From the hourglass to the dunes on the beach, we know that sand can flow downhill. Granular materials naturally behave this way. But researchers have discovered that it is possible to create a granular material that ignores this basic principle: a sand-like substance that flows uphill and climbs stairs and walls.
Like many impressive discoveries, this one came about by chance. Lead author Dr. Samuel Wilson-Whitford noticed the strange movements when he rotated a magnet at the bottom of a vial filled with polymer particles coated with iron oxide. Without the magnet, they behaved like any sand-like medium and behaved as the granular flow equations would expect. But the magnetic field was changing everything.
“Using the equations that describe the flow of granular materials, we were able to show conclusively that these particles do indeed behave like a granular material, except that they flow uphill,” co-author Professor James Gilchrist of Lehigh University said in a statement on the subject.
To explain the motion, the team had to make some unique changes to the detailed flow equations. The angle of repose, the steepest angle of descent that could be created without collapsing the material, had to be negative. The friction coefficient also had to be negative.
“No one would have used these before,” Gilchrist says. “These didn’t exist. But to understand how these particles flow uphill, we calculated what the stresses were that caused them to move in that direction. If you have a negative angle of repose, you need to have cohesion to give a negative drag coefficient. Granular flow equations were never derived to take them into account, but once calculated, a drag coefficient appeared to be negative.”
The magnetic field is shown to be the reason for the irrational movement of these substances, called microcylinders. The magnetic field creates a torque that causes the particles to start spinning, and cohesion is produced so that the particles stick together for a while. A stronger magnetic field increases cohesion, giving them more traction, allowing them to go uphill faster. In this way, the researchers were able to move the material up small stairs.
“This first paper focuses only on how material flows uphill, but our next few papers will look at applications, and some of this research will answer the question: Can these microrollers climb obstacles,” Gilchrist said. “And the answer is yes,” he says and continues: “We are examining these particles to their fullest extent. We experiment with different rotation speeds and different amounts of magnetic forces to better understand their collective motion. “I basically know the titles of the next 14 articles we will publish.”
The study was published in the journal Nature Communications.
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