  Active matter comprised of self-propelled interacting units holds a major
promise for extraction of useful work from its seemingly chaotic
out-of-equilibrium dynamics. Streamlining active matter to produce work is
especially important at microscale, where the viscous forces prevail over
inertia and the useful modes of transport require very specific non-reciprocal
type of motion. Here we report that microscopic active droplets representing
aqueous dispersions of swimming bacteria Bacillus subtilis show unidirectional
propulsion when placed in an inactive nematic medium. Random motion of bacteria
inside the droplet is rectified into a directional self-locomotion of the
droplet by the polar director structure that the droplet itself creates in the
surrounding nematic through anisotropic molecular interactions at its surface.
Droplets without swimming bacteria show no net displacement. The trajectory of
the active droplet can be predesigned as rectilinear or curvilinear by
patterning the molecular orientation of the nematic medium. The effect
demonstrates that swimming at microscale can be achieved at the expense of
broken spatial symmetry of the medium; it can be used in development of
micromachines.
