Shear jamming in granular materials

For frictionless particles with purely repulsive interactions, there is a critical packing fraction $\phi_J$ below which no jammed states exist. Frictional granular particles in the regime of $\phi < \phi_J$ act differently under shear: early experiments by Zhang \& Behringer at Duke University show jammed states can be created by the application of shear stress. Compared to the states above $\phi_J$, the shear-jammed states (SJS) are mechanically more fragile, but they can resist shear. Formation of these states requires the anisotropic contact network as a backbone and these new states must be incorporated into a more general jamming picture (Bi et al Nature 2011). If time permits, I will present some new results from recent experiments at SJTU aimed towards understanding the more detailed nature of SJS and the transition from unjammed states to SJS.