Defect order in active nematics

Using a recently derived model of active defects as quasi-particles, some of us have reported emergent states where +1/2 defects in active nematics organize in structures with large scale polar order. While ordered states of +1/2 defects have been obtained in both experiments and simulations of active nematic hydrodynamics, both polar and nematic alignment of the +1/2 defects have been seen depending on whether active flows are dissipated by friction with a substrate or by viscous stresses. In this talk, we will discuss the interplay between elastic and active torques, active flows and frictional dissipation in controlling defect patterns in these active liquid crystals.