For the granular medium, the researchers started with industrial-grade
blasting powder -- which consisted of tiny, fairly uniform glass beads.
The motion of the granules was recorded on videotape with a CCD
(charge-coupled device) camera and then analyzed one frame at a time.
"Initially, there is a stick-slip motion of the layer as a whole, as it
is dragged up the rising side of the cylinder to a critical angle where
it falls back to the bottom," Fried said. "But as the angular velocity
is increased, a wave-motion sets in, which creates a span-wise variation
in the thickness of the layer -- visible as bright and dark bands of
light transmitted through the granular material. The frontal patterns
resemble fingers."
Next, the researchers added some coarse sand to the glass beads. The
resulting fingers and wave patterns did not differ substantially from
what had been observed with the beads alone.
"Our results clearly demonstrate that fingers can form at the front of a
flowing granular medium even in the absence of segregation induced by
coarse, irregularly shaped particles," said Fried, who presented his
team's findings at the American Physical Society meeting, held March
21-26, in Atlanta. "This suggests that some other mechanism is
responsible."
Because the fingering patterns are similar to those seen in conventional
viscous fluids, Fried said, the explanation may lie in an effective
surface tension generated by cohesive forces between grains. "In fluids,
the frontal fingering instability is driven by a competition between
viscosity and surface tension," Fried said.
"Although granular media are commonly thought to be incapable of
sustaining surface tension, we cannot rule out the possibility that the
fingering patterns result from a similar competition between the
viscosity of the bulk medium and effective surface tension."
- By James E. Kloeppel
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