  We use numerical simulations and linear stability analysis to study the
dynamics of an active liquid crystal film on a substrate in the regime where
the passive system would be isotropic. Extensile activity builds up local
orientational order and destabilizes the quiescent isotropic state above a
critical activity value, eventually resulting in spatiotemporal chaotic
dynamics akin to the one observed ubiquitously in the nematic state. Here we
show that tuning substrate friction yields a variety of emergent structures at
intermediate activity, including lattices of flow vortices with associated
regular arrangements of topological defects and a new state where flow vortices
trap pairs of $+1/2$ defect that chase each other tail. These chiral units
spontaneously pick the sense of rotation and organize in a hexagonal lattice,
surrounded by a diffuse flow of opposite rotation to maintain zero net
vorticity. The length scale of these emergent structures is set by the
screening length $l_\eta=\sqrt{\eta/\Gamma}$ of the flow, controlled by the
shear viscosity $\eta$ and the substrate friction $\Gamma$, and can be captured
by simple mode selection of the vortical flows. We demonstrate that the
emergence of coherent structures can be interpreted as a phase separation of
vorticity, where friction plays a role akin to that of birth/death processes in
breaking conservation of the phase separating species and selecting a
characteristic scale for the patterns. Our work shows that friction provides an
experimentally accessible tuning parameter for designing controlled active
flows.
