Radial and axial segregation in horizontal rotating cylinders studied by Magnetic Resonance Imaging (MRI)

The dynamics of granular materials, mostly radial and axial segregation in horizontal rotating cylinders filled with millet and poppy seeds, is studied by Magnetic Resonance Imaging (MRI). For the first time, full 3D structures and real-time 2D MRI movies showing the progress of segregation over many hours are reported. Data are acquired with sufficiently high temporal and in-plane spatial resolutions (74 ms and 0.94 mm \times 0.94 mm, respectively), giving new insights into the underlying mechanisms. The millet and poppy mixture composition is calibrated based on the signal intensity and is quantified throughout segregation. As for radial segregation, millet and poppy mixture core formation is observed in cylinders of 75% and 82% filling level. The size of the core is calculated and the avalanche layer thickness is therefore determined. 2D MRI movies showing real-time radial segregation suggests that initial radial segregation is due to the stopping and percolation of poppy seeds. Axial segregation is characterized by the formation, traveling and merging of poppy-rich bands. In all cases studied, the formation of poppy-rich bands is observed, after which individual bands start to travel at \sim 3 \mu m s^{-1} until they are within \sim 3 cm of a stationary band. Adjacent bands then merge into a single, enlarged poppy band as millet seeds move out of the merging region. In both radial and axial segregation, core diffusion is shown to be another mechanism pathway for segregation besides the free surface.