Sand Waves: Oscillations and Subdiffusion in Granular Segregation

Unlike fluids, dry granular materials often stubbornly refuse to mix when shaken or stirred. Instead, they sort themselves by size or shape. These “segregation” effects are common in many industrial processes involving grains from cake mixes to gunpowder, but are only rather poorly understood. In this talk, I will describe experiments on granular mixtures that segregate when tumbled in a partially filled, horizontal rotating drum. I will attempt a live demonstration of this counterintuitive phenomenon. The dynamical evolution of segregation can, under certain conditions, be oscillatory. Continuum models of this process posit two coupled fields which oscillate out of phase with one another. We examined several candidate fields and find that all are in phase, in contradiction to a recent order parameter model. We also studied the axial transport in the tube using narrow pulses as initial conditions. Surprizingly, we find that the process is subdiffusive, rather than diffusive as assumed in the models.