  Segregation patterns of size-bidisperse particle mixtures in a
fully-three-dimensional flow produced by alternately rotating a spherical
tumbler about two perpendicular axes are studied over a range of particle sizes
and volume ratios using both experiments and a continuum model. Pattern
formation results from the interaction of size segregation with chaotic regions
and non-mixing islands of the flow. Specifically, large particles in the
flowing surface layer are preferentially deposited in non-mixing islands
despite the effects of collisional diffusion and chaotic transport. The
protocol-dependent structure of the unstable manifolds of the flow surrounding
the non-mixing islands provides further insight into why certain segregation
patterns are more robust than others.
