We report on the experimental studies about a new dual stratification pattern and a trapped kink of a sand pile. When we pour a mixture of Large Rough grains (LRG) and Small Smooth grains (SSG) into a quasi-two dimensional Hele-Shaw cell, a stratification pattern of alternate LRG and SSG is formed spontaneously. In a formation of the stratification pattern, a congestion wave (kink) is observed. Ordinary, the kink is generated at the bottom of a sand pile and then the kink goes toward a top of the sand pile. In our experiment of a large sand pile, we found a new stratification pattern with two wavelengths. A wavelength is short in an upper region of the sand pile, while the other is long in the lower region. When the dual stratification is formed, the kink is generated not only at the bottom (ordinary kink) but also on the way of the sand pile slope (trapped kink). Both two kinks make the short wavelength of the upper region different from the ordinary stratification pattern. Therefore in our experiment of the large sand pile, the dual stratification is formed.
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