Segregation of granular mixtures in a spherical tumbler

Segregation of polydisperse granular materials in rotating containers is a ubiquitous but still not satisfactorily understood phenomenon. This study describes axial segregation of bidisperse granular mixtures of glass beads in a spherical container, rotating about its horizontal axis. Depending on the filling fraction of the mixer and on the composition of the mixture, qualitatively different spontaneously formed patterns are observed. For technical applications, the well-localized segregated bands allow a convenient separation of individual components of the mixtures. It is particularly surprising that the initial compositions of the granular mixtures have a fundamental influence on the location of the segregated bands. This evidences a collective pattern forming mechanism. The spontaneous formation of these bands cannot simply be traced back to individual particle dynamics. Existing models for segregation in spherical mixers are critically examined and extensions are suggested.