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Axial segregation of granular material in a long horizontal rotating drum

Granular mixtures of particles which differ in size, density or surface roughness show the tendency to segregate under a wide variety of flow conditions
A typical example is the segregation along the axis of a partially filled horizontally rotating drum
The system shows a very interesting dynamics:

After a few rotations the small grains forms a core along the axis of the drum (radial segregation).
A few rotations later, this core becomes unstable and a stripe pattern appears (axial segregation).
After the formation of the stripe pattern a so-called coarsening sets in. This means that individual stripes shrink until they vanish, while other stripes grow in size.

Until now, the coarsening dynamic is only poorly understood .

	initially mixed state
	after 500 rotations
	after 23000 rotations

Advantages of the system compared to other segregation experiments:

The energy can be supplied continuously and uniformly over arbitrarily long time periods.
The stripe pattern can be observed straightforward.
The time evolution of the visible stripe pattern can be mapped in space time plots.

Our particular topic of interest is the coarsening of the pattern:

What are the dynamical laws of the coarsening process?
What is the physical mechanism of coarsening?

Space time plot of a stripe pattern, the drum is half filled with a 50:50 mixture of 1.5 and 0.55 mm glass beads

Elucidation of the 3-dimensional particle distribution is one important prerequisite to understand the mechanism of coarsening.
The 3D particle distribution is investigated by nuclear magnetic resonance imaging (MRI). Experiments are performed in collaboration with the IFN Magdeburg (Dr. H. Niessen).

Example: experimental cross section (sagittal section, along the vertical midplane of the tube). The tube is filled with glass beads in water. Bright areas reflect the proton signal from the surrounding water. One acknowledges two stripes of the small bead species separated by a band of large beads. In the core, one observes a channel of small beads connecting the two outer stripes. This channel is essential for the stripe dynamics.

Slice along the axis of the drum

last modified Dec. 21, 2004, T. Finger