Gradient dynamics models for liquid films with soluble surfactant

TL;DR

This paper develops a comprehensive gradient dynamics framework for modeling the complex behavior of liquid films with soluble surfactants, ensuring thermodynamic consistency and capturing various physical effects.

Contribution

It introduces a three-field gradient dynamics model for soluble surfactant-covered films that automatically incorporates convection, diffusion, adsorption, desorption, and evaporation processes.

Findings

Model reduces to classical hydrodynamics in dilute limit.

Energy functional can be extended to include nonlinear effects.

Framework ensures thermodynamic consistency and Onsager reciprocity.

Abstract

In this paper we propose equations of motion for the dynamics of liquid films of surfactant suspensions that consist of a general gradient dynamics framework based on an underlying energy functional. This extends the gradient dynamics approach to dissipative non-equilibrium thin film systems with several variables, and casts their dynamic equations into a form that reproduces Onsager's reciprocity relations. We first discuss the general form of gradient dynamics models for an arbitrary number of fields and discuss simple well-known examples with one or two fields. Next, we develop the gradient dynamics (three field) model for a thin liquid film covered by soluble surfactant and discuss how it automatically results in consistent convective (driven by pressure gradients, Marangoni forces and Korteweg stresses), diffusive, adsorption/desorption, and evaporation fluxes. We then show that in the dilute limit, the model reduces to the well-known hydrodynamic form that includes Marangoni fluxes due to a linear equation of state. In this case the energy functional incorporates wetting energy, surface energy of the free interface (constant contribution plus an entropic term) and bulk mixing entropy. Subsequently, as an example, we show how various extensions of the energy functional result in consistent dynamical models that account for nonlinear equations of state, concentration-dependent wettability and surfactant and film bulk decomposition phase transitions. We conclude with a discussion of further possible extensions towards systems with micelles, surfactant adsorption at the solid substrate and bioactive behaviour.