Transition to Centrifugal Particle Motion in Rotating Drums
  The dynamics and the transition to the centrifugal regime are studied analytically and numerically for particles in rotating drum. The importance of the particle-wall friction coefficient is demonstrated by studying first the motion of one non-rotating particle where three different regimes are found in the transition to the centrifugal motion. When a few rotating particles are considered, they behave similarly to one non-rotating particle in the low friction limit. A critical particle number is necessary to reach the centrifugal regime for which an analytic expression is derived in the limit of negligible inter-particle friction. 
Gerald H. Ristow