  The evolution of the interface propagation in a slowly rotating half-filled
horizontal cylinder is studied using MRI. Initially, the cylinder contains two
axially segregated bands of small and large particles with a sharp interface.
The process of the formation of the radial core is clearly captured, and the
shape and the velocity of the propagating front are calculated by assuming a
one-dimensional diffusion process along the rotation axis of the cylinder and a
separation of time scales associated with segregation in the radial and axial
directions. We found that the interfacial dynamics are best described when a
concentration dependent diffusion process is assumed.
