BALTIMORE, MARYLAND--From chess pieces on a board to nuts in a jar, a good shake ordinarily mixes things up. But vibrations can cause flowing beads to "freeze" into an orderly pattern like atoms in a crystal, a pair of physicists reports. The paradoxical observation could lead to deeper insights into disorderly solids such as glasses.
Pumping energy into a thing usually raises its temperature and jumbles its insides. For example, heating a crystalline solid such as ice scrambles the regular pattern of atoms or molecules within it, eventually causing it to melt into a disorganized liquid. But granular materials--in which grains, marbles, or other macroscopic objects act like atoms or molecules--often defy common sense notions of how matter should behave. In fact, physicists and engineers continue to puzzle over the properties of gravel, powdered medicine, and other grainy substances. Now, Karen Daniels of the North Carolina State University in Raleigh and Robert Behringer of Duke University in nearby Durham have shown that shaking can "freeze" spherical beads into an orderly pattern.
The researchers filled a can 50 centimeters wide and 10 centimeters deep with plastic beads measuring 2.5 millimeters in diameter. The bottom of the can was spring-loaded to gently squeeze the beads upward, and the top of the can rotated, causing the beads near the brim to flow randomly over those below. At the same time, the researchers shook the can up and down and used a high-speed camera to film the motion of the beads. They had hoped to study the poorly understood flow of the beads over one another, Daniels said. But to their surprise, when the shaking was sufficiently vigorous, the flow stopped, and the beads locked into a regular, 3-dimensional array like the atoms in a crystal, she told the meeting of the American Physical Society here yesterday.
"If you think of shaking as raising the temperature, then [the result] creates a cognitive paradox," says Jerry Gollub of Haverford College in Pennsylvania. Sidney Nagel of the University of Chicago in Illinois says that the extra energy of the shaking lets the jumbled grains seek out an orderly state from which they'd otherwise be cut off. Such a cut-off, or jamming, is thought to be key to forming glass, which is a disorderly solid that cannot quite reach an orderly crystalline state. So the surprisingly orderly grains could lead to insights into the puzzling physics of glass.
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