Nanoscale Surfactant Transport: Bridging Molecular and Continuum Models

TL;DR

This paper combines molecular dynamics simulations with continuum models to understand nanoscale surfactant transport, revealing molecular mechanisms, validating continuum approximations, and deriving a new molecular transport equation.

Contribution

It introduces a novel molecular-level equation for surfactant transport and bridges molecular and continuum models at the nanoscale.

Findings

Molecular mechanisms of surfactant migration are identified.

Continuum models remain accurate at the nanoscale under certain conditions.

The study's results align with experimental observations.

Abstract

Surfactant transport is central to a diverse range of natural phenomena, and for many practical applications in physics and engineering. Surprisingly, this process remains relatively poorly understood at the molecular scale. This study investigates the mechanism behind the transport of surfactant monolayers on flat and curved liquid vapor interfaces using nonequilibrium molecular dynamics simulations, which are compared with the continuum transport model. This approach not only provides fresh molecular level insight into surfactant dynamics, but also confirms the nanoscale mechanism of the lateral migration of surfactant molecules along a thin film that continuously deforms as surfactants spread. By connecting the continuum model where the long wave approximations prevail, to the molecular details where such approximations break down, we establish that the transport equation preserves substantial accuracy in capturing the underlying physics. Moreover, the relative importance of the different mechanisms of the transport process are identified. Consequently, we derive a novel, exact molecular equation for surfactant transport along a deforming surface. Finally, our findings demonstrate that the spreading of surfactants at the molecular scale adheres to expected scaling laws and aligns well with experimental observations.