Granular segregation as a critical phenomenon
  We present the results of an experimental study of patterned segregation in a horizontally shaken shallow layer of a binary mixture of dry particles. An order parameter for the segregated structures is defined and the effect of the variation of the combined filling fraction, $C$, of the mixture on the observed pattern formation is systematically studied. The surprising result is that there is a critical event associated with the onset of the pattern, at $C=0.647\pm0.049$, which has the characteristics of a second order phase transition, including critical slowing down. 
Pedro M. Reis and Tom Mullin