Simulating surfactant effects in phase-transforming fluids

Surfactants are critical in natural processes and engineering, but measuring their concentrations in non-equilibrium conditions and in the presence of flow is difficult. Therefore, computational methods are key tools for improving our understanding. Predicting the effect of surfactants on liquid–vapor transformations is particularly challenging due to (1) simultaneous mass transfer, non-equilibrium thermodynamics, and Marangoni stresses, and (2) the phenomenological assumptions underlying many liquid–vapor phase-change models. Starting from the Navier–Stokes–Korteweg equations, a first-principles approach to liquid–vapor phase transformations, we developed a model of liquid–vapor flows with surfactants. We performed simulations of bubbles under equilibrium and of liquid–vapor interface oscillations to demonstrate that the model successfully reproduces surfactant-mediated reductions in surface tension. We also investigated the mechanisms whereby surfactant affects bubble coalescence and condensation. Overall, this work provides a new framework for studying the effect of surfactants on liquid–vapor transformations and suggests multiple areas for future research, including the impact of complex surface chemistries on flow around bubbles and the acoustic response of bubbles with surfactants.