Criticality and Scaling Relations in a Sheared Granular Material
  We investigate a rheological property of a dense granular material under shear. By a numerical experiment of the system with constant volume, we find a critical volume fraction at which the shear stress and the pressure behave as power-law functions of the shear strain rate. We also present a simple scaling argument that determines the power-law exponents. Using these results, we interpret a power-law behavior observed in the system under constant pressure. 
Takahiro Hatano, Michio Otsuki, Shin-ichi Sasa