Confinement-induced horizontal segregation in a vertically shaken granular bed

With appropriate spatial confinement to the upper surface of a vertically shaken granular mixture, an uncommon segregation behavior, horizontal segregation (HS) where two types of particles separate along the horizontal orientation, is found. By quantitatively tuning confined space above the granular bed, we plot different phase diagrams and find a qualitative dissimilarity. In particular, the HS disappears when the effect of confinement becomes weak. Additionally, we gradually evacuate air out of the container, and measure the segregation extent of HS as a function of air pressure. It is found that all the segregation behaviors change to mixed state at a low pressure. We also examine the existence of the HS with different granular mixtures and confinement boundaries. The experimental observation confirms that the formation of the HS depends on the interstitial air and confinement.