Pre-avalanche structural rearrangements in granular packing
Seminar Room 1, Newton Institute
AbstractIt has long been known that, under increasing inclination, a granular medium displays a transition from a solid like (static regime) to a fluid like (flow regime) phase. A complete understanding of this transition seems yet unavailable. However, predicting this transition is important for geophysics, numerous industrial applications, and for understanding the dynamics of granular materials and the unjamming transition.An important question is whether reliable precursors can be found when approaching the critical angle. The main finding reported in previous studies is that the density of surface rearrangements gradually increases when increasing the inclination angle. For small angles (~10), events are rare and occur randomly on the surface while, close to the critical angle (~25), quasi-periodic rearrangements involving larger and larger numbers of grains were observed. Our experimental work consists in studying the dynamics of a 3D granular packing constituted by glass beads contained in a rectangular box which is inclined up to the threshold of instability. With an image processing technique and tracking methods, we obtain the number of surface rearrangements and we follow the trajectories of grains as a function of the angle inclination. When the angle increases, four types of events are observed. Weak rearrangements take place at random on the surface for small angles (<10) and quasi-periodic events involving large numbers of grains occur close to the threshold angle. In addtion we observe yet unreported collective sliding events about 3 degrees below the critical angle at which an avalanche takes place. With tracking methods, we analysed the properties of these rearrangements and we measured the speeds at which groups of grains move locally. We discuss the "solid like" or "fluid like" behaviour of grains depending on the angle of inclination. Moreover, we emphasize the presence of strong non trivial correlations of grain motions during rearrangements. We also compare our results with those obtained recently by V. Zaitsev et al, who studied the dynamics of rearrangements in the bulk of the pack with a nonlinear acoustic technique. In particular, we find very similar results about the quasi-periodicity of precursors and we show the presence of memory effects as a signature of irreversible rearrangement processes during the inclination procedure. Finally we define a simple cellular automaton type model whose aim is to simulate the growing process of rearrangements at the surface of the pack. This model, although phenomenological, shows a behaviour very similar to the experimental one. It confirms the interest of this type of simulation in the study of granular media.
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