External boundaries and internal shear bands in granular convection

Granular material convects in response to vertical vibrations of sufficient peak acceleration. Particles flow upward in the center of a container with rough, vertical walls and downward in a thin stream along the sides. Canting the side walls outward from the vertical beyond a transition angle reverses the sense of the flow and changes its shape: particles flow upward in a broad region above the walls and downward in the center. We have studied this transition experimentally and find evidence for two opposing mechanisms, net downward shear at a rough vertical wall and 'ratcheting' at an oblique wall, which combine to determine the effect of the boundaries on particle motion. The direction of convective flow is not solely a function of the external boundaries, however. Continuous vibration can induce internal shear bands at which the flow is always observed to move downward, in which case a competition between the internal shear and the shear at the external boundaries determines the overall pattern of convection.