Results are presented of an experimental investigation into patterned size segregation of binary granularmixtures in a thin rotating drum that is half full. It is observed that streaks of small particles are formed withinregions of large ones where the integer number of streaks is fixed over a range of rotation rate of the drum.Different patterns form in adjacent parameter ranges and the dynamics associated with the exchange betweenneighboring states is analyzed using angular spatiotemporal diagrams. These help to reveal properties of themerging mechanism for streaks of small particles. We report experimental evidence that the merging of streaksis mediated by the movement of a surplus material in a direction opposite to that of the rotation of the drum.The excess material is distributed throughout the pattern and the extra streak eventually disappears.
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