Experiments on Mixing and Segregating Granular Materials

When our intuition is based on liquid behaviour, granular solids flow and mix in ways both novel and unexpected. We describe 2- and 3-dimensional experiments in tumbling containers tracking mixing and segregation of granular solids. The unique aspect of these experiments is that they are designed to view the grain motion. Experiments in thin rotating containers partially filled with coloured particles motivate model which separates the dynamics and geometry of the flow and which quantitatively accounts for the measured mixing rates. These kinds of experiments should be considered test beds of granular mixing from which to add complexities, such as baffles, cohesion, and particle property differences, in a controlled manner. Model limitations and a computational challenge based on further experiments are also discussed. Non-invasive magnetic resonance imaging shows us the transport of granular solids in a slowly rotating tube with and without segregation effects.