Particle physics experiments

Laser experiments shed light on landslide physics

Laser experiments shed light on landslide physics

by Julia Rosen Tuesday June 16th, 2015

The researchers used beads filled with laser-activated dye to reveal the behavior of granular materials during landslides. Credit: Felice Frankel and Joshua Dijksman.

How does cereal come out of the box? Why do wheat grains get stuck in a hopper? What happens to the ground when a slope collapses during a landslide? And, more broadly, what do these various phenomena have to do with each other?

The answer is that they are all governed by the same laws of physics, those which control the behavior of granular materials. A detailed understanding of these laws eluded physicists for centuries. But now, researchers working in the lab have developed a new way to explore the mathematical relationships that underlie particle interactions. The results could one day help explain the mechanics of avalanches, landslides and a myriad of other geophysical and industrial processes.

Ever since Stephen Hales studied the packing model of peas in 1727, physicists have searched for a way to relate what happens on the small scale between two particles – which is relatively easy to measure – to what happens on the large. scale between many particles – which is not. Such equations for materials like liquids and solids “have been around for 100 years or more, and they work very well,” says Robert Behringer, a Duke University physicist and co-author of the study, published in Nature Communications. . But there is no well accepted model for granular materials.

So Behringer and two former Duke postdoctoral fellows, Nicolas Brodu and Joshua Dijksman, combined their lab and data visualization skills to create an experiment that would allow them to study complex, multi-particle interactions in exceptional detail. They started with about 500 marble-sized hydrogel spheres filled with laser-activated dye and stuffed them into a case the size of a tissue box. Then they used a plate to slowly compress the beads from above, like a trash compactor.

To imagine what happened during the experiment, the team shot a flat laser beam through the box, which illuminated a cross section of beads. They scanned the laser back and forth, taking pictures and then stitching them together to create a 3D image, like an MRI. By scanning the box repeatedly as the experiment progressed, the researchers were able to observe how the particles deformed and rearranged as the pressure weighing them down increased.

It is well known that granular materials become stiffer with compression (think how a sandy beach feels solid under your feet, but easily crumbles between your fingers). The experiment revealed why: some of the stiffness comes from the deformation of the individual grain boundaries, but they can’t give much. In addition, the number of contacts between the grains also increases, says Behringer. “As more contacts form, the system can withstand further compression,” he explains. Such knowledge will help researchers formulate equations describing the physics of granular systems.

Although previous studies have used similar techniques — like particle imaging or bead compression — none have combined these methods into a single experiment, says Brian Tighe, a physicist at Delft University of Technology in the Netherlands. -Bas, who did not take part in the work. Tighe says the new results are fundamental – “it’s not the work that’s going to tell you immediately when a landslide is going to start” – but they will help scientists test their models in a way that hasn’t been possible so far. If your model performs well in these basic situations, he says, “you trust it much more” when simulating geophysical processes such as landslides and avalanches. Ultimately, what’s really new about this study, says Tighe, is that “they made something invisible visible.”