  Motivated by results for the propagation of active-passive interfaces of
bacterial Serratia marcescens swarms [Nat. Comm., 9, 5373 (2018)] we use a
hydrodynamic multiphase model to investigate the propagation of interfaces of
active nematics on substrates. We characterize the active nematic phase of the
model and discuss its description of the statistical dynamics of the swarms. We
show that the velocicity correlation functions and the energy spectrum of the
active turbulent phase of the model scale with the active length, for a range
of activities. In addition, the energy spectrum exhibits two power-law regimes
with exponents close to those reported for other models and bacterial swarms.
Although the exponent of the rising branch of the spectrum (small wavevector)
appears to be independent of the activity, the exponent of the decay changes
with activity systematically, albeit slowly. We characterize also the
propagation of circular and flat active-passive interfaces. We find that the
closing time of the circular passive domain decays quadratically with the
activity and that the structure factor of the flat interface is similar to that
reported for swarms, with an activity dependent exponent. Finally, the effect
of the substrate friction was investigated. We found an activity dependent
threshold, above which the turbulent active nematic forms isolated islands that
shrink until the system becomes isotropic and below which the active nematic
expands, with a well defined propagating interface. The interface may be
stopped by a fricion gradient.
