  While active systems possess notable potential to form the foundation of new
classes of autonomous materials, designing systems that can extract functional
work from active surroundings has proven challenging. In this work, we extend
these efforts to the realm of designed active liquid crystal/colloidal
composites. We propose suspending colloidal particles with Janus anchoring
conditions in an active nematic medium. These passive Janus particles become
effectively self-propelled once immersed into an active nematic bath. The
self-propulsion of passive Janus particles arises from the effective $+1/2$
topological charge their surface enforces on the surrounding active fluid. We
analytically study their dynamics and the orientational dependence on the
position of a companion $-1/2$ defect. We predict that at sufficiently small
activity, the colloid and companion defect remain bound to each other, with the
defect strongly orienting the colloid to propel either parallel or
perpendicular to the nematic. At sufficiently high activity, we predict an
unbinding of the colloid/defect pair. This work demonstrates how suspending
engineered colloids in active liquid crystals may present a path to extracting
activity to drive functionality.

Soft Condensed Matter