A Magnetic 'Flea' That Spins and Hovers

(Inside Science) -- It was just another day in the lab when David Fairhurst first noticed one of his trusted fleas was acting a little unusual. Fairhurst is a fluid physicist, not a biologist, and the “flea” in question was a little magnetic pill used to stir up liquids. One of its inventors, Edward McLaughlin, had named the device a “flea” due to its occasional jumpy moments, but what Fairhurst observed was something different -- the flea was spinning and hovering steadily in the middle of the beaker.

Fairhurst followed his curiosity about the odd behavior, and uncovered new insights into the operating principles of this humble staple of chemistry labs. The findings could one day pave the way to improved microfluidic pumps that move tiny amounts of liquid for applications ranging from inkjet printing to drug delivery.

An unassuming 'flea'

The first step Fairhurst, who works at Nottingham Trent University in the U.K., took was to look up papers about the motion of magnetic stirrers -- but he found nothing, not even about the jumpy movement that inspired the “flea” moniker.

“I was stunned actually. I mean, no one has looked into this? You're kidding,” Fairhurst recalled.

After fiddling with the settings of the external magnetic motor, as well as testing the flea in liquids with different levels of viscosities, Fairhurst and his colleagues found that the flea would start jumping around if the external magnetic motor tried to spin the flea faster than the liquid allowed it to move. However, under certain conditions, when the viscosity of the liquid and the speed of the external motor are tuned just right, the flea would hover steadily in the middle of the beaker in a gravity-defying feat.

Using a high-speed camera, they captured the movement of the flea in its semistable hovering state, and discovered that the flea would waggle as it spun -- roughly 60 degrees one way, and then 10 degrees back. They also added tiny reflective particles into the liquids to study the flow pattern generated by the flea.

The waggling motion of the flea, unintentionally generated by the external motor when it fell out of sync with the flea’s movement, enabled the stirrer to propel itself off the bottom of the beaker and also spew liquid radially outward. The findings were published in the journal Physical Review Letters.

The waggling motion of a magnetic stirrer allows it to hover above the bottom of a beaker.
Credit: Physical Review Letters/ American Physical Society

Tiny pumps with big applications

The new findings could impact the field of microfluidics, which focuses on the manipulation of fluids on a microscopic scale, said Don DeVoe, a microfluidics engineer from the University of Maryland, College Park who was not involved in the study. “Their technique is unique in the sense that one can control these oscillating streaming flows with an external magnetic motor.”

One of the main challenges in microfluidics is fitting all the components needed to move liquids onto a tiny device called a chip, and according to both Fairhurst and DeVoe, the new paper could provide new approaches for making smaller and simpler micropumps.

“The most common way to pump fluids in microfluidic devices today is using off-chip pumps, which are typically fairly expensive and bulky and power-hungry,” said DeVoe.

The magnetically driven flea may provide a way for integrating pumps onto the chips. It would still require an external magnetic motor to power the pump, but could provide a tubeless solution that eliminates the need to connect every pumping channel to an external pump.

“Maybe not right away, but one could imagine taking advantage of this basic concept in future designs,” said DeVoe.

Perhaps one day, this unintended bug in the magnetic flea’s design will become a desirable feature for something the inventors had never envisioned.