Watch sand defy gravity and flow uphill thanks to “negative friction”

There’s hardly ever time to write down about each cool science-y story that comes our method. So this 12 months, we’re as soon as once more operating a particular Twelve Days of Christmas sequence of posts, highlighting one science story that fell by means of the cracks in 2023, every day from December 25 by means of January 5. At the moment: how making use of magnetic forces to particular person “micro-roller” particles spurs collective movement, producing some fairly counter-intuitive outcomes.

We intuitively perceive that the sand pouring by means of an hourglass, for instance, varieties a neat roughly pyramid-shaped pile on the backside, wherein the grains close to the floor stream over an underlying base of stationary particles. Avalanches and sand dunes exhibit related dynamics. However scientists at Lehigh College in Pennsylvania have found that making use of a magnetic torque can really trigger sand-like particles to collectively stream uphill in seeming defiance of gravity, in accordance with a September paper printed within the journal Nature Communications.

Sand is fairly fascinating stuff from a physics standpoint. It is an instance of a granular materials, because it acts each like a liquid and a stable. Dry sand collected in a bucket pours like a fluid, but it will probably help the load of a rock positioned on high of it, like a stable, despite the fact that the rock is technically denser than the sand. So sand defies all these tidy equations describing numerous phases of matter, and the transition from flowing “liquid” to a inflexible “stable” occurs fairly quickly. It is as if the grains act as people within the fluid type, however are able to abruptly banding collectively when solidarity is required, reaching a bizarre form of “energy in numbers” impact.

Nor can physicists exactly predict an avalanche. That is partly due to the sheer variety of grains of sand in even a small pile, every of which is able to work together with a number of of its speedy neighboring grains concurrently—and people neighbors shift from one second to the subsequent. Not even a supercomputer can monitor the actions of particular person grains over time, so the physics of stream in granular media stays a significant space of analysis.

However grains of sand that collectively stream uphill? That’s merely weird conduct. Lehigh College engineer James Gilchrist manages the Laboratory for Particle Mixing and Self-Group and stumbled upon this odd phenomenon whereas experimenting with “micro-rollers”: polymer particles coated in iron oxide (a course of referred to as micro-encapsulation). He was rotating a magnet underneath a vial of micro-rollers sooner or later and seen they began to pile uphill. Naturally he and his colleagues needed to examine additional.

For his or her experiments, Gilchrist et al. hooked up neodymium magnets to a motorized wheel at 90-degree intervals, alternating the outward dealing with poles. The equipment additionally included a pattern holder and a USB microscope in a hard and fast place. The micro-rollers had been ready by suspending them in a glass vial containing ethanol and utilizing a magnet to separate them from mud or any uncoated particles. As soon as the micro-rollers had been clear, they had been dried, suspended in contemporary ethanol, and loaded onto the pattern holder. A vibrating motor agitated the samples to supply flattened granular beds, and the motorized wheel was set in movement to use magnetic torque. A gaussmeter measured the magnetic subject energy relative to orientation.

The outcomes: every micro-roller started to rotate in response to the magnetic torque, creating pairs that briefly shaped after which cut up, and rising the magnetic pressure elevated the particle cohesion. This in flip gave the micro-rollers extra traction and enabled them to maneuver extra shortly, working in live performance to counterintuitively stream uphill. Within the absence of that magnetic torque, the miro-rollers flowed downhill usually. The torque-induced motion was so surprising that the researchers coined a brand new time period to explain it: a “unfavorable angle of repose” brought on by a unfavorable coefficient of friction.

“Up till now, nobody would have used these phrases,” said Gilchrist. “They didn’t exist. However to grasp how these grains are flowing uphill, we calculated what the stresses are that trigger them to maneuver in that course. When you’ve got a unfavorable angle of repose, then you need to have cohesion to offer a unfavorable coefficient of friction. These granular stream equations had been by no means derived to think about this stuff, however after calculating it, what got here out is an obvious coefficient of friction that’s unfavorable.”

It is an intriguing proof of precept that would sooner or later result in new methods to manage how substances combine or separate, in addition to potential swarming microrobotics purposes. The scientists have already began constructing tiny staircases with laser cutters and videotaping the micro-rollers climbing up and down the opposite. One micro-roller cannot overcome the peak of every step, however many working collectively can accomplish that, per Gilchrist.

DOI: Nature Communications, 2023. 10.1038/s41467-023-41327-1  (About DOIs).

Itemizing picture by Lehigh College