Researchers make sand that flows uphill

A team of researchers including a chemist at the University of Leicester has discovered that sand can actually flow uphill.

The team’s findings have been published in the journal Nature Communications. A corresponding video shows what happens when torque and an attractive force is applied to each grain—the grains flow uphill, up walls, and up and down stairs.  

The researchers, led by Lehigh University in Pennsylvania, United States, say the highly unusual discovery could unlock many more lines of inquiry that could lead to a vast range of applications, from healthcare to material transport and agriculture.

“After using equations that describe the flow of granular materials,” says James Gilchrist, the Ruth H. and Sam Madrid Professor of Chemical and Biomolecular Engineering in Lehigh’s P.C. Rossin College of Engineering and Applied Science and one of the authors of the paper, “we were able to conclusively show that these particles were indeed moving like a granular material, except they were flowing uphill.” 

The paper’s lead author, Dr Samuel Wilson-Whitford who conducted the work at Lehigh University and is now based at the University of Leicester School of Chemistry, captured the movement entirely by serendipity in the course of his research into microencapsulation. When he rotated a magnet beneath a vial of iron oxide-coated polymer particles called microrollers, the grains began to heap uphill.

Dr Wilson-Whitford and Professor Gilchrist began studying how the material reacted to the magnet under different conditions. When they poured the microrollers without activating them with the magnet, they flowed downhill. But when they applied torque using the magnets, each particle began to rotate, creating temporary doublets that quickly formed and broke up. The result, says Gilchrist, is cohesion that generates a negative angle of repose due to a negative coefficient of friction.

“Up until now, no one would have used these terms,” he says. “They didn’t exist. But to understand how these grains are flowing uphill, we calculated what the stresses are that cause them to move in that direction. If you have a negative angle of repose, then you must have cohesion to give a negative coefficient of friction. These granular flow equations were never derived to consider these things, but after calculating it, what came out is an apparent coefficient of friction that is negative.”

Dr Samuel Wilson-Whitford from the University of Leicester School of Chemistry said: “We can think about the benefits of this phenomenon on the small or industrial scales. The handling of granular material, such as sugar and powders, is hugely important in industrial processes and as such the ability to reverse or alter the flow behaviour of grains will be useful to industrial powder processes such as separation and mixing. On the small scale, these materials can be used in the field of microrobotics, where very small objects such as these microrollers are directed to perform tasks. These tasks could be anything from displacement of matter to injectable robotic medicines, and as we are already seeing the swarms of rollers can collectively navigate complex structures. Really, the possibilities are beyond count.”

Increasing the magnetic force increases the cohesion, which gives the grains more traction and the ability to move faster. The collective motion of all those grains, and their ability to stick to each other, allows a pile of sand particles to essentially work together to do counterintuitive things—like flow up walls, and climb stairs. The team is now using a laser cutter to build tiny staircases, and is taking videos of the material ascending one side and descending the other. A single microroller couldn’t overcome the height of each step, says Professor Gilchrist. But working together, they can.

Potential applications could be far ranging. The microrollers could be used to mix things, segregate materials, or move objects. And because these researchers have discovered a new way to think about how the particles essentially swarm and work collectively, future uses could be in microrobotics, which in turn could have applications in healthcare. 

• Microrollers flow uphill as granular media is published in Nature Communications, DOI: 10.1038/s41467-023-41327-1
• Associated funders include The John Hopkins University Applied Physics Laboratories, the National Science Foundation (1931681), the McClurg Endowment Faculty Development Fund of the Department of Chemical and Biomolecular Engineering at Lehigh University. Equipment in Lehigh's Institute for Functional Materials and Devices (I-FMD) was used in the research.