Defying gravity: Team discovers sand that can flow uphill

Researchers at Lehigh University have made a groundbreaking discovery – sand that can flow uphill. This finding has the potential to redefine our understanding of granular materials and their behavior.

The research also opens the door to numerous applications ranging from healthcare to material transport.

New phenomenon

The Lehigh team demonstrated that when torque and an attractive force are applied to sand grains, they can flow uphill, ascend walls, and even traverse stairs. 

This behavior defies our traditional understanding of gravity-driven granular flow and could signify the start of a new chapter in the study of granular materials.

“After using equations that describe the flow of granular materials, we were able to conclusively show that these particles were indeed moving like a granular material, except they were flowing uphill,” said study co-author Professor James Gilchrist.

Dr. Samuel Wilson-Whitford, the lead author of the paper and a former research associate at Gilchrist’s laboratory, first stumbled upon this phenomenon while researching microencapsulation. 

He found that when a magnet was rotated beneath a vial containing iron oxide-coated polymer particles, known as microrollers, the grains began heaping uphill.

Understanding the mechanics

The researchers found that activating the microrollers with the magnet resulted in the particles rotating and forming temporary doublets. This dynamic leads to cohesion that yields a negative angle of repose due to a previously unheard-of negative coefficient of friction.

“Up until now, no one would have used these terms,” said Professor Gilchrist. “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.”

Potential applications

The implications of this discovery are far-reaching. The microrollers can potentially be utilized to mix substances, segregate materials, or move objects. 

The newfound understanding of their collective behavior suggests that they could be employed in microrobotics, which could revolutionize healthcare.

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, but working together they can, said Professor Gilchrist.

Future research 

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

Gilchrist has submitted a paper that delves into using these microrollers for delivering nutrients through soil, emphasizing their potential in agricultural applications.

“We’re studying these particles to death, experimenting with different rotation rates, and different amounts of magnetic force to better understand their collective motion. I basically know the titles of the next 14 papers we’re going to publish.”

The research is published in the journal Nature Communications.

Video Credit: Laboratory for Particle Mixing and Self-Organization, Lehigh University

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