Rolling at right angles: Magnetic anisotropy enables dual-anisotropic active matter

We report on an experimental active matter system with motion restricted to four cardinal directions. Our particles are magnetite-doped colloidal spheres driven by the Quincke electrorotational instability. The absence of a magnetic field (|B|=0) leads to circular trajectories interspersed with short spontaneous runs. Intermediate fields (|B|≲20mT) linearize the motion along the axis perpendicular to B. At high magnetic fields, we observe the surprising emergence of a second, distinct linearization along the axis parallel to B. With numerical simulations, we show that this behavior can be explained by anisotropic magnetic susceptibility.