  The impact of droplets on a liquid pool is ubiquitous in nature and important
in many industrial applications. A droplet impacting on a liquid pool can
result in the pinch-off of a regular bubble or entrap a large bubble under
certain impact conditions. In this study, the cavity deformation and the bubble
entrapment during the impact of droplets on a liquid pool are studied by
combined experimental measurements and numerical simulations. The time
evolution of the free surface profile obtained in the numerical simulation is
in good agreement with the experimental results. The cavity created by the
droplet impact affects the pinch-off of regular bubbles and the entrapment of
large bubbles. The regular bubble pinch-off is the direct consequence of the
capillary wave propagating downward along the interface of the cavity and
merging at the bottom of the cavity. In contrast, the large bubble entrapment
is due to the merging of the liquid crowns at the mouth of the cavity. Gravity
and environmental pressure play important roles in cavity deformation and
bubble entrapment after droplet impact on liquid pools. The maximum depth of
the cavity decreases as the gravitational effect becomes stronger. The
phenomenon of regular bubble pinch-off may disappear as the gravity decreases.
We find that the regular bubble pinch-off can transform into large bubble
entrapment when increasing environmental pressure. The size of the large bubble
entrapped decreases with increasing the environmental pressure due to the
larger difference in the pressure and the vorticity around the liquid crown at
increased environmental pressure. Finally, the regime map of bubble entrapment
after the droplet impact is obtained.
