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Updated: Nov 29, 2025

Magnetic Tweezers for the Measurement of Twist and Torque
Published on: May 19, 2014
Counterrotation of magnetic beads in spinning fields
Jean Farago1, Thierry Charitat1, Alexandre Bigot1
1Université de Strasbourg, CNRS, Institut Charles Sadron UPR-22, Strasbourg, France.
Abstract:
A magnetic stirrer, an omnipresent device in the laboratory, generates a spinning magnetic dipolelike field that drives in a contactless manner the rotation of a ferromagnetic bead on top of it. We investigate here the surprisingly complex dynamics displayed by the spinning magnetic bead emerging from its dissipatively driven, coupled translation and rotation. A particularly stunning and counterintuitive phenomenon is the sudden inversion of the bead's rotational direction, from corotation to counterrotation, acting seemingly against the driving field, when the stirrer's frequency surpasses a critical value. The bead counterrotation effect, experimentally described by Chau et al. [J. Magn. Magn. Mater. 476, 376 (2019)JMMMDC0304-885310.1016/j.jmmm.2018.12.073], is here comprehensively studied, with numerical simulations and a theoretical approach complementing experimental observations.
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