Pre-avalanche dynamics

Coworkers: G. Debregeas, J.-M. di Meglio, T.J. Senden

We use an X-Ray imaging technique to probe the stability of an inclined granular pack in a rotating drum geometry. In agreement with previous numerical studies, we observe an intermittent dynamics in the pack well before the surface reaches the critical avalanche angle. We show that this pre-avalanche dynamics corresponds to a progressive mechanical adaptation of the structure of the pack in the vicinity of the surface, with a characteristic depth being controlled by the system size. The observed behavior proved to be independent of the material and grains shape.

Setup

Experiments are done in a cylindrical plexiglass drum 55mm in diameter with an internal space of 55 mm in thickness. The drum is half filled with several types of grains. To prevent a solid rotation of the sample, a disordered monolayer of grains is glued to the inner spacer. The drum is co-axially mounted inside the central aperture of a precision rotation stage. The motion of the stage allows for very smooth rotation of the drum using continuous acceleration and a maximal rotation speed of 0.5°/sec. The micro-focus X-ray source is placed 2300 cm from the drum and provides a weakly divergent beam which crosses the stage aperture, producing a radiograph on a scintillator-coupled CCD camera.

Typical experiment: