  We have investigated by rotating drum experiments the influence of a small
amount of fine particles on the stability of a granular heap. The fine
particles are shown to have a strong and ambivalent influence. For low fine
particle content, the heap destabilizes by avalanches, and the fine particles
tend to fluidize the heap. In contrast, for high fine particle content, they
increase the cohesion of the heap, which destabilizes through stick-slip at the
drum wall. We interpret and model the fluidification in the avalanching regime,
which we show is independent of humidity, by granular lubrication
considerations, whereas the stick-slip behavior, highly dependent on humidity,
is understood by a solid friction model.
