Studying granular materials will allow scientists to control flow

University scientists have harnessed a new combination of cutting-edge technologies to solve an important mystery in the study of granular materials and have reported their findings in the July 27 issue of the international journal Nature.

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 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.

A high-speed video frame, taken at one thousandth of a second, captures a glimpse of mustard seeds viewed through the transparent bottom of the experimental apparatus. The black lines trace the movement of individual particles over the preceding 200 frames.

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 a static to a 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 Chicago Ph.D. student in physics. 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; Peter Eng, a research scientist at the University’s Center for Advanced Radiation Sources; and Sidney Nagel, the Louis Block Professor in Physics.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 used by doctors 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. The researchers also utilized 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,” said Mueth. “However, for irregular or rough particles the flow becomes very complicated. The idea that particle shape influences velocity and the way things flow wasn’t really known before.”

The National Science Foundation and the University’s Materials Research Science and Engineering Center supported the study. For more information, see http://arnold.uchicago.edu/~jaeger/granular2.