Segregation of combined size and density varying binary granular mixtures in a slowly rotating tumbler

Granular mixtures composed of particles which differ in size or density readily segregate when there is relative motion between particles, such as in a slowly rotating tumbler. In the past, several studies have considered segregation due to different sized particles or different density particles. The physical mechanisms that lead to segregation are different in these two cases. Few studies have considered the important (practical) situation where particles differ both in size and density, which is what we consider here. Depending on particle sizes and densities the two mechanisms either oppose or enhance each other. We consider both cases here and find when the two mechanisms oppose each other size segregation is generally stronger than density segregation. We also determine stability regions for the various patterns that form. When the mechanisms enhance each other we find, under some specific conditions, streak patterns form. We argue that one requires a bed microstructure which fluctuates in strength which subsequently leads to avalanching. We find stable streaks only occur when the smaller, denser particles are blocky in shape while the larger, less dense particles are spherical.