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Counterrotation of magnetic beads in spinning fields

Monday 5 October 2020, by mcube

Jean Farago, Thierry Charitat, Alexandre Bigot, Romain Schotter, and Igor Kulić
Phys. Rev. E 102, 042201 – Published 2 October 2020.

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)], is here comprehensively studied, with numerical simulations and a theoretical approach complementing experimental observations.
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