Jamming, Yielding and Rheology of Weakly Vibrated Granular Media
  We establish that the rheological curve of dry granular media is non-monotonic, both in the presence and absence of external mechanical agitations. In the presence of weak vibrations, the non-monotonic flow curves govern a hysteretic transition between slow but steady and fast, inertial flows. In the absence of vibrations, the non-monotonic flow curve governs the yielding behavior of granular media. Finally, we show that non-monotonic flow curves can be seen in at least two different flow geometries and for several granular materials. 
J. A. Dijksman, G.H. Wortel, L.T.H. van Dellen, O. Dauchot, M. van   Hecke
cond-mat.soft