Movie S1.

Unidirectional alignment of the active nematic. The normally turbulent active nematic dynamics is indirectly organized into unidirectional patterns of laminar flow by the action of a magnetic field. Transition from the nematic (36 °C) to the smectic-A (25 °C) phase of the passive liquid crystal 8CB, leads to the formation of a “bookshelf” configuration of the mesogen molecules under the influence of a homogeneous magnetic field (4 kG). The interface presents now strong uniaxial anisotropy of the shear stress, which forces the active nematic underneath. The biofilaments assemble now in stripes surrounded by defect lanes, which organize antiparallel flows along the direction of the less viscous axis, perpendicular to the magnetic field (H).