Confinement-induced Horizontal Segregation In A Vertically Shaken Granular Bed

Many experimental and theoretical studies mainly concentrate on granular materialswith a free upper surface. By contrast, the studies on the segregation behaviors of a vibratedgranular mixture with upper-boundary confinement are still lacking. This paper mainlystudies the confinement-induced horizontal segregation in a vertically shaken granular bedin a rectangular container. A novel segregation behavior, horizontal Brazil nut effect(HBNE), in the binary granular mixture under a vertical vibration, is found, besides Brazilnut effect(BNE), Reverse Brazil nut effect(RBNE), Periodic segregation(PS).We define a gap d, the distance from the confined boundary to the container base, toquantitatively describe the extent of the confinement. Firstly, by tuning the gap, we plot thephase diagrams to depict the various segregation behaviors under the different confinement,and find the HBNE and RBNE finally disappears when the gap becomes wider (narrower).In addition, we fix the condition of vibration, through continuous changing the distance d,we obtain the evolution of granular spacial configuration: RBNE-HBNE-BNE. We alsoexamine the existence of the HBNE with different materials.Finally, we analyze several effect factors on segregation. The HBNE can be foundwhen the size ratio is1:2and density ratio from2.55to5.08; there is no HBNE if thedensity ratio is1and size ratio from2to4. The confinement could influence the flow of thegranular bed, by changing the material of confined boundary, we find the material is not thedeterminant causing the HBNE. We evacuate air out of the container and discover that allthe segregation behaviors change to mixed states, separation degree of HBNE is given withthe pressure variation, which show the importances of the interstitial air.