2023-09-21 10:40:46
An upward flow of Janus microrollers driven by magnetic drive, including an illustration of the direction of rotation of the particles – NATURE COMMUNICATIONS
MADRID, 21 Sep. (EUROPA PRESS) –
Lehigh University researchers have discovered that sand can flow uphill, with applications, from healthcare to materials transportation and agriculture.
A corresponding video shows what happens when a torque and attractive force are applied to each grain: the grains flow uphill, up walls, and up and down stairs.
“After using equations describing the flow of granular materials” says James Gilchrist, professor of Chemical and Biomolecular Engineering and one of the authors of the paper, “we were able to show conclusively that these particles were indeed moving like a granular material, except that they flowed uphill.”
The researchers say this very unusual discovery could unlock many more lines of research that could lead to a wide range of applications, from healthcare to materials transportation and agriculture.
The paper’s lead author, Dr Samuel Wilson-Whitford, a former postdoctoral research associate in Gilchrist’s Laboratory of Particle Mixing and Self-Organisation, captured the motion entirely by chance in the course of his research into microencapsulation. When he spun a magnet beneath a vial of iron oxide-coated polymer particles called microrollers, grains began to accumulate uphill.
Wilson-Whitford and 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 spin, creating temporary doublets that quickly formed and broke apart. The result, Gilchrist says, is a cohesion that generates a negative angle of repose due to a 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 flow uphill, we calculated what the stresses are that make them move in that direction. If you have a negative angle of repose, then you must have cohesion to give a negative angle of repose.” “Friction coefficient. These granular flow equations were never derived to consider these things, but after calculating them, What resulted is an apparent friction coefficient that is negative.”
Increasing magnetic force increases cohesion, giving the grains more traction and the ability to move faster. The collective movement 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 climb walls and climb stairs. The team is now using a laser cutter to build small stairs and is recording videos of the material ascending one side and descending the other. A single microroller couldn’t surpass the height of each step, Gilchrist says. But working together, they can do it.
“This first article simply focuses on how material flows uphill, but our next articles will look at applications, and part of that exploration is answering the question: Can these micro rollers climb obstacles? And the answer is yes”.
The potential applications could be far-reaching. Microrollers could be used to mix things, segregate materials, or move objects. Because these researchers have discovered a new way of thinking about how particles essentially swarm and work collectively, future uses could be in microrobotics, which in turn could have applications in healthcare. Gilchrist recently presented a paper exploring its use in soil as a means of delivering nutrients through a porous material.
“We are studying these particles thoroughly,” he says, “experimenting with different rotation speeds and different amounts of magnetic force to better understand their collective motion. “Basically, I know the titles of the next 14 articles we’re publishing.”
#sand #move