Role of friction in multidefect ordering

We use continuum simulations to study the impact of friction on the ordering of defects in an active nematic. Even in a frictionless system, +1/2 defects tend to align side-by-side and orient antiparallel reflecting their propensity to form, and circulate with, flow vortices. Increasing friction enhances the effectiveness of the defect-defect interactions, and defects form dynamically evolving, large scale, positionally and orientationally-ordered structures which can be explained as a competition between hexagonal packing, preferred by the -1/2 defects, and rectangular packing preferred by the +1/2 defects.