Radial striping in granular mixtures: a positive feedback mechanism

When mixtures of particles differing only in size (S-systems) or only in density (D-systems) are rotated in a circular drum, the components quickly un-mix, and a simple cylindrically symmetric segregation pattern forms. In D-systems the circularly symmetric segregation (`moon') pattern is stable for all drum fill levels. In contrast, in S-systems this simple segregation pattern will further evolve into a dramatic radial striping (`sun') pattern for drum fill levels near 50 percent. While the simple radial segregation pattern is considered well-understood, the mechanism driving the striped segregation pattern is not. In this talk we present experimental evidence that the growth of the stripes is driven by a positive feedback mechanism based on a concentration dependence of the velocity of the particles. This model is tested and supported using computational experiments using a continuum model based on these experimental observations.