Evolution of the local packing density in a sheared granular material

The local particle packing density, $\eta$, in a bed of poppy seeds sheared in a vertical, split-bottom cylindrical container is measured using magnetic resonance imaging. Wide shear zones away from the wall are generated when a thin concentric disk at the bottom of the cylinder, and with radius less than the cylinder radius, is rotated slowly. Experiments reveal that, initially, a shear band forms at the outer edge of the disk which decreases in radius with decreasing depth. Material near the shear zone dilates with a corresponding decrease in $\eta$ of about $15\%$. The radial extent of the dilated region increases approximately logarithmically with increasing rotation in shallow layers. In relatively deep layers, the shear zone is at first entirely below the free surface, but with further rotation grows vertically creating a low $\eta$ core. We discuss the relation between the previously reported universal velocity profiles across the shear zone and the spatial and temporal evolution of the packing density.