Engineers at Lehigh University announced the discovery of a phenomenon in a statement. A phenomenon that defies conventional wisdom—Magnetized sand flowing uphill.
The findings of this research, published in the journal Nature Communications, reveal a spectacle that challenges everything we thought we knew about granular materials.
James Gilchrist, the Ruth H. and Sam Madrid Professor of Chemical and Biomolecular Engineering at Lehigh’s P.C. Rossin College of Engineering and Applied Science. And one of the authors of the paper, stated, “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.”
A Serendipitous Discovery Magnetized Sand
Dr. Samuel Wilson-Whitford, the lead author of the paper. A former postdoctoral research associate in Gilchrist’s Laboratory of Particle Mixing and Self-Organization stumbled upon this extraordinary phenomenon during his research into microencapsulation.
The serendipitous nature of this revelation adds an element of wonder to the story. It all began when he introduced a magnet beneath a vial of iron oxide-coated polymer particles known as micro rollers.
Under the influence of the magnet, these micro rollers displayed an uncanny behavior—they began to defy gravity by moving uphill.
Wilson-Whitford, along with Gilchrist, embarked on a systematic study of this material’s response to magnetic forces under various conditions. When the micro rollers were poured without magnetization, they flowed downhill.
How Magnetized Sand Works
However, on applying torque using magnets, each particle was observed to rotate. Forming temporary doublets that exhibited cohesion and resulted in a negative angle of repose due to a negative coefficient of friction.
“Up until now, no one would have used these terms. 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,” elaborated Gilchrist.
“If you have a negative angle of repose, then you must have the 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.”
A World of Possibilities
Increasing the magnetic force increases the cohesion, giving grains more traction and speed to their movement. The collective motion and adhesion among the grains enable piles of sand particles to collaboratively perform surprising feats, such as flowing up walls and climbing stairs. The team is analyzing the movement of material across tiny staircases built using a laser cutter.
“This first paper just focuses on how the material flows uphill, but our next several papers will look at applications,” said Gilchrist. “Part of that exploration is answering the question, can these micro rollers climb obstacles? And the answer is yes.”
The applications of this discovery appear boundless. Microrollers could revolutionize industries by changing the way we mix things, segregate materials, and move objects. This revelation could also open up new vistas in micro-robotics and revolutionize healthcare.
“We’re studying these particles to death, experimenting with different rotation rates, and different amounts of magnetic force to understand their collective motion better. I know the titles of the next 14 papers we’re going to publish,” Gilchrist enthused.
The sands of science are shifting, and they are flowing in unexpected directions.
Summary
- This research explores how to make sand flow uphill by applying torque to individual grains using a rotating magnetic field.
- Normally, friction in flowing sand quickly dissipates energy, limiting movement to the top surface.
- By giving each grain an individual “spin,” they stick together temporarily and overcome gravity, forming a heap with a negative angle of repose (meaning it slopes upwards).
- The researchers identified two “regimes” of uphill flow depending on the applied torque and bulk movement.
- They developed a model that accurately predicts the negative friction using cohesion between particles.
- Interestingly, even though individual grains act actively (responding to the magnetic field), the overall uphill flow behaves like a dissipative granular system (losing energy over time).
In essence, this research shows that by manipulating individual grains at the microscopic level, you can create macroscopic phenomena like sand defying gravity.
