We examine the impact of a solid sphere into a fine-grained granular bed. Using highspeed X-ray radiography we track both the motion of the sphere and local changes in the bed packing fraction. Varying the initial packing density as well as the ambient gas pressure, we find a complete reversal in the effect of interstitial gas on the impact response of the bed. The dynamic coupling between gas and grains allows for easier penetration in initially loose beds but impedes penetration in more densely packed beds.

As a result, when there is interstitial gas is present the solid sphere sinks deeper into a loosely packed bed than when the gas is removed, but sinks less deep into a more densely packed bed than when the gas is removed!   High-speed imaging of the local packing density shows that these seemingly incongruous effects have a common origin in the resistance to bed packing changes caused by interstitial air. Effectively, interstitial gas (e.g., air) tends to oppose any change in packing fraction, whether it is dilation or compaction.

This initially counter-intuitive phenomenon is shown in the images below, assembled from fast x-ray radiographs of the interior of the granular bed, taken at Argonne's APS (the image above is the second row of the complete set below). The colors correspond to the bed packing density: yellow-green = initial packing fraction before the impact, orange to red = compaction, blue = dilation. Time increases left to right, with the far left image corresponding to the initial impact of the sphere. Top row: loose bed, with air (sphere drills deep hole into bed). 2nd row: loose bed, no air (notice the shock-like compaction front). 3rd row: dense bed, with air (sphere dilates bed but stops quickly); 4th row: dense bed, no air (sphere dilates bed more, stops later)

1. •John R. Royer, Bryan Conyers, Eric I. Corwin, Peter J. Eng and Heinrich M. Jaeger, “The Role of Interstitial Gas in Determining the Impact Response of Granular Beds”, Europhysics Letters 93, 28008 (2011). pdf