Service de Physique de l'Etat Condensé

Granular matter
Olivier Dauchot logo_tutelle 

Introduction


Dry granular materials are very simple: they are large conglomerations of discrete macroscopic particles. If they are non-cohesive, then the forces between them are essentially only repulsive so that the shape of the material is determined by external boundaries and gravity.

Yet despite this seeming simplicity, granular materials behave differently from any of the other standard and familiar forms of matter : solids, liquids or gases, and should therefore be considered as an additional state of matter in its own right. At the root of this unique status are three important aspects: the existence of static friction, the fact that temperature is effectively zero and, for moving grains, the inelastic nature of their collisions. In some cases, such as a sandpile at rest with a slope less than the angle of repose, static friction produces solid-like behavior: the material remains at rest even though gravitational forces create macroscopic stresses on its surface. If the pile is tilted several degrees above the angle of repose grains start to flow, like in a fluid.
However, this flow is clearly not that of an ordinary fluid because it only exists in a boundary layer at the pile's surface. Also, unlike in an ordinary fluid, kT plays no role in a granular material and entropy considerations can easily be outweighted by dynamical effects that now become of paramount importance. Unless perturbed by external disturbances, each metastable configuration of the material will last indefinitely, and no thermal averaging over nearby configurations will take place. Because each configuration has its unique properties, the reproducibility of granular behavior, even on large scales and certainly near the static limit, can only be defined in terms of ensemble averages.

In order to better understand the dynamical and rheological properties of granular materials, we have investigated surface flows in a rotating drum, then studied the response of a granular pile to a localised surface disturbance. More recently, we have considered the relaxation of a granular pile, following the occurence of an avalanche. Also, we have looked at some application to dunes dynamics and industrial powders flows.

 

Major results


Surface flows
We investigate steady granular surface flows in a rotating drum, measuring the velocity profile in the flowing layer at the center of the drum, the flowing layer thickness and the static/flowing boundary profiles (to learn more about it...)

Relaxation and response to a localized perturbation
The micro-displacements generated by a small localized overload at the free surface are visualized experimentally inside a packing of steel beads. For a triangular packing, beads rearrangements remain confined in two inverted triangles on both sides of the applied overload. This pattern disappears for stronger disorder. We have studied the relaxation of the pile towards mechanical equilibrium following an avalanche. We first have shown that the dynamics exhibits non-trivial relaxations and is much slower than expected given the "microscopic" time-scale. Perturbation experiments have allowed us to confirm the emergence of strong spatial correlations when increasing the pile slope, which are reminiscent of the clusters that we observed in flows. (to learn more about it...)

Glassy behaviour and dynamical heterogeneities in granular media
Following tracers, we show that the particle motion is subdiffusive and that the trajectories exhibit strong cage effect of the kind observed in colloidal glasses. We also characterize the relaxation heterogeneities and demonstrate that the rigidity transition is of a critical kind (to learn more about it ...)

Applications: dunes dynamics and metallic powders
We have set up a wind channel where to conduct experiments with sand and we have shown experimentally that an initial sand pile, under a wind flow charged in sand, flattens and exhibits a typical barchan shape before disappearing. In collaboration with CEA/Grenoble (DRT/DTEN), we have studied problems related to metallic powder flows in industrial processes. (to learn more about it ...)

 

Maj : 04/11/2009 (289)

 

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