Wave-induced motion of magnetic spheres

TL;DR

This study experimentally investigates how magnetized spheres move under traveling magnetic waves, revealing backward motion, chaos, and speeds exceeding the wave, explained by a model involving friction, air resistance, and magnetic torque.

Contribution

It introduces new experimental observations of backward and chaotic motion of magnetic spheres driven by traveling magnetic waves, with a theoretical model explaining these phenomena.

Findings

Backward motion occurs at high wave speeds.

The transition to backward motion is subcritical and can be chaotic.

Sphere speed can surpass the wave speed by an order of magnitude.

Abstract

We report an experimental study of the motion of magnetized beads driven by a travelling wave magnetic field. For sufficiently large wave speed, we report the existence of a backward motion, in which the sphere can move in the direction opposite to the driving wave. We show that the transition to this new state is strongly subcritical and can lead to chaotic motion of the bead. For some parameters, this counter-propagation of the sphere can be one order of magnitude faster than the driving wave speed. These results are understood in the framework of a model based on the interplay among solid friction, air resistance and magnetic torque.