Brownian forces in sheared granular matter
  We present results from a series of experiments on a granular medium sheared in a Couette geometry and show that their statistical properties can be computed in a quantitative way from the assumption that the resultant from the set of forces acting in the system performs a Brownian motion. The same assumption has been utilised, with success, to describe other phenomena, such as the Barkhausen effect in ferromagnets, and so the scheme suggests itself as a more general description of a wider class of driven instabilities. 
Andrea Baldassarri, Fergal Dalton, Alberto Petri, Stefano Zapperi,   Giorgio Pontuale, and Luciano Pietronero