Watch sand resist gravity and flow upward due to negative friction

There’s very little time to write about every cool science story we come across. So this year, we’re once again launching a special “Twelve Days of Christmas” series of posts, highlighting a science story that’s been missed each day in 2023, from December 25 to January 5. Today: How magnetism is applied to individual “micro-roller” particles to stimulate collective motion, producing some rather counterintuitive results.

For example, we intuitively understand that sand poured through an hourglass will form a neat, roughly pyramid-shaped pile at the bottom, with particles near the surface flowing over a base of stationary particles below. Avalanches and sand dunes exhibit similar dynamics. But according to a paper published in the journal Nature Communications in September, scientists at Lehigh University in Pennsylvania found that applying magnetic torque can actually cause sand-like particles to flow upward collectively, seemingly defying gravity.

From a physics perspective, sand is fascinating stuff. This is an example of a granular material because it behaves like both a liquid and a solid. Dry sand collected in a bucket pours like a liquid, but it can support the weight of a rock placed on top of it like a solid, even though rock is technically denser than sand. As a result, sand defies all those neat equations that describe the various phases of matter, and the transition from flowing “liquid” to rigid “solid” occurs rather quickly. It’s as if the particles act as individuals in fluid form, but are able to suddenly unite together when unity is needed, achieving a strange “power in numbers” effect.

Physicists also can’t accurately predict avalanches. Part of the reason is that even a small pile of sand grains is so numerous that each grain interacts with several immediately adjacent grains at the same time, and these neighbors carry over from one moment to the next. Even supercomputers cannot track the movement of a single particle over time, so the physics of flow in granular media remains an important area of ​​research.

But what about the grains of sand that flow collectively toward the hillside? This is simply bizarre behavior. James Gilchrist, an engineer at Lehigh University who runs the Particle Mixing and Self-Organization Laboratory, stumbled upon the strange phenomenon while experimenting with “microrollers”: polymer particles coated with iron oxide. for the process of microencapsulation). One day, he spun a magnet under a vial of tiny rollers and noticed they began to pile up. Of course, he and his colleagues had to investigate further.

For their experiments, Gilchrist et al. Attach neodymium magnets to the electric wheel at 90-degree intervals, alternating outward-facing poles. The unit also includes a fixed-position sample holder and USB microscope. Microrollers were prepared by suspending them in a glass bottle containing ethanol and using magnets to separate them from dust or any uncoated particles. Once the microroller is clean, it is dried, suspended in fresh ethanol, and loaded onto a sample holder. A vibration motor stirs the sample to create a flat bed of particles, and a motorized wheel is set in motion to apply magnetic torque. A gaussmeter measures the strength of a magnetic field relative to the direction.

The result: Each microroller begins to spin in response to magnetic torque, forming pairs that briefly form and then break apart. Increasing magnetic force increases the cohesion of the particles. This in turn gives the micro-rollers more traction, allowing them to move faster, working together to flow uphill in counterintuitive ways. In the absence of magnetic torque, the micro rollers flow downward normally. The effect caused by torque is so unexpected that researchers have coined a new term to describe it: “negative angle of repose” caused by a negative coefficient of friction.

So far, no one has used those terms, Gilchrist said. They don’t exist. But to understand how these particles flow upward, we calculated the stresses that cause them to move in that direction. If the angle of repose is negative, cohesion is necessary to produce a negative coefficient of friction. These particle flow equations were never derived to take these things into account, but after calculations, a negative apparent friction coefficient was obtained.

It’s an interesting proof of principle that may one day lead to new ways of controlling how matter is mixed or separated, as well as potential applications for swarming microrobotics. Scientists have begun using laser cutters to build miniature staircases and film tiny rollers climbing up and down them. According to Gilchrist, one tiny roller can’t overcome the height of each step, but a lot of collective work can.

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

Listing image courtesy Lehigh University