Mechanism of secondary entrapment of bubble.*

When a drop strikes the water's free surface, a cavity forms beneath the surface. During the collapse of this cavity, a bubble entrapment is observed and this is generally called a "secondary bubble entrapment" or "irregular entrapment," since bubble formation may or may not occur under the same impact conditions. The mechanism of this secondary bubble entrapment differs slightly from the mechanism of the primary bubble entrapment (regular entrapment) mechanism. This is due to an increase in the Weber number and Froude number, which increases the maximum cavity depth and changes the cavity shape. This bubble formation is only possible if the crown collapse causes wavefronts to travel down the cavity walls at the same speed. These wavefronts arrest the cavity retraction slightly above the maximum cavity depth, steepen the cavity walls, and transform the cavity base into a single stepped cone. When the wavefronts converge near the cavity's axis of symmetry, the single stepped cone base pinches off as a bubble and leaves the cavity as a truncated cone. The speed of the wavefronts traveling down the cavity walls would probably depend on the drop shape at the time of impact on the free surface of the water.