Quantifying orientational interactions among defects in active nematics

In passive two-dimensional systems, from superfluid films to nematic layers, topological defects play a key role in controlling continuous order-disorder transitions. Inspired by this, some of us have recently derived an effective description of the dynamics of topological defects in 2D active nematics as quasiparticles, where the unbound defects are modeled as a gas of self-propelled (+1/2) and passive (-1/2) particles with Coulomb interactions and aligning torques. We have now tested this model against defect trajectories obtained from numerical simulations of 2D active nematic hydrodynamics. Specifically, we demonstrate that the polar +1/2 defects interact via torques that tend to align the defect polarization with the elastic force they experience from other defects, as in previously studied models of flocking.