  Using high-speed photography coupled with optical interference, we
experimentally study the air entrapment during a liquid drop impacting a solid
substrate. We observe the formation of a compressed air film before the liquid
touches the substrate, with internal pressure considerably higher than the
atmospheric value. The degree of compression highly depends on the impact
velocity, as explained by balancing the liquid deceleration with the large
pressure of compressed air. After contact, the air film expands vertically at
the edge, reducing its pressure within a few tens of microseconds and producing
a thick rim on the perimeter. This thick-rimmed air film subsequently contracts
into an air bubble, governed by the complex interaction between surface
tension, inertia and viscous drag. Such a process is universally observed for
impacts above a few centimeters high.
