21 August
2003
Against the grain: Brownian motion in a non-equilibrium system
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Back in the 1920s, Walter Gerlach and E. Lehrer observed rotational brownian motion of a fine wire immersed in an equilibrium environment, a gas. Their simple experiment led to a key idea of equilibrium statistical mechanics, that the complex many-particle problem of particles colliding with the wire can be represented by two macroscopic parameters, viscosity and temperature. Can this idea also describe systems far from equilibrium? A non-equilibrium version of the classic experiment (see cover) gives the answer "yes", finding effective values for these parameters. A sensitive torsion oscillator was suspended in millimetre-sized grains, fluidized by strong external vibrations. The oscillator acts as an elastically bound brownian particle, and the vibrated grains as the non-equilibrium medium.
Observing
brownian motion in vibration-fluidized granular matter
G. D'ANNA, P. MAYOR, A. BARRAT, V. LORETO & FRANCO
NORI
Nature 424, 909–912 (2003);
doi:10.1038/nature01867
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Granular materials: Shaken sand — a granular
fluid?
PAUL UMBANHOWAR
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.
Nature 424, 886–887 (2003);
doi:10.1038/424886a
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