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See Inside July 2005

Grain Freeze

MALE AFRICAN ELEPHANT



BRYAN CHRISTIE DESIGN

Heat up most forms of matter, and they flow more easily. But grainy masses seem to be an exception. Bob Behringer and Karen Daniels of Duke University filled a cylindrical hopper with marble-sized plastic beads. At rest the beads resemble a crystalline solid, like ice. When the investigators sheared the beads by turning the top of the container, the added energy “melted” the crystal into a more jumbled, flowing state, as expected. But when they injected more energy by vibrating the hopper, the beads oddly “refroze.” Hard thumping causes the beads to momentarily achieve weightlessness, which may allow them to snuggle up closer together, the pair hypothesizes in the April 29 Physical Review Letters. The results contradict the intended goal of vibratory feeders for processing powders and grains, which “work under the presumption that shaking makes grains flow more readily,” notes grain expert Troy Shinbrot of Rutgers University.

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