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

TL;DR

This study investigates how time-dependent driving forces affect interface fluctuations and surface tension in nonequilibrium liquids, revealing conditions where capillary wave theory applies and quantifying the increase in surface tension due to energy input.

Contribution

It provides the first detailed analysis of interface fluctuations under time-dependent forces in driven liquids, demonstrating how surface tension can be tuned by external driving.

Findings

Interface fluctuations follow capillary wave theory at certain driving amplitudes in repulsive systems.

Surface tension increases linearly with driving force, more than doubling from equilibrium.

CWT applicability depends on the nature of interactions and driving amplitude.

Abstract

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.