SPONTANEOUS STRATIFICATION IN GRANULAR MIXTURES

Granular materials(1-5) segregate according to grain size when exposed to periodic perturbations such as vibrations(6-12). Moreover, mixture s of grains of different sizes can also spontaneously segregate in the absence of external perturbations: when such a mixture is simply pour ed onto a pile, the large grains are more likely to be found near the base, while the small grains are more likely to be near the top(13-20) . Here we report another size-separation effect, which arises when we pour a granular mixture between two vertical plates: the mixture spont aneously stratifies into alternating layers of small and large grains whenever the large grains have larger angle of repose than the small g rains. We find only spontaneous segregation, without stratification, w hen the large grains have smaller angle of repose than the small grain s. The stratification is related to the occurrence of avalanches: duri ng each avalanche, the grains separate into a pair of static layers, w ith the small grains forming a sublayer underneath the layer of large grains.