Passive Janus particles are self-propelled in active nematics

The pursuit of systems capable of extracting work from active media has proven to be a particularly challenging task. In this work, we extend these efforts to the realm of active liquid crystal composites and present a design for passive particles that become effectively self-motile when embedded in an active fluid. We study a colloidal particle with Janus anchoring conditions immersed in an active nematic liquid crystal. The colloid surface enforces an effective +1/2 topological charge in the surrounding active fluid, which gives rise to an effective self-propulsion of the Janus particle. We analytically study this self-propulsion, linking its orientational dependence on the position of a companion -1/2 defect. We predict that the colloid/defect pair remains bounded at small activity, with the defect firmly orienting the colloid to propel parallel or perpendicular to the nematic. Conversely, if the activity is sufficiently high, we predict an unbinding of the colloid/defect pair. This work demonstrates how engineered colloids suspended in active liquid crystals may present a path to functionality.