Harnessing a twist wall instability to propel a rotated nematic colloid

We study a ferromagnetic disk colloid confined in a nematic liquid crystal filled cell with planar anchoring. When rotated by an external magnetic field, the disk translates or swims, a motion forbidden in isotropic fluids. Furthermore, the disk's rate of translation depends on its anchoring conditions. In particular, a disk with hybrid anchoring translates faster than one with all homeotropic anchoring, and this translation is accompanied by a complex reorganization of the nematic director's structure as the disk swims that does not occur for all homeotropic colloids. As the hybrid disk rotates, its companion defect elongates along one edge and sweeps across the disk's face at a critical rotation angle. We study these dynamics in the context of the instability of a topological twist defect. We find that the companion defect is a twist wall whose formation is embedded in the three-dimensional director field around the disk. We identify the mechanism that pins the defect along the disk's sharp edges that demarcate a boundary between opposite twist handedness. These findings will inform new strategies in micro-robotics and reconfigurable materials that exploit far-from-equilibrium structures in nematic liquid crystals.