Gallery: Diffusion in granular drainage
This sequence of images is taken from a granular drainage experiment at Clark University. Alternating bands of different colored beads are poured into a large, thin container with front and back walls made of glass. The images have been color-enhanced for clarity.
A slit is opened in the center of the container base, allowing the particles to fall out, and the following pattern is observed. We see very clearly a parabolic region of flow, diffusing on a scale of the width of the container.
What is suprising, however, is that during the flow, particles diffuse on a much smaller length scale than the rate at which the flow profile diffuses. Even in frame (c), after much drainage has taken place, the bands of color remain mainly coherent. Theories such as the Void Model predict that these two should be the same, since they propose that particles move independently of their neighbors. This image shows that in the regime of slow, dense flow, geometrical packing effects are extremely important, since a particle is locked into formation with its neighbors. The recently proposed Spot Model based on correlated motion of particles across several particle diameters successfully captures these two length scales.