Active matter composed of self-propelled interacting units holds a major promise for the extraction of useful work from its seemingly chaotic dynamics. Streamlining active matter is especially important at the microscale, where the viscous forces prevail over inertia and transport requires a non-reciprocal motion. Here we report that microscopic active droplets representing aqueous dispersions of swimming bacteria Bacillus subtilis become unidirectionally motile when placed in an inactive nematic liquid-crystal 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 creates in the surrounding nematic through anisotropic molecular interactions at its surface. Droplets without active swimmers show no net displacement. The trajectory of the active droplet can be predesigned by patterning the molecular orientation of the nematic. The effect demonstrates that broken spatial symmetry of the medium can be the reason for and the means to control directional microscale transport.

由自作用的相互作用单元组成的活性物质具有从其看似混乱的动力学中提取有用工作的主要前景。在微观尺度上，使活性物质流线化尤为重要，因为在微观尺度上，粘性力胜过惯性，而运输则要求不可逆运动。在这里我们报告，代表游泳细菌枯草芽孢杆菌水分散体的微观活性液滴当置于非活性向列型液晶介质中时，单向运动。液滴内细菌的随机运动通过极性导向器结构校正为液滴的定向自移动，该极性导向器结构是液滴通过其表面的各向异性分子相互作用在周围的向列中形成的。没有活跃游泳者的飞沫没有净位移。活性液滴的轨迹可以通过图案化向列的分子取向来预先设计。该结果表明，介质空间对称性的破坏可能是导致定向微尺度传输的原因和手段。