Segregation and stability of a binary granular heap

We measure stability of two-dimensional granular mixtures in a rotating drum and relate grain configurations to stability. We use two types of grains which differ in both size and shape, with the larger grains reaching a larger average angle before an avalanche. In our mixtures, the smaller grains cluster near the center of the drum, while the larger grains remain near the outer edge, a pattern suggesting that grain size rather than avalanche angle determines the segregation behavior. 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.