August 1, 2001
Predicting behavior of granular materials
Materials Center scientists have harnessed a new combination of cutting-edge technologies to solve an important mystery in the study of granular materials.
These seemingly simple materials, which include dry sand and powders, have perplexed physicists because they exhibit flow behavior far different from ordinary solids, liquids and gases. Yet understanding the peculiar behavior of granular materials is vital for predicting and controlling them under a variety of industrial, civil engineering and scientific conditions, said Heinrich Jaeger, Professor in Physics at the University of Chicago and study co-author." This simple material displays some of the most complex behavior we're dealing with in the physics of fluids and solids," Jaeger said.
Among other findings, the experiments verified suspicions that the collapse of granular materials can be traced to structural failure along a narrow region, called a shear band, of material measuring approximately 10 particles wide. The experiments explain for the first time how, across this shear band, particles go from the static to the flowing state and, once movement begins, how much material will flow and in what fashion.
The flow of granular materials has defied explanation until now because it has been difficult to look inside the material. In addition, granular materials need to be understood as individually moving particles, though physicists generally prefer to work with the averages of fluctuating phenomena.
"Here the fluctuations become so large that they dominate the behavior of the system as a whole," said Daniel Mueth, the article's lead author and a Ph.D. student in physics at the University. Mueth's other co-authors are George Debregeas, a former Grainger Fellow at the University, now a research scientist at the Institut Paul Sadron in Strasbourg, France; Greg Karczmar, Associate Professor in Radiology at the University; Peter Eng, a research scientist at the Center for Advanced Radiation Sources at the University; and Sidney Nagel, the Louis Block Professor in Physics at the University.
The team used three techniques in combination to comprehensively document, with an unprecedented level of precision, the velocities, positions and packing densities of flowing mustard seeds and poppy seeds during separate tests. One of the techniques (Magnetic Resonance Imaging) is the same doctors use to measure blood flow in the body. Trace amounts of oil in the seeds made it possible to use MRI in the experiments, Jaeger said. Also used were X-ray tomography using the Advanced Photon Source at Argonne National Laboratory and high-speed video.
Without MRI and X-ray tomography, Jaeger said, there would be no way to track the movement and interaction of individual grains with such high resolution. The high resolution derived from the synergistic combination of the three experimental techniques also, for the first time, made it possible to detect the different dynamics associated with different particle shapes. "For smooth, spherical particles the flow is reasonably simple and similar to other systems we're familiar with. However, for irregular or rough particles the flow becomes very complicated," Mueth explained. "The idea that particle shape influences velocity and the way things flow wasn't really known before."
Figure 1 is a sketch of the University of Chicago granular materials experiment as reported in the July 27, 2000 issue of the international journal Nature. The experiment consists of two concentric cylinders with diameters of approximately 2 and 3 inches and filled with seeds to a height of nearly 2.5 inches. The inner cylinder was rotated at velocities ranging from less than 1 up to 45 revolutions per minute to get the seeds moving.
Figure 2. Distorted stripe patterns show movement of material following rotation of the inner cylinder using a technique called spin-tagging, which images a magnetic characteristic of the water molecules in the seeds.
Figure 3. High-speed video frame taken at a thousandth of a second of mustard seeds viewed through the transparent bottom of the experimental container. The black lines trace out the movement of individual particles over the preceding 200 frames.
Figure 4. X-ray tomography image showing a horizontal slice of mustard seeds buried in the experimental container.
Seth B. Darling, created 08/01