In this article, we will discuss a strange phenomenon that occurs when applying magnetic forces to sand particles coated with iron oxide, where these particles move collectively upwards despite the force of gravity. This phenomenon was discovered by researchers at Lehigh University in Pennsylvania, and they published their findings in the journal Nature Communications in September.
Sand as a Strange Granular Material
Sand is an interesting material from a physics perspective, as it behaves like both a liquid and a solid at the same time. Dry sand flows in a bucket like a liquid, but it can support the weight of a rock placed on top like a solid, even though the rock is denser than the sand. Sand challenges the governing equations of various phases of matter, rapidly transitioning from “liquid” to “solid.” The particles appear to behave as individuals in the liquid state but can suddenly gather when unity is necessary, achieving a strange kind of “strength in numbers.”
Sand Moving Upwards Collectively
Does sand move upwards collectively? This is truly strange behavior. Engineer James Gilchrist from Lehigh University discovered this odd phenomenon while experimenting with “micro-rollers”: polymeric particles coated with iron oxide. He noticed that when a magnet spun beneath a bottle containing the micro-rollers, the particles began to gather upwards. Of course, he and his colleagues had to further explore this phenomenon.
Experiments and Results
For their experiments, Gilchrist and his team fixed neodymium magnets onto a rotating wheel at 90-degree angles, alternating the poles directed outward. The setup also included a sample holder and a stationary USB microscope. The micro-rollers were prepared by suspending them in a glass bottle containing ethanol and using a magnet to separate them from dust or any uncoated particles. Once the micro-rollers were cleaned, they were suspended in pure ethanol and loaded onto the sample holder. A vibrating motor moved the samples to produce flat granular beds, and the rotating wheel was restarted to apply magnetic torque. A gaussmeter measures the magnetic field relative to the direction.
The results: Each micro-roller began to rotate in response to the magnetic torque, leading to the formation of short-lived pairs that then broke apart, and increasing magnetic strength enhanced the cohesion of the particles. This, in turn, gave the micro-rollers greater stability and enabled them to move faster, working together to flow upwards against expectations. In the absence of magnetic torque, the micro-rollers flow naturally upwards. The process induced by magnetic torque was so unexpected that the researchers coined a new term to describe it: “negative comfort angle” caused by the negative friction coefficient.
Potential Applications
This is an intriguing piece of evidence that could someday lead to new ways of controlling the mixing or separation of materials, as well as applications in collective microrobotics. Scientists have already started building small ladders using laser cutters and observing the micro-rollers as they climb and descend. A single micro-roller cannot overcome the height of each step, but many of them can do so collectively, as Gilchrist states.
DOI: Nature Communications, 2023. 10.1038/s41467-023-41327-1 (About DOIs).
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