Activity driven defect ordering in an active nematic on an anisotropic substrate

We investigate the effect of an anisotropic substrate on the turbulent dynamics of a simulated two dimensional active nematic. This results in an effectinve anisotropic friction and viscosity, with the orientation of the anisotropy being defined by the substrate. In this system we observe the emergence of global nematic order of topological defects that is controlled by the degree of anisotropy in the viscosity and the magnitude of the active stress. No such alignment is seen in passive liquid crystals with anisotropic viscosity confirming that ordering is driven by the active stress. We then closely examine the active flow generated by a single defect to show that the kinetic energy of the flow is orientation dependent, resulting in a torque on the defect to align them with the anisotropy in the substrate.