Active Model B${}^{-}$ from Mass-Conserving Reaction-Diffusion Systems

We show that the late-time dynamics of a minimal three-component mass-conserving reaction–diffusion system reduce to a scalar active field theory, Active Model B${}^{-}$ (AMB${}^{-}$), in which a density-dependent interfacial coefficient $\kappa (\phi )$ turns negative at high density. This drives a finite-wavelength instability and stabilises microphase-separated patterns, in contrast to the unbounded coarsening of two-component mass-conserving systems. Unlike Active Model B${}^{+}$, AMB${}^{-}$ retains a chemical potential that remains a state function, inherited from the underlying conservation law, but admits no equation of state for the pressure.