  Active systems, from bacterial suspensions to cellular monolayers, are
continuously driven out of equilibrium by local injection of energy from their
constituent elements and exhibit turbulent-like and chaotic patterns. Here we
demonstrate both theoretically and through numerical simulations, that the
crossover between wet active systems, whose behaviour is dominated by
hydrodynamics, and dry active matter where any flow is screened, can be
achieved by using friction as a control parameter. Moreover we discover
unexpected vortex ordering at this wet-dry crossover. We show that the self
organisation of vortices into lattices is accompanied by the spatial ordering
of topological defects leading to active crystal-like structures. The emergence
of vortex lattices, which leads to the positional ordering of topological
defects, suggests potential applications in the design and control of active
materials.
