Why do Brazil nuts end up on top?An important tool in medicine that was developed by physicists -- magnetic resonance imaging (MRI) -- has come back to the physics laboratory. University physicists, using MRI, have for the first time obtained a three-dimensional picture of how granular materials flow in a cylinder. Their results are reported in the March 17 issue of the journal Science.
Four Chicago physicists -- Heinrich Jaeger, Assistant Professor in Physics; Sidney Nagel, Professor in Physics; Edward Ehrichs, Research Associate in the James Franck Institute; and James Knight, a graduate student in Physics -- are investigating the physical properties of such granular materials as sand, agricultural grains and pharmaceutical powders. Sometimes these materials flow as easily as liquids, while at other times they appear as rigid as highly ordered solids. Knowing more about the physics of these materials has important implications for industries as diverse as pharmaceuticals, mining, road construction and agriculture.
In previous investigations, Nagel and Jaeger demonstrated that when a granular material in a cylinder is agitated, the convection generated brings material from the bottom of the container to the top through the center of the cylinder, and then back down in a narrow band along the sides of the container. The "convection band" heading back down is so narrow that larger grains get caught at the top, while smaller ones easily travel back down to the bottom. This has important implications for industries working with mixed materials composed of grains of different sizes. It is also the reason the Brazil nuts in a jar of mixed nuts always end up at the top, even though they are heavier than the smaller peanuts and cashews in the mix.
But while it is relatively easy to observe the end result of shaking, it had been, up until now, impossible to see what is happening inside the cylinder. With the collaboration of two colleagues, Greg Karczmar, Assistant Professor in Radiology, and Vadim Kuperman, Research Associate in Radiology, the physicists employed the MRI technique to watch how grains at different levels in the cylinder behaved when the cylinder was shaken.
Using a cylinder filled with white poppy seeds -- seeds were used because they contain oil and thus have free protons that can be imaged readily by MRI -- the scientists shook the cylinder repeatedly and took MRI pictures at the end of each shake.
They found that the poppy seeds near the bottom of the cylinder moved very slowly, while those at the top moved much more quickly, indicating that the velocity of the flow is depth-dependent. The grains that moved the fastest of all were those near the top that were moving down the sides of the container.
"This is the first time that anybody has been able to measure the velocity profile noninvasively inside a container," said Jaeger. "Using MRI we can look inside this pile when it is shaken and find out all the details of what happens. Finally, the theorists have some real data that they can start comparing with their models." Jaeger said that in addition to the implications for anyone who is designing systems for containing or transporting granular materials, the work has important implications for physicists. "This is the first time that anybody has been able to study this kind of problem without disturbing the system," he said. "These seeds provide a model system for understanding a lot of other interactions as well, including friction -- which is actually a very complicated, nonlinear process that is not well understood by physicists."
Now that MRI has been demonstrated to be an effective, high-resolution tool for observing the behavior of granular materials, the researchers hope to use the technique to study how granular materials compact and how grains of different sizes -- mixed nuts, for example -- separate.
Ehrichs, Jaeger, Nagel and Knight are members of the Materials Research Science & Engineering Center, a National Science Foundation (NSF) Science and Technology Center at the University. This research was supported by grants from the NSF and the Department of Energy. Individual researchers received support from the Grainger Foundation, the David and Lucile Packard Foundation, the NSF and the Alfred P. Sloan Foundation.
-- Diana Steele