Reconfigurable flows and defect landscape of confined active nematics

Abstract

The physics of active liquid crystals is mostly governed by the interplay between elastic forces that align their constituents, and active stresses that destabilize the order with constant nucleation of topological defects and chaotic flows. The average distance between defects, also called active length scale, depends on the competition between these forces. Here, in experiments with the microtubule/kinesin active nematic system, we show that the intrinsic active length scale loses its relevance under strong lateral confinement. Transitions are observed from chaotic to vortex lattices and defect-free unidirectional flows. Defects, which determine the active flow behaviour, are created and annihilated on the channel walls rather than in the bulk, and acquire a strong orientational order in narrow channels. Their nucleation is governed by an instability whose wavelength is effectively screened by the channel width. These results are recovered in simulations, and the comparison highlights the role of boundary conditions.Active nematics refers to systems made of a collection of elongated units, each of which consumes ambient or stored energy in order to move. The authors experimentally and numerically study an active nematic system in confinement finding a defect-free regime of shear flow, and defect nucleation under certain boundary conditions, highlighting the importance of topological defects in controlling confined active flows.