Two types of avalanche behaviour in granular media

Abstract

The nature of the transition between static and flowing regimes in granular media 1,2 provides a key to understanding their dynamics. When a pile of sand starts flowing, avalanches occur on its inclined free surface. Previously, studies 3 of avalanches in granular media have considered the time series of avalanches in rotating drums 4 , or in piles continuously fed with material. Here we investigate single avalanches created by perturbing a static layer of glass beads on a rough inclined plane. We observe two distinct types of avalanche, with evidence for different underlying physical mechanisms. Perturbing a thin layer results in an avalanche propagating downhill and also laterally owing to collisions between neighbouring grains, causing triangular tracks; perturbing a thick layer results in an avalanche front that also propagates upwards, grains located uphill progressively tumbling down because of loss of support. The perturbation threshold for triggering an avalanche is found to decrease to zero at a critical slope. Our results may improve understanding of naturally occurring avalanches on snow slopes 5 where triangular tracks are also observed.The experiments are done on an inclined plane covered with velvet cloth. This surface is chosen so that the glass beads (180-300 m in diameter), our granular material, have a larger friction with it than between themselves. A thin layer of grains can thus remain static on the plane up to a larger angle than if it were on a grain pile. We set the plane to an angle J (larger than the pile angle J 0 ) and pour glass beads abundantly at the top. The moving beads leave behind a static layer of uniform thickness h(J) (arrow leading to point a in Fig. 1; the geometry of the system is shown in Fig. 1 inset). This effect is explained by the variation of the friction of the successive grain layers with their distance to the surface of the inclined slope 6 : it is maximum for the bottom layer, and decreases continuously to the value for a thick pile. The top static layer is that which has a large enough friction coefficient on the underlying layers m(h) to come to a stop. At inclination angle J, the friction coefficient of this top static layer is then mðhÞ ¼ tanJ. The measurements h(J) of Fig. 1 thus give the variation of the coefficient of friction with depth 7 . We obtain a simple exponential decay as in ref. 7:

Friction law for dense granular flows: application to the motion of a mass down a rough inclined plane

“…In between these two angles there exist metastable states, where avalanches can be triggered by perturbations. The complex and rich dynamics of the avalanches has been recently studied by Daerr and Douady (1999) and Daerr (2001).…”

Measurement of Correlations between Low-Frequency Vibrational Modes and Particle Rearrangements in Quasi-Two-Dimensional Colloidal Glasses

“…4 suggests that, sufficiently far above the jamming transition, rearrangements tend to occur at soft spots even in our thermal system, consistent with the idea that soft spots are robust, intrinsic defects in disordered solids that control where local failure occurs under mechanical load. The experiments thus raise the possibility of using vibrational properties to identify regions susceptible to failure in packings of constituents ranging from nanoparticles to macroscopic grains, perhaps even including systems subject to natural events such as earthquakes and avalanches [21,22] .FIG. 4 (color).…”

Stopping dynamics of a steady uniform granular flow over a rough incline

“…This sudden variation of boundary condition creates a shock wave travelling at a velocity u shock larger than the front velocity u 0 , then the height and the velocity decrease slowly. Finally, a deposit lays on the rough plane of uniform height h stop [18,19]. It is known that h stop (θ) is a function of the slope angle θ and depends on the properties of grains and of the bottom surface of the plane.…”