Rene' Mikkelsen, Michel Versluis, Devaraj van der Meer, Ko van der Weele, and Detlef Lohse

A steel ball dropped onto loose, very fine sand creates an upward jet exceeding the release height of the
ball. It is generated by the gravity-driven collapse of the void created by the ball: the focused pressure
pushes the sand straight up into the air. Using a 2-dimensional experimental setup and high-speed imaging,
the collapse of the void is visualized. For high impact velocities the void collapse is seen to entrain
air. These experimental observations are accounted for within a Rayleigh-Plesset type model. The entrained
air bubble slowly rises through the sand, and upon reaching the surface causes a granular eruption.