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
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
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