Rolling in circles: the mysterious dynamics of superparamagnetic Quincke rollers

Quincke rotation occurs when a dielectric particle in a conducting fluid rotates spontaneously in response to an externally applied DC electric field E. Above a threshold value E_c, for particles near a surface, friction converts rotation into rolling in the plane orthogonal to E at a constant speed set by the field. Typical Quincke systems showcase individual random walks, with collective motion emerging at sufficient densities. We introduce an additional degree of freedom using superparamagnetic (SPM) particles, and observe markedly different dynamics. With no magnetic field B, particles execute tight circular trajectories or even orbits. These are more unstable at higher E fields, as periods of circular motion may be interspersed with short “walks”. We also find that the rolling speed now scales with |E|^α, α ≥ 2. Introducing a homogeneous in-plane B in addition to E, linearises the circular motion, causing flows perpendicular to the magnetic field lines. For a fixed E ≥ E_c, rolling speed remains independent of the applied B field. Unlike other recent work on SPM rollers, we observe no enhanced particle interactions, e.g. chaining. While the origin of the circular motion remains elusive, the switch to linear motion in a non-zero B field suggests the particles’ intrinsic magnetic properties are critical in determining their dynamics.