Fast X-Ray Pictures of Sand Jets

Granular materials -- possessing both solid-like and liquid-like characteristics -- exhibit much strange emergent behavior even in the simplest of experiments. When, for example, a heavy sphere is dropped into a bed of sand, what happens, if you look carefully enough, can still surprise seasoned researchers.

Heinrich Jaeger of the University of Chicago and his colleagues watched the jets kicked up by the sphere: they used high-speed video and ordinary light to view the outside of the jets and high-speed radiography -- with X-rays supplied by the Advanced Photon Source at Argonne National Laboratory in Illinois -- of the jet interior. The impact kicked up a bizarre two-tiered jet structure: a thick shaft at the bottom and, projecting up out of the top, a further and thinner shaft (see figures on the Jaeger group's Web page).

That the jets are so well collimated is a surprise: why doesn’t the sand just fly out at all angles? In moving up in a sort of directed beam, with very little lateral motion, it seems to act like an ultracold gas, at least in the sideways direction. Another surprise is the twofold jet structure. The lower, thicker jet is surely sculpted by collisions between sand grains and air molecules, since it gets progressively scantier until, at pressures close to vacuum, it goes away altogether, leaving only the thinner spiky jet.

The jet interior pictures are unprecedented: taken with an exposure rate of 5000 frames per second, the X-ray flux provided the equivalent of a 50-watt halogen lamp illumination -- only at x-ray wavelengths. The X-ray pictures proved that air squeezed among the grains was the driving force in forcing up the thick stage of the jet formation, and not as one might have expected a force for dissipating the jet.

Royer et al., Nature Physics, December 2005.
Nature Physics is a new journal that began publication in October 2005.

The Jaeger group's Web page