Dynamic angle of repose as a function of mixture concentration: Results from MRI experiments and DEM simulations

Certain binary mixtures of large and small granular materials exhibit a reversible axial segregation when rotated in a drum mixer. That is, they axially segregate into bands of relatively pure single concentration when rotated at higher speeds and remix at lower speeds. We report measurements of the dynamic angle of repose as a function of mixture concentration for speeds where the mixture segregates and where it remixes using Nuclear Magnetic Resonance Imaging (MRI) techniques. These measurements show that the dynamic angle of repose increases monotonically with small bead concentration at rotation speeds for which the mixture axially segregates. At speeds where the mixture does not segregate, the dynamic angle of repose does not vary with mixture concentration. These results are consistent with simulations using a Discrete Element Method (DEM) for modeling the granular dynamics.