The shape and structure of a granular jet depends on the ambient air pressure
The above movie was shot at a few thousand frames per second with a Phantom v7 high-speed video camera. It is played back much more slowly so the details of the jet become observable with the naked eye. At the beginning of the experiment, a 1" diameter steel sphere is dropped into fine, loose sand from a height of about 1m. The sand consists of 50micron diameter glass spheres. Prior to the sphere drop, the bed of sand was fluidized and brought into its loose state by circulating nitrogen gas through it from below. The gas flow was then stopped and the system evacuated to the desired pressure. Results for three different air pressures are shown.
copyright: John Royer, Eric Corwin, Heinrich Jaeger, University of Chicago, 2005