Avalanche segregation of granular media

“Avalanche segregation” occurs when an initially homogeneous binary mixture of granular media is poured near the closed edge of a “quasi-two-dimensional” Hele-Shaw cell consisting of two vertical transparent plates held a narrow distance apart; the resulting sandpile spontaneously forms alternating segregated layers. Digital images taken with a CCD camera and Fourier transformed to obtain the structure function of the banding pattern, are used to study the wavelength selection mechanism and degree of segregation as the plate separation of the Hele-Shaw cell is systematically increased. For a given flow rate, the degree of segregation, as reflected in the amplitude of the peak in the structure function, decreases as the plate separation increases, and the peak in the structure function shifts to lower wavelengths.