Fast x-ray pictures of sand jets

Granular materials, which have both solid-like and liquidlike characteristics, can exhibit unexpected behavior even in the simplest experiments. When, for example, researchers at the University of Chicago dropped a heavy sphere into a bed of loose fine sand, the results surprised them. They used high-speed video to view the splashdown (left image) and resulting jets (right), and discovered that in a vacuum, the jet was much shorter and thinner than in ambient conditions (shown here). Furthermore, as the gas pressure was reduced toward a vacuum, a two-stage structure emerged with a more pin-like jet perched atop a thicker one. The physicists then combined the video with high-speed x-ray radiographs taken at the Advanced Photon Source at Argonne National Laboratory to view the interior of the sand bed as the jet formed. The x rays revealed that simple gravity-driven collapse of the void produced by the descending ball could explain the fine jet but not the large one. Further examination showed that as the void pinched off near the top and sand filled it in from the sides, enclosed gas was pressurized and drove the large jet up and out. (J. R. Royer et al., Nat. Phys. 1 , 164, 2005 http://dx.doi.org/10.1038/nphys175 .)