It turns out that 8000 tiny plastic disks in a rotating drum could help scientists develop a technique to forecast avalanches or earthquakes through sound.
False-colour image of grains in a drum. Image: Ted Brzinski (NCSU)
A team of researchers studying granular materials at North Carolina State University set up an experiment to recreate stick-slip failure events - jolts of force from two things sliding against one another. These are the kinds of events that can lead to earthquakes, avalanches and landslides on much larger scales. The researchers were able to pick out sound signatures not just of the failure, but of the straining leading up to failure.
"While acoustic emissions have previously been known to coincide with the failure of granular media, our method provides a new capability: Assessment of the progress of a system en route to failure," the researchers write in the journal Physical Review Letters.
Now, these are 8000 pea-sized plastic beads in a vat, not sand and earth along a fault line, so we don't want to oversell the study's results. But the researchers realised that information about vibrations in a material with an added external force may encode information about the state of the material. They loaded a slowly spinning cylinder with special plastic disks. Twelve crystal sensors on the cylinder's outer wall could detect how much force was exerted by the outward-pushing beads.
The rotating produces occasional stick-slip failures: The disks lock into place from friction against one another, but eventually they all jostle, around once per minute. The researchers were able to correlate vibrations in the disks with their collective behaviour inside the rotating drum.
Again, this is very different from an earthquake, but on a larger scale, the researchers predict they might be able to use sound patterns to forecast larger-scale events like avalanches or landslides. They won't be able to predict exact times, but they could at least offer probabilities.
One researcher not involved in the study, Lisa Manning from Syracuse University, told science writer Mark Buchanan at Physics that this was important work. "It suggests a new method for assessing the internal state of granular packing and shows that the vibrations inside change a lot when the material gets close to failing."
Furthermore, the plastic beads that the scientists use to do this work are really cool. Karen Daniels, one of the scientists behind the study, once brought them to a talk of hers I attended. If you squeeze on them, they change the direction of light passing through them. You can only see the effect through a polarised filter, but essentially, squeezed disks appear to have bright lines that mimic the direction of the force passing through them.
She and a team of undergrads are currently using the disks to try to figure out the best way for a lander to dock on an asteroid, which have low gravity and a lot of dust grains on their surface. The disks stand in for the grains.
Sometimes, understanding the fundamental behaviour of large systems requires some strange setups that don't look much like the thing they're modelling. But insights from this bead-filled plastic drum might one day be important for potentially life-saving forecasting.