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Great shakes: why pebbles wind up atop sand.

As just about any devotee of the sandbox has observed, vigorously shaking a bucket that holds a dry mixture of pebbles and sand inevitably brings the pebbles to the top. Other mixtures of granular materials show the same segregation effect when shaken. Even if the components differ only in size and not in composition or density, the larger particles usually end up on top.

Now, researchers have obtained experimental evidence that this phenomenon involves more than just the local rearrangement of particles. Vertically vibrating a cylinder packed with glass beads induces a collective stop-and-go motion resembling a convection cycle -- that carries beads upward through the cylinder's middle and downward in a tbin layer along its inner surface.

Tbus, a large glass bead immersed in smaller beads can readily travel upward. But when it reaches the top, it can't join in the much narrower downward flow of beads along the wall. It gets trapped at the top.

James B. Knight, Heinrich M. Jaeger, and Sidney R. Nagel of the University of Chicago describe their experiments in the June 14 PHYSICAL REVIEW LETTERS.

The researchers used a glass cylinder, 35 millimeters in diameter, mounted on a shaker that gave it a vertical shove, or "tap," once per second. Open at the top, the cylinder was filled with spherical, 2-millimeter-wide glass beads, some of which were dyed so that their motion could be observed.

Previous computer simulations had suggested that vibrations would create substantial voids beneath any large particles present. Smaller particles would then slip into these voids during subsequent shakings. These rearrangements would slowly drive the large particles upward (SN: 8/8/92, p.86).

"What we found was completely different," Nagel says.

Nagel and his co-workers attribute the convective motion they observed to friction between the cylinder wail and adjacent glass beads. Earlier computer simulations had failed to take this factor into consideration.

The researchers also designed and tested a container with slanting sides that reversed the direction of the beads' motion (see diagram). In this conical container, a large bead descends to the bottom and stays there.

Many questions remain unanswered, including what effect a cylinder's width may have on the motion of the beads. Sadly, packagers of mixed nuts don't have enough information yet to select a tin of just the right shape to keep their product thoroughly mixed.
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Title Annotation:larger particles move to top of cylinder due to convection motion caused by friction between cylinder wall and adjacent particles
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Jun 26, 1993
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