Watch sand defy gravity and move uphill because of “detrimental friction”

There’s not often time to put in writing about each cool science-y story that comes our manner. So this yr, we’re as soon as once more working a particular Twelve Days of Christmas collection 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. Right this 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, types a neat roughly pyramid-shaped pile on the backside, wherein the grains close to the floor move 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 move uphill in seeming defiance of gravity, in line 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 might probably help the load of a rock positioned on prime of it, like a stable, although the rock is technically denser than the sand. So sand defies all these tidy equations describing varied 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 kind, however are able to abruptly banding collectively when solidarity is required, attaining a bizarre type of “power 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 can work together with a number of of its rapid neighboring grains concurrently—and people neighbors shift from one second to the following. Not even a supercomputer can observe the actions of particular person grains over time, so the physics of move in granular media stays a significant space of analysis.

However grains of sand that collectively move 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 beneath a vial of micro-rollers sooner or later and observed they began to pile uphill. Naturally he and his colleagues needed to examine additional.

For his or her experiments, Gilchrist et al. connected 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 have 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 have been clear, they have been dried, suspended in recent ethanol, and loaded onto the pattern holder. A vibrating motor agitated the samples to provide flattened granular beds, and the motorized wheel was set in movement to use magnetic torque. A gaussmeter measured the magnetic area power relative to orientation.

The outcomes: every micro-roller started to rotate in response to the magnetic torque, creating pairs that briefly fashioned after which break up, and rising the magnetic power 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 move uphill. Within the absence of that magnetic torque, the miro-rollers flowed downhill usually. The torque-induced motion was so sudden that the researchers coined a brand new time period to explain it: a “detrimental angle of repose” attributable to a detrimental coefficient of friction.

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

It is an intriguing proof of precept that would sooner or later result in new methods to regulate how substances combine or separate, in addition to potential swarming microrobotics functions. 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