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Coupling between chemical fuel consumption and phase separation can lead to condensation at a nonequilibrium steady state, resulting in phase behaviors that are not described by equilibrium thermodynamics. Theoretical models of such “chemically driven fluids” typically invoke near-equilibrium approximations at small length scales. However, because dissipation occurs due to both molecular-scale chemical reactions and mesoscale diffusive transport, it has remained unclear which properties of phase-separated reaction–diffusion systems can be assumed to be at an effective equilibrium. Here, we use microscopic simulations to show that mesoscopic fluxes are dependent on nonequilibrium fluctuations at phase-separated interfaces. We further develop a first-principles theory to predict nonequilibrium coexistence curves, localization of mesoscopic fluxes near phase-separated interfaces, and droplet size-scaling relations in good agreement with simulations. Our findings highlight the central role of interfacial properties in governing nonequilibrium condensation and have broad implications for droplet nucleation, coarsening, and size control in chemically driven fluids. 

Chemically driven fluids can demix to form condensed droplets that exhibit phase behaviors not observed at equilibrium. In particular, nonequilibrium interfacial properties can emerge when the chemical reactions are driven differentially between the interior and exterior of the phase-separated droplets. Here, we use a minimal model to study changes in the interfacial tension between coexisting phases away from equilibrium. Simulations of both droplet nucleation and interface roughness indicate that the nonequilibrium interfacial tension can either be increased or decreased relative to its equilibrium value, depending on whether the driven chemical reactions are accelerated or decelerated within the droplets. Finally, we show that these observations can be understood using a predictive theory based on an effective thermodynamic equilibrium.