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As shown in the figure, the two differenct sized
beads are initially homogeneously mixed, by shaking
and rocking the cylinder. The smaller (d ~ 0.7 mm )
beads are darker in the picture, while the larger (d ~
3 mm) are clear (see intro for setup description).
After rotation about the horizontal axis at about 15
rpm for approximately 5 minutes, clear bands of
segregated material form. The motor speed was
then decreased to 3 rpm. After approximately one
hour, the bands had disappeared, and the system
had returned to its homogeneously mixed state as
shown. When the speed of the motor was again
increased to 15 rpm for another 5 minutes, the
segregated bands returned, though the precise
pattern, that is, the width and location of the bands,
differed. While the location and size of the axially segregated bands varies from run to run and mixture to mixture, for those mixtures that exhibit a segregation effect, there are reproducible features at the beginning and end of each run. Upon beginning rotation of the mixture, some of the small beads move toward the axis of rotation almost immediately to form a radially segregated mode. This has been observed previously by other groups, and is attributed to the variation in compactivity extending from the axis of rotation to the freely flowing top surface. Another feature consistently observed is that generally the first two axially segregated bands form at the two ends of the cylinder, while the remainder of the segregated bands (typically two or three for the cylinder and mixture shown in the figure) appear in no particular order, their location depending on axial concentration fluctuations, as described in the papers. This initial segregation at the end plates is almost certainly a boundary effect, as confirmed by varying the length of the cylinder and the roughness of the endplates.
Finally, the initial banding pattern as indicated in the figure is not stable, but when the mixture is rotated for an extended period of time at 15 rpm the bands evolve almost invariably to a final pattern of one central band of large beads with bands of small beads adjacent on either side and narrow bands of large beads at both end plates. The time dependent evolution of the segregated bands is complicated, and the mechanism that motivates it remains poorly understood. Similarly, band movement and merging has been seen for the binary mixtures of granular media described here. (View movie of merging, .avi format, 521K). The arrangement of bands changes from one moderately stable pattern to the next, each with fewer bands than the last. After the initial pattern forms, on the order of 10 to 20 minutes passes while the bands may be moving very slowly axially without merging. The bands then evolve into the next configuration, in general, with one fewer of each band (large and small beads, each), as two bands of one type merge and the one between them disappears.
For references and a more in depth discussion please see our published reversible axial segregation papers.
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