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Nature 424, 886-887 (21 August 2003) | doi:10.1038/424886a

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Granular materials: Shaken sand — a granular fluid?

Paul Umbanhowar1

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The connection between random grain motion and viscosity in shaken sand — a strongly non-equilibrium system — has been probed. Curiously, the link is similar to that found in an ordinary liquid in thermal equilibrium.

By measuring both the free and forced oscillations of a rigid pendulum immersed in an ordinary liquid, the temperature and viscosity of the liquid can be determined1. This is due, in part, to a relation from equilibrium statistical mechanics known as the fluctuation–dissipation theorem2, which, in a precursor to its modern form, was devised by Einstein3 to explain the diffusive Brownian motion of small particles suspended in liquids4.

  1. Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3112, USA.
    Email: umbanhowar@nwu.edu