Spontaneous formation of dual stratification patterns in a large quasi-two-dimensional sand pile.

By pouring a mixture of two types of grains into a large vertical cell with a narrow space, we discovered a dual stratification pattern consisting of two different wavelengths at the upper and lower regions of the resulting sand pile. In the formation of this pattern, we also observed an additional type of kink-a wave that moves toward the top of the sand pile along the slope. The kink, herein called a trapped kink, is essential for the formation of dual stratification patterns. Asymmetric probability distributions are obtained from measurement of the position where the kinks are generated. We proposed a phenomenological model to describe the kink generation process. The results analyzed by this model agree with experimental distributions.