Active nematics near walls*

Topology and anchoring boundary conditions are effective tools to control static defect configurations in equilibrium liquid crystals. In contrast, in active liquid crystals, motile topological defects randomly nucleate and annihilate by local energy consumption, making their dynamics weakly impacted by the topological constraints. Here we show that geometrical confinement can be an effective means to achieve control on the dynamical defect configuration of an active system. In particular, we study a 2-dimensional active nematic, consisting of microtubule bundles sheared by kinesin dimers, in the vicinity of a lateral boundary. While positive and negative 1/2 defects equally populate the bulk, we find that the lateral boundary is exclusively populated by negative defects, which exhibit an exotic behavior : despite being like-charged, they attract each other and eventually fuse together. We show that geometrical patterning of the lateral boundary can allow for control over defect nucleation and induce directional flows. These results provide promising tools for designing autonomous microfluidic transport devices and micromachines.