  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.
