Pattern formation during mixing and segregation of flowing granular materials

Powder mixing plays an important role in a number of industries ranging from pharmaceuticals and food to ceramics and mining. Avalanches provide a mechanism for the stretching and folding needed to mix granular solids. However, unlike fluids, when particles dissimilar in size, density, or shape flow, they can spontaneously demix or segregate. Using magnetic resonance imaging, we track the transport of granular solids in a slowly rotating tube both with and without segregation effects. Compared with experiments in a 2-dimensional rotating disk partially filled with colored particles, the mixing kinematics and the granular pattern formation in a tube are changed by an axial flow instability. From simple physical principles we argue how size and density segregation mechanisms can be made to cancel, allowing good mixing of dissimilar particles, and we show experiments verifying this. Further experiments isolate the axial transport in the slowly rotating tube. Axial transport can appear faster with segregation than without.