  We use numerical simulations to investigate the hydrodynamic behavior of the
interface between nematic (N) and isotropic (I) phases of a confined active
liquid crystal. At low activities, a stable interface with constant shape and
velocity is observed separating the two phases. For nematics in homeotropic
channels, the velocity of the interface at the NI transition increases from
zero (i) linearly with the activity for contractile systems and (ii)
quadratically for extensile ones. Interestingly, the nematic phase expands for
contractile systems while it contracts for extensile ones, as a result of the
active forces at the interface. Since both activity and temperature affect the
stability of the nematic, for active nematics in the stable regime the
temperature can be tuned to observe static interfaces, providing an operational
definition for the coexistence of active nematic and isotropic phases. At
higher activities, beyond the stable regime, an interfacial instability is
observed for extensile nematics. In this regime defects are nucleated at the
interface and move away from it. The dynamics of these defects is regular and
persists asymptotically for a finite range of activities. We used an improved
hybrid model of finite differences and lattice Boltzmann method with
multi-relaxation-time collision operator, the accuracy of which allowed us to
characterize the dynamics of the distinct interfacial regimes.
