In a vertically vibrated binary granular mixture, sandwich-like segregation pattern is observed; a layer of large heavy particles is sandwiched in between two layers of small particles. This pattern is different from the double-layer structure, known as the “Brazil nut” effect, where the large heavy particles populate on the top. Our experiments show that such a three-layer structure is a stable pattern. It occurs when the vibration acceleration is beyond a critical threshold. Two different segregation processes are observed. The mechanisms for these two segregation processes are discussed. The dependence of the segregation pr ocesses on the driving frequency f and acceleration Γ, and the size ratio and volume ratio of the particles are also experimentally studied.
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