Duke University physicist Bob Behringer has been known to create stress in his lab. He doesn’t consider it a good day unless he’s done some pushing around, and maybe a little shoving, too.
But that’s a good thing, because his victims aren’t grad students, but little hydrogel beads. And how they react to this bullying could one day help save lives – not only during natural disasters such as landslides and avalanches, but also the man-made kinds that occur in some on-the-job accidents.
Inside the university’s Center for Nonlinear and Complex Systems, Behringer and his colleagues study the way granule materials similar to sand or snow interact and react when pushed, pressed squeezed or otherwise stressed during coexistence.
It’s not always the way you would expect. Often, there’s an actual change that’s taking place inside each granule, and that may play a part in why, when forced together, they can react differently, sometimes behaving like one big solid while other times taking on properties more similar to a liquid.
It’s the reason a scoop of dusty soil can scatter in a breeze, but once it lands on the ground can also be packed and pressed firmly enough to walk on.
Or why a handful of powdered snow is harmless by itself, but a mountainside full of it, when under the spell of gravity, can create an interaction between flakes that changes their collective behavior, creating force chains that act more like liquids.
Understanding force, physical change
The reason why granule materials sometimes interchange between solid and liquid states is something physicists are still working to figure out.
“In their own right, granule materials are extraordinarily common and on the other hand, the way they behave is extraordinarily poorly understood,” said Behringer, who hopes their research will help further the understanding of what’s happening during these interactions.
In the lab, Behringer uses a Plexiglas tank filled with hundreds of clear water-filled beads to conduct his experiments because they mimic other granule substances like soil, sand, and snow, and because he can see the changes occurring inside them. Pistons work to shift and squeeze, and pinch and push the beads while sensors and cameras measure the force and physical changes the granules are undergoing.
“We can look in and tickle the system and then we can literally see at the grain scale how the grains are responding, both in terms of the motion, but also the force they feel,” he said.
Information they learn may someday be passed on to engineers who can then design safeguards to protect lives at the onset of an avalanche, landslide or earthquake.
The research may also be used to design safer and more efficient storage facilities for industries – from pharmaceutical and agricultural to electronics and steel – that use bulk granule materials, such as silicon, grains and binding agents during manufacturing.
In the past 50 years, at least 900 accidents have occurred where workers have been engulfed in granule materials while operating hoppers, according to Purdue University researchers.
Understanding why granule materials react differently on different occasions may one day improve the design of equipment, such as hoppers and silos.
But there’s still a lot of pushing and prodding in his lab before that becomes a reality, said Behringer.
“Can I tell them today an equation that will help them?” he said. “I think the answer is no, but I think we’re on the right track.”