  For a limited set of impact conditions, a drop impacting onto a pool can
entrap an air bubble as large as its own size. The subsequent rise and rupture
of this large bubble plays an important role in aerosol formation and gas
transport at the air-sea interface. The large bubble is formed when the impact
crater closes up near the pool surface and is known to occur only for drops
which are prolate at impact. Herein we use experiments and numerical
simulations to show that a concentrated vortex ring, produced in the neck
between the drop and pool, controls the crater deformations and pinch-off.
However, it is not the strongest vortex rings which are responsible for the
large bubbles, as they interact too strongly with the pool surface and
self-destruct. Rather, it is somewhat weaker vortices which can deform the
deeper craters, which manage to pinch off the large bubbles. These observations
also explain why the strongest and most penetrating vortex rings emerging from
drop impacts, are not produced by oblate drops but by more prolate drop shapes,
as had been observed in previous experiments.
