The team’s findings were published in a journal Nature Communication. The accompanying video shows what happens when torque and gravity are applied to each grain; The grains flow uphill, up the walls, and up and down the stairs.

“After using the equations that describe the flow of granular materials,” says James Gilchrist, the Ruth H. and Sam Madrid Professor of Chemical and Biomolecular Engineering at Lehigh PC Rossin School of Engineering and Applied Science and one of the paper’s authors. ‘We were able to show conclusively that these particles do indeed behave like granular material, except that they flow uphill.’

Researchers say this highly unusual discovery could open up many other avenues of research that could lead to a wide range of applications, from healthcare to material transportation to agriculture.

The lead author of the paper is Dr. D. D., a former research assistant at the Gilchrist Particle Entanglement and Self-Organization Laboratory. Samuel Wilson-Whitford caught this movement during his research on microencapsulation. When he rotated a magnet at the bottom of a vial filled with iron oxide-coated polymer particles called microcylinders, the grains began shooting uphill.

Wilson-Whitford and Gilchrist began investigating how the material responded to the magnet under different conditions. When they poured the microrolls without activating them with a magnet, they flowed downwards. But when they applied torque with magnets, each particle began to spin, creating temporary pairs that quickly formed and broke apart. The result, according to Gilchrist, is cohesion, which creates a negative natural camber angle due to the negative coefficient of friction.

“Until now, no one would have used these terms,” he says. “They didn’t exist. But to understand how these grains flowed upward, we calculated the stresses that moved them in that direction. If you have a negative camber angle, cohesion has to be present to give a negative coefficient of friction. These granular flow equations were never derived to take these things into account, However, after calculation, the resulting imaginary friction coefficient turns out to be negative.”

Increasing the magnetic force increases cohesion, which gives the particles more traction and the ability to move faster. The collective movement of all these grains and their ability to stick together allows a group of sand particles to actually work together to do crazy things, like flow up walls and up stairs. The team now uses a laser cutter to create small staircases, filming the material as it moves up one side and down the other. Gilchrist says a microcylinder cannot cover the height of every step. They can only achieve this by working together.

“This first paper focuses only on how the material flows upward, but our next few papers will look at applications, and part of that research will be answering the question: Can these microfilms cross barriers? And the answer is yes.”

Potential applications can be very diverse. Micro rollers can be used to mix things, separate ingredients, or move objects. Because these researchers discovered a new way of thinking about how particles actually pack together and work collectively, it could have future applications in microrobots and, in turn, healthcare. Gilchrist recently presented a paper exploring their use in soil as a means of delivering nutrients through porous material.

“We are studying these particles to death,” he says, “experimenting with different spin speeds and different magnetic force magnitudes to better understand their collective motion. I basically know the titles of the next 14 papers we will publish.” Source