  Sliding and rolling are two outstanding deformation modes in granular media.
The first one induces frictional dissipation whereas the latter one involves
deformation with negligible resistance. Using numerical simulations on
two-dimensional shear cells, we investigate the effect of the grain rotation on
the energy dissipation and the strength of granular materials under quasistatic
shear deformation. Rolling and sliding are quantified in terms of the so-called
Cosserat rotations. The observed spontaneous formation of vorticity cells and
clusters of rotating bearings may provide an explanation for the long standing
heat flow paradox of earthquake dynamics.
