  We create controllable active particles in the form of metal-dielectric Janus
colloids which acquire motility through a nematic liquid crystal film by
transducing the energy of an imposed perpendicular AC electric field. We
achieve complete command over trajectories by varying field amplitude and
frequency, piloting the colloids at will in the plane spanned by the axes of
the particle and the nematic. The underlying mechanism exploits the sensitivity
of electro-osmotic flow to the asymmetries of the particle surface and the
liquid-crystal defect structure. We present a calculation of the dipolar force
density produced by the interplay of the electric field with director anchoring
and the contrasting electrostatic boundary conditions on the two hemispheres,
that accounts for the dielectric-forward (metal-forward) motion of the colloids
due to induced puller (pusher) force dipoles. These findings open unexplored
directions for the use of colloids and liquid crystals in controlled transport,
assembly and collective dynamics.
