  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.
