epl draft Shearing of frictional sphere packings

We measure shear response in packings of glass beads by pulling a thin, rough, metal plate vertically through a bed of volume fraction φ, which is set, before the plate is pulled, in the range 0.575 to 0.628. The yield stress is velocity independent over 4 decades and increases exponentially with φ, with a transition at φ ≈ 0.595. An analysis of the measured force fluctuations indicates that the shear modulus is significantly smaller than the bulk modulus. Introduction. – Granular matter is distinguished in part by its response to shear stress. We examine here the response to shear of beds of glass beads of diameter 200 μm. The granular bed is prepared (by fluidization and sedimentation in water) in a well-defined homogeneous volume fraction φ in the range 0.575−0.628 . We measure the dependence on volume fraction of the response of the bed to shear by pulling a thin, rough, metal plate embedded in the material. We find a transition indicated by a change of slope of yield force as a function of φ at φ ≈ 0.595, consistent with the phase transition at φ between 0.59 and 0.60 reported in [1]. In the Discussion Section we argue that the transition in the yield slope can be identified with the dilatancy onset, the change in sign of the volume response of a sheared system. Further, we suggest that the yielding itself can be viewed as the transition between a solid-like mechanically stable state and a granular fluid state. In recent years this transition has often been discussed in the context of the Jamming paradigm [2] proposed by Liu and Nagel [3]. We show in the Discussion section how the yield stress can be used to distinguish the regimes in a jamming phase diagram [3,4] for frictional particles. Experiment. – Our experimental set-up (Fig. 1(a)) consists of a vertical glass tube (diameter 38.7 mm, height 550 mm) filled with water and soda-lime glass beads (diameter d = 200±10 μm, average sample height 300 mm). The granular bed is fluidized by a computer-controlled gear pump that produces 1 min long flow pulses, which are followed by a 2 min settling time during which the bed completely sediments [1]. This yields a homogeneous bed V