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Impact on soft sand: Void collapse and jet formation

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Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)

Abstract

A steel ball dropped on loose, very fine sand creates a jet which can exceed the release height of the ball. Our experiments show that the series of events is as follows: On impact of the ball, sand is blown away in all directions (``splash'') and an impact crater forms. When this cavity is collapsing, a jet emerges and is driven straight into the air. In the sand, an air bubble has formed which rises slowly towards the top, causing a granular eruption. We address the problem and the phenomenon by first comparing it with similar effects in liquid. We then show the results of molecular dynamics simulations and construct a simple continuum model for this system which shows the same features as seen in experiment. Finally, we examine the role of the air for the phenomenon and present results on impact events on the same sand in vaccuum. Surprisingly, the jet is getting much weaker.

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