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Shaking up powder physics.

Shaking and tapping a container of flour to compress the powder into a smaller volume is a familiar kitchen technique. But scientists have until now lacked theoretical models that explain on a microscopic level how and why this volume change occurs -- and why, in some instances, shaking and tapping can actually increase a powder's volume. In the July 15 PHYSICAL REVIEW LETTERS, Anita Mehta of the Cavendish Laboratory in Cambridge, England, and G.C. Barker of the AFRC Institute of Food Research in Norwich, England, report success in modeling what happens to individual grains in a pile of powder subjected to vibration.

In their theoretical model, the researchers represent a granular pile as an assembly of grain clusters. Strong vibrations carrying sufficient energy can force the ejection of a grain from one cluster, which leads to its subsequent capture by a neighboring cluster. In contrast, the energy of weak vibrations goes into the reorganization of grains within a cluster to minimize voids. Three-dimensional computer simulations based on this model reveal how these two energy-dependent competing effects determine a vibrated pile's final slope and roughness, and change a powder's bulk properties and collective behavior. The results show that for a given type of vibration, differences in the vibration intensity can promote grain reorganizations that change the powder's volume. Small vibrations tend to decrease a powder's volume, whereas large vibrations force a substantial restructuring of the powder that may typically lead to an increased volume.
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Title Annotation:a model of what happens to individual grains in a pile of powder subjected to vibration
Publication:Science News
Date:Jul 27, 1991
Words:243
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