How cohesion controls the roughness of a granular deposit.

Cohesive granular materials often form clusters of grains, which alter their flowing properties. How these clusters form and evolve is difficult to visualize in the bulk of the material, and thus to model. Here, we use a proxy to investigate the formation of such clusters, which is the rough surface of a cohesive granular deposit. We characterize this roughness and show how it is related to the cohesion between beads. Specifically, the size of this roughness increases with the inter-particle cohesion, and the profile exhibits a self-affine behaviour, as observed for crack paths in the domain of fractography. In addition to providing a simple method to measure the inter-particle cohesion from macroscopic parameters, these results give better comprehension of the formation of clusters in cohesive granular materials.

Correlated motion in the bulk of dense granular flows.

Numerical simulations of two-dimensional stationary dense granular flows are performed. We check that the system obeys the h_{stop} phenomenology. Focusing on the spatial correlations of the instantaneous velocity fluctuations of the grains, we give evidence of the existence of correlated motion over several grain diameters in the bulk of the flow. Investigating the role of contact friction and restitution, we show that the associated typical length scale lambda is essentially independent of the grain properties. Moreover, we show that lambda is not controlled by the packing compacity. However, in agreement with previous experimental work, we observe that the correlation length decreases with the shear rate. Computing the flows inertia number I , we show a first-order dependence of lambda on I .

Friction versus texture at the approach of a granular avalanche.

We perform an analysis of the granular texture of a granular bed close to stability limit. Our analysis is based on a unique criterion of friction mobilization in a simulated two-dimensional packing. In this way, we recover the bimodal character of granular texture and the coexistence of weak and strong phases in the sense of distinct contacts populations. Moreover, we show the existence of a well-defined subset of contacts within the weak contact network. These contacts are characterized by their important friction and form a highly coherent population in terms of fabric. They play an antagonistic role with respect to force chains. Thus, we are able to discriminate between incoherent contacts and coherent contacts in the weak phase and to specify the role that the latter plays in the destabilization process.

Preavalanche instabilities in a granular pile.

We investigate numerically the transition between static equilibrium and dynamic surface flow of a 2D cohesionless granular system driven by a continuous gravity loading. This transition is characterized by intermittent local dynamic rearrangements and can be described by an order parameter defined as the density of critical contacts, i.e., contacts where the friction is fully mobilized. Analysis of the spatial correlations of critical contacts shows the occurrence of "fluidized" clusters which exhibit a power-law divergence in size at the approach of the stability limit. The results are compatible with recent models that describe the granular system during the static/dynamic transition as a multiphase system.