Impact dynamics: Void collapse and jet formation

Devaraj van der Meer, University of Twente

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.