Negative friction sees sand flow uphill

Sand that seemingly defies gravity by flowing uphill, has been created by researchers at Lehigh University in the US.

They believe the discovery could be useful in applications from healthcare to material transport and agriculture. The team’s findings are  published in Nature Communications.

Piles of black granule sand force diagram
Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-41327-1.

The bizarre motion was discovered accidentally when Dr Samuel Wilson-Whitford was researching microencapsulation – a process with gives tiny particles or droplets useful properties by coating them in other materials.

Polymer granules coated in iron oxide did something unexpected when a magnet beneath them was rotated. They began to heap uphill.

When not activated by the magnet, the “microrollers” acted as expected and rolled downhill.

But the team noticed that the rotating magnet also generated a torque in the granules which caused them to rotate in turn. In a pile, the granules interacted with each other creating a “cohesion” which saw the particles move uphill.

Lehigh University professor James Gilchrist says that a single grain of sand couldn’t do this, but through collective motion the particles can perform their gravity-defying stunts.

He says the experiment is the first evidence for a negative angle of repose (uphill, rather than down) due to a negative coefficient of friction.

“To understand how these grains are flowing uphill, we calculated what the stresses are that cause them to move in that direction,” Gilchrist explains. “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.”

In all known circumstances, friction coefficients are positive.

Friction – the interaction of particles on the surface of objects – is responsible for sparks that cause fire and animal and vehicle locomotion.

Gilchrist’s team is experimenting with the strength of the effect.

Increasing the magnetic force increases cohesion and makes the particles move faster. The collective motion of the grains means that piles of sand can be made to flow up walls and climb stairs.

“We’re studying these particles to death, experimenting with different rotation rates and different amounts of magnetic force to better understand their collective motion,” Gilchrist says.

“This first paper just focuses on how the material flows uphill, but our next several papers will look at applications. Part of that exploration is answering the question: can these microrollers climb obstacles? And the answer is yes.”

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