Journal of Conference Abstracts

A Phase-transition between 'Gas-like' Dynamics and Elastic-plastic Deformation in Granular Materials

Einat Aharonov (einat@ldeo.columbia.edu) & Dave Sparks (sparks@ldeo.columbia.edu)

Lamont-Doherty Earth Observatory, Rt 9w, Palisades, NY 10964, U.S.A.

Although the dynamics of granular materials have long been a topic of research, they still defy quantitative understanding. One of the difficulties in understanding the dynamics of a collection of grains arises because there is no single, simple rheological description: At dense grain packing stresses are transmitted by long-lasting frictional contacts, while at low volume fractions, stresses are primarily transmitted by short-lived collisions. In shearing granular material the packing density can change significantly in both space and time.

We performed systematic two-dimensional discrete element modeling of granular media undergoing shearing. We find a transition between the frictional and collisional regimes which is characterized by a change in the average properties of the shearing material. This transition has the characteristics of a "melting-freezing" phase-transition, similar to that observed in other physical materials undergoing a disorder-order transition, such as thin liquid films. We will show that shearing of a granular media with a high volume fraction results in elastic-plastic behavior at low velocities, with power-law, stick-slip rearrangement events. Shearing of a low volume fraction results in gas-like behavior with underlying 'grain chaos'. We demonstrate that the stresses and stress-ratio in the system are a function of packing density and show different dependences on the shearing rate on the two sides of the phase transition. The ratio of shear to normal stress may be strongly dependent on velocity in the frictional regime, and shows no dependence on velocity in the collisional regime. Our results are in agreement with results from experiments in rapidly-shearing low-volume fraction granular media, which show no velocity dependence, and also with much slower, higher-volume fraction experiments simulating creep on faults, which show velocity-strengthening. We shall discuss the implications of our model results to large-scale geological systems, and in particular to landslides and fault-gouge.