Crystallization in load-controlled shearing flows of monosized spheres.

Abstract

Identical, inelastic spheres crystallize when sheared between two parallel, bumpy planes under a constant load larger than a minimum value. We investigate the effect of the inter-particle friction coefficient of the sheared particles on the flow dynamics and the crystallization process with discrete element simulations. If the imposed load is about the minimum value to observe crystallization in frictionless spheres, adding small friction to the granular assembly results in a shear band adjacent to one of the planes and one crystallized region, where a plug flow is observed. The ordered particles are arranged in both face-centered cubic and hexagonal-closed packed phases. The particles in the shear band are in between the crystalline state and the fluid state, but the latter is never reached, which results in a large shear resistance. As the particle friction increases, the shear band disappears, and the ordering in the core region is destroyed. A significant portion of the particles are in a fluid state with a zero shear rate, leading to a substantial and unexpected reduction in the shear resistance with respect to the frictionless case. If the imposed load is increased well above the minimum from the onset of crystallization, we observe the formation of one shear band in the core, where the particles are again between the crystalline state and the fluid state, surrounded by two crystallized regions near the boundaries, in which most of the particles are in the face-centered cubic phase and translate as a rigid body with the boundaries themselves. In this case, the macroscopic shear resistance is independent of the particle friction.