The impact of solid objects into a granular bed has been studied since the 1800’s, yet many fundamental questions remain unresolved.  The complex response of the bed to sudden impact differs substantially from impact in ordinary solids and liquids.  We investigate how this response is mediated by the presence of interstitial gas between the grains.  Using high-speed x-ray radiography we track the motion of a steel sphere through the interior of a bed of fine, loose granular material.  We find a crossover from nearly incompressible, fluid-like behavior at atmospheric pressure to a highly compressible, dissipative response once most of the gas is evacuated. 

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1. •John R. Royer, Eric I. Corwin, Peter J. Eng, Heinrich M. Jaeger, “Gas-Mediated Impact Dynamics in Fine-Grained Granular Materials”, Phys. Rev. Lett. 99, 038003 (2007).   cond-mat/0702339