  We experimentally study the mixing of binary granular systems in a horizontal
rotating cylinder. When materials have the same size and differ by dynamic
angle of repose only, we observe an axial transport of matter that generates
transient radial segregation. The system then evolves towards homogeneity. If
materials differ by density also radial segregation becomes steady. A mechanism
is suggested where radial segregation is promoted by axial differences of
dynamic angle of repose. This differs from the free surface segregation
suggested so far to explain radial segregation.
