Transients in sheared granular matter

Abstract

Abstract. As dense granular materials are sheared, a shear band and an anisotropic force network form. The approach to steady state behavior depends on the history of the packing and the existing force and contact network. We present experiments on shearing of dense granular matter in a 2D Couette geometry in which we probe the history and evolution of shear bands by measuring particle trajectories and stresses during transients. We find that when shearing is stopped and restarted in the same direction, steady state behavior is immediately reached, in agreement with the typical assumption that the system is quasistatic. Although some relaxation of the force network is observed when shearing is stopped, quasistatic behavior is maintained because the contact network remains essentially unchanged. When the direction of shear is reversed, a transient occurs in which stresses initially decrease, changes in the force network reach further into the bulk, and particles far from the wheel become more mobile. This occurs because the force network is fragile to changes transverse to the force network established under previous shear; particles must rearrange before becoming jammed again, thereby providing resistance to shear in the reversed direction. The strong force network is reestablished after displacing the shearing surface ≈ 3d, where d is the mean grain diameter. Steady state velocity profiles are reached after a shear of 30d. Particles immediately outside of the shear band move on average less than 1 diameter before becoming jammed again. We also examine particle rotation during this transient and find that mean particle spin decreases during the transient, which is related to the fact that grains are not interlocked as strongly.

Memory of the unjamming transition during cyclic tiltings of a granular pile

“…Whatever the density of critical contacts and the inclination when the packing starts to relax, ν vanishes after a complex transient dynamics. The time needed for critical contacts to relax is larger for slopes close to θ a : this confirms the complex relaxation towards equilibrium of a granular packing [12,22,23], and this is consistent with observations of long-range correlations close to the unjamming transition [22,27,30,31,32]. The relaxation of the critical contacts for rigorous static conditions shows that the critical state is a transient state.…”

“…At the transition from loading to unloading, critical contacts can not any more sustain the sliding condition, so that the frictional state of the packing is immediately removed, attesting the end of coherent shearing, as observed in the case of experimental shear reversal [23,37].…”

“…Important non equilibrium effects of structurally disordered granular systems, beside the slow relaxation to equilibrium [12,22,23], are : bistability and hysteresis [24,25,26] as the result of a memory effect due to the internal history dependence of systems out of equilibrium and the lack of an unique metastable state [2] ; and a jerky response to the external driving force with impulselike events (local rearrangements of grains or avalanches), as the result of the exploration of local minimal energy configurations [27,28]. A related important question is whether a diverging length-scale exists on the jammed side of the transition [29].…”

“…In contrast to cyclic shear solicitations [17,23,37,38,39,40], we focus here -in the continuation of [27,32,33]-on quasi-static cyclic rotations of dense cohesionless granular systems under gravity. Depending on the amplitude of the cyclic solicitations, the granular system can approach the unjamming transition, without overpass it, so as to keep a solid-like state and a quasi-static evolution.…”

Particle dynamics in dense shear granular flow

“…Recently several experiments on shear granular flows have been performed in the Couette geometry, both in two-dimensional [9][10][11] and three-dimensional [12][13][14] systems. The Couette geometry with uniformly shearing boundary provides a steady-state flow which can be maintained very long and has symmetry for azimuthal averaging of many important quantities.…”