of liquid drops has conventionally been thought to have just
two regimes when the drops are brought together slowly in vacuum or air: a
viscous regime corresponding to the Stokes-flow limit and a later
inertially-dominated regime. Recent work [Proc. Natl. Acad. Sci. 109, 6857
(2012)] found that the Stokes-flow limit cannot be reached in the early moments
of coalescence, because the inertia of the drops cannot be neglected then.
Instead, the drops are described by an "inertially limited viscous" regime,
where surface tension, inertia, and viscous forces all balance. The dynamics
continue in this regime until either viscosity or inertia dominate on their
own. I use an ultrafast electrical method and high-speed imaging to provide a
detailed description of coalescence near the moment of contact for drops that
approach at low speed and coalesce as undeformed spheres. These measurements
support a description of coalescence having three regimes. Signatures both
before and after contact identify a threshold approach-speed for deformation of
the drops by the ambient gas.
