Regular and Anomalous Motion of Individual Magnetic Quincke Rollers Under Rotating Magnetic Field

TL;DR

This study investigates the motion of magnetic Quincke rollers under a rotating magnetic field, revealing both regular helical trajectories and unexpected anomalous counterclockwise motion influenced by magnetic and hydrodynamic factors.

Contribution

It introduces a comprehensive model explaining the emergence of anomalous motion in magnetic Quincke rollers under rotating magnetic fields.

Findings

At low frequencies, particles exhibit clockwise helical trajectories.

Higher frequencies induce helical wavy trajectories with varying curvature.

Anomalous counterclockwise trajectories occur under specific conditions, contrary to the field direction.

Abstract

We report the motion of individual magnetic Quincke rollers composed of silica particles doped with superparamagnetic iron oxide nanoparticles, whose activity arises from the coupling between Quincke rolling and an externally applied rotating magnetic field. We applied a clockwise (CW) rotating magnetic field of magnitude approximately 11 mT and rotational frequencies ranging from 0.2 to 2.75 Hz. At low frequencies, the dominant mode of motion is a CW helical trajectory. Circular trajectories emerge as a limiting case of this helical motion, in which lateral translation vanishes and the particle traces overlapping closed loops in the xy-plane. At higher frequencies, a second regular mode becomes prevalent, characterized by helical wavy trajectories in which the particle follows a CW helical path with a spatially varying curvature. Under specific conditions, however, we observe the unexpected emergence of anomalous counterclockwise (CCW) trajectories, in which individual particles roll in a direction opposite to that of the applied CW rotating magnetic field. A theoretical model incorporating electrostatic interactions, far-field hydrodynamic coupling, and a magnetic dipole approximation indicates that the anomalous behavior results from the interplay among the magnitude and orientation of the initial magnetic dipole moment, the frequency of the rotating magnetic field, and the magnitude of the initial translational velocity. Together, these factors determine the likelihood of a particle exhibiting regular or anomalous rotational motion.