Since rotating horizontal cylinders are often used as mixers, ob-servations of axial segregation seem counterintuitive. Previous studies have shown axial segregation takes place in a partially filled cylinder, typically at 50percent solid volume fraction. The dynamic angle of repose was interpreted as a control parameter of the two competing effects of diffusion and pref-erential drift. In this paper, we report a surprising result based on a set of experiments where the cylinder was almost completely packed by the mixed particles. Even in this extreme situation where a flowing surface layer was virtually nonexistent, mixed particles formed axially segregated bands as well as radial core of small particles. A visual observation on particle mo-tion shows a series of collapses of local structures (micro-collapses) and this may be a driving force for the particle migration.
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