Interface roughening in nonequilibrium phase-separated systems

Interfaces of phase-separated systems roughen in time due to capillary waves. Because of fluxes in the bulk, their dynamics is nonlocal in real space and is not described by the Edwards-Wilkinson or Kardar-Parisi-Zhang (KPZ) equations, nor their conserved counterparts. We show that in the absence of detailed balance, the phase-separated interface is described by a new universality class that we term |q|KPZ. We compute the associated critical exponents via one-loop renormalization group, and corroborate the results by numerical integration of the |q|KPZ equation. Deriving the effective interface dynamics from a minimal field theory of active phase separation, we finally argue that the |q|KPZ universality class generically describes liquid-vapor interfaces in active systems.