Watch the sand defy gravity and flow uphill due to negative friction

There is rarely time to write about every cool science story that comes our way. So this year, we’re once again running a special Twelve Days of Christmas series of posts, highlighting a science story that fell through the cracks in 2023, every day from December 25 to January 5. Now: how applying magnetic forces to individual ” The micro-roller” particles encourages collective motion, producing some rather counter-intuitive results.

raising / Engineering researchers at Lehigh University discovered that sometimes sand can flow upwards.

Lehigh University

We intuitively understand that the sand poured through an hourglass, for example, forms a smooth almost pyramid-shaped pile at the bottom, where the grains near the surface flow over an underlying base of stationary particles. Avalanches and sand dunes show similar dynamics. But scientists at Lehigh University in Pennsylvania have discovered that applying a magnetic torque can actually make sand-like particles collectively flow upward in apparent opposition to gravity, according to a September paper published in the journal Nature Communications.

Sand is an interesting substance from a physics point of view. This is an example of a granular material, because it behaves like a liquid and a solid. Dry sand collected in a bucket pours like a fluid, but it can support the weight of a rock placed on top of it, like a solid, even though the rock is technically denser than in the sand. So the sand defies all the smooth equations that describe the different phases of matter, and the transition from flowing “liquid” to a hard “solid” occurs rapidly. It is as if the grains act as individuals in liquid form, but are able to suddenly merge when unity is needed, achieving a unique kind of “strength in numbers” effect.

Nor can physicists accurately predict an avalanche. That is due to the number of grains of sand even in a small pile, each of which interacts with many neighboring grains simultaneously and those neighbors move from one moment to the next. Even a supercomputer cannot track the movements of individual grains over time, so the physics of flow in granular media remains an important area of ​​research.

But the grains of sand that collectively flow uphill? That’s rare behavior. Lehigh University engineer James Gilchrist directs the Laboratory for Particle Mixing and Self-Organization and discovered this strange phenomenon while experimenting with “micro-rollers”: polymer particles coated in iron oxide (a process called micro-encapsulation). He was spinning a magnet in the bottom of a bowl of micro-rollers one day and noticed that they started to pile up in a hill. Naturally he and his colleagues had to investigate further.

For their experiments, Gilchrist et al. attached neodymium magnets to a motorized wheel at 90-degree intervals, alternating poles facing outward. The apparatus also includes a sample holder and a USB microscope in a fixed position. Micro-rollers are prepared by suspending them in a glass vial containing ethanol and using a magnet to separate them from dust or any uncoated particles. Once the micro-rollers were clean, they were dried, suspended in fresh ethanol, and loaded into the sample holder. A vibrating motor moves the samples to create flattened granular beds, and the motorized wheel is driven to apply magnetic torque. A gaussmeter measures the strength of the magnetic field relative to the orientation.

Upward granular flow of microrobotic microrollers. Source: Lehigh University.

The results: each micro-roller begins to rotate in response to the magnetic torque, creating pairs that are briefly formed and then split, and increasing the magnetic force increases the particle cohesion. This in turn gives the micro-rollers more traction and makes them move faster, working in concert to counterintuitively flow uphill. Without the magnetic torque, the miro-rollers slide down normally. The action caused by the torque is so unexpected that the researchers coined a new term to describe it: a “negative angle of repose” due to the negative coefficient of friction.

So far, no one has used these terms, Gilchrist said. They don’t exist. But to find out how these grains flow upwards, we calculate what stresses cause them to move in that direction. If you have a negative angle of repose, you must have unity to give a negative coefficient of friction. These granular equations of flow are never taken into consideration of these factors, but after calculating them, what comes out is the apparent coefficient of friction which is negative.

This is an interesting proof of principle that may one day lead to new ways to control how substances are mixed or separated, as well as potential applications in microrobotics. Scientists have started making small stairs with laser cutters and videoing the micro-rollers going up and down the other side. A micro-roller cannot overcome the length of each step, but many working collectives can, according to Gilchrist.

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

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