Directional self-locomotion of active droplets enabled by nematic environment

Abstract: 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.