Interface height fluctuations and surface tension of driven liquids with time-dependent dynamics

Interfaces in phase-separated driven liquids are one example of how energy input at the single-particle level changes the long-length-scale material properties of nonequilibrium systems. Here, we measure interfacial fluctuations in simulations of two liquids driven by time-dependent forces, one with repulsive interactions and one with attractive interactions. The time-dependent forces lead to currents along the interface, which can modify the scaling of interface height fluctuations with respect to predictions from capillary wave theory (CWT). We therefore characterize the whole spectrum of fluctuations to determine whether CWT applies. In the system with repulsive interactions, we find that the interface fluctuations are well-described by CWT at one amplitude of the driving forces but not at others. In the system with attractive interactions, they obey CWT for all amplitudes of driving, allowing us to extract an effective surface tension. The surface tension increases linearly over two orders of magnitude of the driving forces, more than doubling its equilibrium value. Our results show how the interfaces of nonequilibrium liquids with time-dependent forces are modified by energy input.