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Axial Segregation
Avalanche Dynamics
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Avalanche Dynamics

Experimental Setup

The Hele-Shaw cell consists of two vertical Plexiglas sheets 20 cm high and 30 cm long, mounted parallel to each other onto a Plexiglas base. One of the vertical sheets is bolted to the base. The distance between the vertical sheets is controlled by a 20 cm long strip spacer that closes one edge of the cell and two small block spacers at the opposite edge. By bolting the vertical plates together with various sized spacers, the plate separation d is varied from 3 mm to 24 mm. A 50/50 mixture by mass of sand and sugar is poured near the closed edge of this cell, using a titration bulb with a rotating stopcock to control flow rate. The sand has been dyed blue/black for contrast in the photos without affecting the segregation process.

FFT analysis of dataDigital photos of experimental runs are taken with a monochrome CCD camera (Cohu 4910) in conjunction with a Scion LG-3 frame grabber and a Power Macintosh 7100/80. Data image analysis is performed with the public domain program Image from the NIH. Figure a shows a digital image of the banding pattern near the top of the sand pile, as indicated by the white box in the experimental setup figure. The program Image can plot the pixel intensity against the horizontal position (fig. b). Using white sugar and black sand lets us correlate the pixel intensity to the concentration of sand near the transparent vertical plate of the Hele-Shaw cell. The resulting pixel intensity graph is fast Fourier transformed to yield the structure function for the banding pattern (fig. c). The location of the peak in the plot of FFT amplitude against wave vector in figure (c) gives the average wavelength of the segregated bands, while the amplitude of the peak provides a quantitative measure of the degree of stratification. Both the degree of stratification and the average wavelength of the bands decrease in a nonlinear manner as the plate separation is increased.

Influence of plate separation of the cell walls

Influence of flow rate of mixture pouring into the cell

Construction of a phase diagram indicating the necessary conditions for segregation to occur

Comparing results of a different mixture of granular materials

Computer simulations of the segregation

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