When a marble or ball-bearing is dropped onto a bed of fine, loose sand, one first observes a broad splash of sand at impact. Then, a tall jet of granular material shoots up vertically. Experiments at the Chicago MRSEC in collaboration with researchers from the APS at Argonne have tracked the birth and evolution of these granular jets using the fastest x-ray based imaging performed to date (6000 video frames per second). The images show granular jets emerging from a bed of fine glass spheres after impact by a heavy steel sphere dropped from above. Results for four different ambient air pressures are given. Note the two-stage jet shape visible at intermediate pressures.
These jets are among the most spectacular manifestations of liquid-like behavior in granular materials. Although resembling similar phenomena in ordinary liquids, these jets form in the absence of any cohesive forces or surface tension yet their overall shape depends on the ambient pressure. The measurements at Argonne provide a new understanding of how granular jets are formed and establish how ambient gas pressure affects them.
 Formation of granular jets observed by high-speed X-ray radiography, J.R. Royer, E.I. Corwin, A. Flior, M.L. Cordero, M.L. Rivers, P.J. Eng, and H.M. Jaeger, Nature Physics, 1, 3, 164-167 (2005).