Statistical mechanics of static granular media
Idea
Granular materials like sand and sugar, pills and peanuts consist of particles
with diameters between 100 μm and a couple of cm. Typical batches of those
materials involves a large number (> 106) of individual particles.
Normally we are not interested in the behavior of individual particles, but
would want a statistical mechanics type description with
macroscopic state variables. The feasibility of such a description for
static granular media is at present an open question, though there are
encouraging first results.
Exploring the phase space with flow pulses
Experimental setup: During a flow pulse produced by a syringe pump
the granular sample expands and the particles move randomly. After the
pulse the bed sediments into a new mechanically stable configuration.
The individual particles in granular materials are massive enough that their
potential energy is at least 10 orders of magnitude larger than their thermal
energy. So the exploration of a phase space spanned by all the mechanically
stable configurations requires external, periodic driving. Here at the CNLD
we use flow pulses in a water-fluidized bed: during the flow pulse the
fluidized bed expands and the individual grains have a non-zero kinetic
energy.
After the pulse the particles sediment into a new mechanically stable
configuration with a volume fraction Φ between 0.55 and 0.63;
Φ can be controlled by the flow rate during the pulse.
This peparation method is
history independent,
which is a necessary (but not sufficient) condition for ergodicity.
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Last modified: Mon Jan 15 22:12:20 CST 2007
