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Abstract:
Three fundamental segregation and pattern formation processes are known in granular mixtures in a
rotating cylindrical drum: radial segregation, axial banding, and coarsening of the band pattern. While
the mechanism for the first effect is well understood and for the second effect, several models have
been proposed, the long-term coarsening mechanism remained unexplained so far.Wedemonstrate
that the unidirectional flow between the bands in an axially segregated pattern is driven by small
differences in size of the small beads at the band edges. Due to a process of microsegregation inside
each band of small particles, which was so far unrecognized, this difference in diameter will be effective
in all experiments with polydisperse beads. In consequence the stability of individual bands can be
easily controlled by minor alterations of their composition. Our results make evident that a new
mechanism as the driving force behind the axial particle flow has to be sought.Wesuggest possible
hypotheses for such a mechanism.
