Granular jets

Dr. Devaraj van der Meer, Physics of Fluids, Twente University

A steel ball dropped onto loose, very fine sand ("dry quicksand")
creates an upward jet exceeding the release height of the ball. There is
a striking similarity with the impact of an object in a liquid: The jet
is generated by the gravity-driven collapse of the void created by the
ball, and 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. The entrained air bubble slowly rises
through the sand, and upon reaching the surface causes a granular
eruption. The experimental observations are quantitatively explained by
a Rayleigh-type model. Parallels are drawn with impacts on a planetary
scale.