Strong-weak network anisotropy switching and hysteresis in three-dimensional granular materials

Reduction of the bulk modulus with polydispersity in noncohesive granular solids

We study the effect of grain polydispersity on the bulk modulus in noncohesive two-dimensional granular solids. Molecular dynamics simulations in two dimensions are used to describe polydisperse samples that reach a stationary limit after a number of hysteresis cycles. For stationary samples, we obtain that the packing with the highest polydispersity has the lowest bulk modulus. We compute the correlation between normal and tangential forces with grain size using the concept of branch vector or contact length. Classifying the contact lengths and forces by their size compared to the average length and average force, respectively, we find that strong normal and tangential forces are carried by large contact lengths, generally composed of at least one large grain. This behavior is more dominant as polydispersity increases, making force networks more anisotropic and removing the support, from small grains, in the loading direction thus reducing the bulk modulus of the granular pack. Our results for two dimensions describe qualitatively the results of three-dimensional experiments.

Consequences of anomalous diffusion in disordered systems under cyclic forcing

We study the particle scale response of a 2D frictionless disk system to bulk forcing via cyclic shear with reversal amplitude γ_{r}. We find a subdiffusive γ_{r}-dependent regime, which is consistent with models of anomalous diffusion with scale-invariant cage dynamics, and a crossover to diffusive grain motion at high γ_{r}. Analysis of local displacements of a particle relative to its cage of neighbors reveals a key distinction from thermal systems. Particles are moved by fluctuations of their cage of neighbors rather than rattling in their cage, indicating a distinct cage-breaking mechanism.

Force fabric and macroscopic friction in two-dimensional granular materials

We study the relationship between the local grain-grain friction and the force texture in a granular pile to its macroscopic emergent frictional values. We account for the force geometrical properties by a model that assumes the existence of slide planes that define macroscopic friction angles. The model depends on the measured angular distribution of contact forces and a single parameter, to be fitted, that is a function of the roughness of the slide plane. The model is tested by a state-of-the-art model of a two-dimensional granular pile built by depositing grains under gravity and submitted to an external uniaxial stress.