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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. 
Digital 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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