Synopsis: Levitating in a Fluid

Researchers have identified a regime in which a magnetic stir bar can be made to levitate while it spins in a fluid. ­­
Synopsis figure
K. Baldwin/Nottingham Trent University

Working in the lab one day, David Fairhurst noticed that his equipment seemed awry. The magnetic “flea” stirrer he was using to mix a polymer with water wasn’t spinning along the beaker’s bottom as normal but was instead levitating in the beaker’s center. Intrigued, Fairhurst, of Nottingham Trent University in the UK, and colleagues set out to study this phenomenon. They found that the behavior arises from a combination of magnetic and fluid forces that act on the flea as it spins at high speed. This insight could aid in the development of artificial swimmers and microfluidic pumps.

To study the behavior, the team used a standard magnetic stirrer that produced a rotating magnetic field that spun a bar magnet—the flea—submerged in a fluid-filled beaker. At low rotation speeds, the magnetic field and the flea remained aligned. The poles of the flea's magnet and those of the magnet creating the rotating field pointed in opposite directions, and the flea spun smoothly along the beaker’s base. For higher rotation speeds, however, viscous forces drag the flea out of alignment with the rotating magnetic field, until the magnetic force acting on the flea flips from being attractive to repulsive. This repulsive force can then lift the flea off the beaker’s base.

At high rotation speeds, the team also observed the flea waggling—oscillating in a circular pattern perpendicular to its axis—while it spun. Using computer simulations, they showed that this waggling stabilizes the levitating flea’s motion in very viscous fluids, a finding they confirmed in experiments. The waggle pumped fluid outward from the flea’s center, creating a centripetal force that held it steady. In fluids with low viscosity, however, the waggle drew fluid toward the flea, knocking it from its stable state.

This research is published in Physical Review Letters.

Correction (9 August 2018): The text was revised to give a more accurate explanation of why higher rotation speeds lead to lift of the flea.

–Michelle Hampson

Michelle Hampson is a freelance writer based in Toronto.


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