Mechanics Seminar, MIT
Monday, November 29, 2004
4-5pm in room 3-370

Title:  The Dynamics of Random Packings in Granular Flow

Speaker: Martin Z. Bazant (Mathematics, MIT)

Abstract:

How do random packings flow? Dilute ``packings'' (gases) flow by the
accumulated effect of independent, random collisions. Dense, ordered
packings (crystals) flow collectively via the motion of defects, such
as vacancies, interstitials, and dislocations. Similarly, existing
theories of the dense, disordered packings in granular drainage are
based on either gas-like inelastic collisions or crystal-like void
diffusion, but experiments show that a fundamentally different
approach is needed. Here, we propose that dense random packings flow
cooperatively in response to diffusing ``spots'' of free volume. The
Spot Model is very simple to simulate and may be analyzed in the
continuum limit (via a non-local stochastic differential
equation). With only a few fitting parameters, it predicts the mean
flow, spatial velocity correlations, cage breaking, diffusion, and
packing structure, in good agreement with experiments and molecular
dynamics simulations. The results suggest that flowing random packings
have a universal structural features.