  We measure stability of two-dimensional granular mixtures in a rotating drum
and relate grain configurations to stability. For our system, the smaller but
smoother grains cluster near the center of the drum, while the larger, rougher
grains remain near the outer edge. One consequence of the size segregation is
that the smaller grains heavily influence the stability of the heap. We find
that the maximum angle of stability is a non-linear function of composition,
changing particularly rapidly when small grains are first added to a
homogeneous pile of large grains. We conclude that the grain configuration
within the central portion of the heap plays a prominent role in stability.
