Three-dimensional shear in granular flow
  The evolution of granular shear flow is investigated as a function of height in a split-bottom Couette cell. Using particle tracking, magnetic-resonance imaging, and large-scale simulations we find a transition in the nature of the shear as a characteristic height $H^*$ is exceeded. Below $H^*$ there is a central stationary core; above $H^*$ we observe the onset of additional axial shear associated with torsional failure. Radial and axial shear profiles are qualitatively different: the radial extent is wide and increases with height while the axial width remains narrow and fixed. 
Xiang Cheng, Jeremy B. Lechman, Antonio F. Barbero, Gary S. Grest,   Heinrich M. Jaeger, Greg S. Karczmar, Matthias E. M\"ob...