Scientists Discover Sand-Like Material That Can Flow Uphill
If you saw grains of sand rolling uphill you might be forgiven for thinking you were watching a Christopher Nolan movie. But scientists have recently figured out how a sand-like material can be made to flow back up slopes, without rewinding time.
Researchers from Lehigh University in the US have engineered tiny particles they call microrollers by coating microscopic grains of polymethyl methacrylate plastic in iron oxide. Activating a rotating magnetic field near a sloped mound of the particles was found to exert a twisting force or torque on each individual microroller. Coupled with a fluctuating attraction between the grains, the ‘sand’ drove itself up slopes and even overcame obstacles against the pull of gravity.
“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,” says chemical and biomolecular engineer James Gilchrist.
The researchers had to come up with new terms to describe the unexpected action. The uphill flow is referred to as a negative angle of repose, caused by a negative coefficient of friction that actually boosts movement rather than slowing it down as would usually be the case.
As a magnetic torque is applied to these microrollers, they rotate around their axes, forcing the particles to temporarily come together and act in unison. This rolling and subsequent cohesion allows the grains to climb up a slope.
A stronger magnetic field means more cohesion, the researchers report, so the grains gain more traction and move faster. The collective effects are impossible if the grains are simply operating independently.
While it’s still early days for the research, there are many potential uses for it. Scientists could potentially control the way substances mix or separate, and the approach could be used in microrobotics and drug delivery.
The team has some solid ideas about what to tackle next with its discovery, and work on constructing a tiny staircase for these weirdly traveling grains to be able to climb has already begun.
“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?” says Gilchrist. “And the answer is yes.”
“We’re studying these particles to death, experimenting with different rotation rates, and different amounts of magnetic force to better understand their collective motion.”
The research has been published in Nature Communications.