Emergent elasticity in shear-jammed granular materials under cyclic shear

While granular packings formed by a shear-jamming process may collapse completely under external perturbations, a recent experiment demonstrated that an "ultrastable" state can be produced by subjecting a shear-jammed sample to small-amplitude cyclic shear. In such states, all the particle positions and contact forces are reproduced after each shear cycle so that a strobed image of the stresses and particle positions appears static, and the behavior of the system is nearly perfectly elastic. To gain insight into the surprising emergence of elasticity in this disordered, frictional, shear-jammed system, we calculated the spatial correlations of the stress tensor in ultrastable states and compared them to the predictions of the Vector Charge Theory of Granular mechanics (VCTG). The local stresses are measured using photoelastic techniques. We show that our experimental results can be fit well by VCTG, assuming a uniaxial symmetry of the contact networks. The fits reveal that the response of the ultrastable states to additional applied stress is substantially more isotropic than that of the original shear-jammed states. Our work provides additional insight into the effect of oscillatory deformation in modifying the mechanical properties of a jammed granular packing.