We are studying slow dense flow of granular materials using several experimental techniques to gain access to various system properties.
The 3D Couette cell is shown in the sketch below. It consists of two two vertical, coaxial cylinders. The outer cylinder is kept stationary while the inner cylinder is rotated at constant velocity. A granular material is placed between the two cylinders, and glued to each cylinder surface to provide a rough and reproducable boundary condition.
This sketch also shows the horizontal spin-tagging stripes used to measure the velocity profile with Magnetic Resonance Imaging (MRI) experiments.
X-Ray tomography is a powerful to for non-invasively measuring density in 3D materials. It has a very high spacial precision, which also makes it very good for imaging.
The following image is a single horizontal slice through the Couette cell, filled with mustard seeds.
Generally, the x-ray tomography data is acquired in 3D data sets. Static 3D volume data sets were visualized using ray tracing software and "slicing" through the sample in various ways.
Because we are in the slow, dense flow regime, the flow is rate-independent. Thus, we can construct a time-sequence (movie) of the particles as they flow by taking static snapshots, between which we rotate the inner cylinder wall by small amounts. The time sequence below is a horizontal slice through the cell constructed this way.
This work was supported by the National Science Foundation under Grant NSF-CTS 9710991.