Impact dynamics: Void collapse and jet formation
Abstract: A steel ball dropped onto loose, very fine sand 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, which is accounted for within a Rayleigh-type model. The entrained air bubble slowly rises through the sand, and upon reaching the surface causes a granular eruption.