Spatial control of topological defects through activity patterning

Active nematics are liquid crystals composed of elongated active particles that individually consume energy to exert stresses along their orientation, driving the fluid out of equilibrium. At high activity, topological defects in the nematic order parameter proliferate and drive self-sustained spatiotemporal chaotic flows. Given the intimate relation between active flows and defect textures, we ask how spatial variations in active stresses can be used to pattern active defects. By combining numerical simulations of active nematodynamic equations with a recently derived hydrodynamic description of defects as an interacting active Coulomb gas, we show how an active strip in a passive region can trap and sort defects. Spatial gradients in activity act like electric fields that can sort defects by topological charge. Remarkably, by tuning the sharpness of the activity gradient, we find a dramatic flip in the topological charge distribution along active-passive interfaces, that we explain via an interplay between defect self-propulsion and diffusion. Persistent kink-walls allow richer patterns with stable defect strings and active flows, paving the way for spatial control of defect patterns in active fluids.