Note to editors: Robert Behringer's talk is scheduled for 12:48 p.m. Pacific Time on Monday, March 16 in room 502A of the Los Angeles Convention Center as part of the American Physical Society's March Meeting.
LOS ANGELES -- The grains of granular materials can abruptly concentrate stress by assembling themselves into brief but dramatic chains shaped much like jagged lightning bolts as they randomly push against each other within tightly compacted spaces, according to a Duke University physicist who has devised ways to observe this behavior in the laboratory.
Robert Behringer said this unexpected way granular particles build up and then dissipate collective tension may help explain why passive containers such as coal-filled hoppers sometimes self-destruct for no apparent reason. These stress fluctuations may also contribute to the behavior of other natural phenomena, the Duke physics professor said in an interview.
"You can imagine that when these forces build up and are then released, it's like a tiny earthquake," he said. In fact, some scientists believe there may even be links between the dynamics of granular materials and earthquakes.
Examples of typical granular materials include piles of boulders and coal lumps to cereals and soil particles. Examples of flowing granular materials include avalanches and shifting sand dunes.
Behringer discussed his experiments conducted both at Duke and in France in a report prepared for presentation Monday at the American Physical Society's March meeting.
Many scientists have long been fascinated by granular particles. Some say their ability to "flow" much like liquids even though they are composed of solids makes them seem like a separate state of matter.
Experiments by Behringer and others also have documented that granular materials can form bizarrely regular geometric patterns when subjected to rapid shaking.
While Behringer's work is supported by the National Science Foundation, some of it is also underwritten by NASA.
"NASA supports research in granular materials because it envisions doing mining on the moon or other places where gravity is weak," he said, noting that the apparent discovery of ice within some lunar soils may give the space agency added impetus. Scientists suspect granular particles may behave differently under low gravity than they do on Earth, he explained.
But Behringer's experiments on stress in the materials probe how granular particles interact under normal gravity, such as when pieces of coal or grains of wheat pile up within the dark confines of a silo.
When engineers design silos, they assume that such particles merely slide when bumped by their fellow particles. Under this scenario, the sliding particles simply slip over each other in ways that dissipate any acquired energy, he said.
Behringer's experiments found more complicated dynamics. Actually, when they are pressed as compactly as they would be in a confined space, "the grains suddenly wake up and they stop slipping," he said. "They don't just slip. They begin to roll, as if they are clever enough to rotate as well as move," he added. He compared the process to pushing on an automobile to make its tires revolve.
While grains would dissipate energy if they just slid among themselves, these rotating particles store up the energy of motion instead, he said. What's more, a network of such "tense" grains can form jagged stress chains that stand out among the larger sea of particles. As the particles continue to interact, these chains then dissipate suddenly, only to be replaced by other chains in other locations.
"These stress chains act to carry force from one place to another place, and they can come and go with very great speed," he said. "Therefore, they have the potential to do damage, because they are localized and because they come and go without a lot of warning."
In his experiments, Behringer used a device that allows such stress chains to be viewed in the laboratory, as well as videotaped in computer-enhanced false colors.
The device consists of a wheel in the center of a shallow circular container, which is filled with small translucent cylinders as flexible as hard rubber. These experimental "granular materials" are made of a special compressible plastic that bends light as it undergoes tension. The tiny cylinders also were marked with a line on one end, which allowed scientists to establish that some rotated instead of simply sliding.
When the device is turned on, the wheel slowly moves and transmits its energy to the surrounding "grains," which respond by bumping into each other and compacting. Their optical properties then change whenever and wherever they build up stresses.
Experimenters can see the resulting flashing stress chain patterns by illuminating the apparatus from underneath and observing it through cross-polarizing filters. Those filters screen out any light whose optical properties have not been stress-distorted.
Scientists can videotape the process and use a computer to convert the patterns of light into color.
As dramatic as the images are, the apparatus is far more than just an attention-getting display. Using a calibration device, investigators also can use it to painstakingly measure the forces on individual grains, as well as following how the grains move and orient themselves within their essentially two-dimensional world.
Behringer acknowledged that most granular particles exist in three-dimensional space, and that most are made of hard substances rather than squeezable ones. But he said separate 3-D experiments with hard glass beads have produced some analogous results. Some of the 3-D work was described in the Oct. 7, 1996, issue of Physical Review Letters.
Behringer has conducted stress fluctuation experiments both at Duke and at the Ecole Superieur de Physique et Chemie Industrielle of Paris, where he spent a 1997 sabbatical. His major collaborators include Christian Veje, a graduate student in Paris, and Daniel Howell, a graduate student at Duke. ###