Electro-osmosis at surfactant-laden liquid-gas interfaces: beyond standard models

TL;DR

This study uses molecular dynamics simulations to explore electro-osmosis at liquid-gas interfaces with surfactants, revealing limitations of existing models and proposing an extended predictive model for zeta potential.

Contribution

The paper introduces an extended model for electro-osmosis at surfactant-laden liquid-gas interfaces, accounting for surfactant and ion dynamics beyond standard theories.

Findings

Zeta potential varies with surfactant coverage, sometimes exceeding or falling below classical predictions.

Surfactants and bound ions move as rigid bodies, affecting electric force transmission.

The extended model accurately predicts experimental zeta potentials.

Abstract

Electro-osmosis (EO) is a powerful tool to manipulate liquids in micro and nanofluidic systems. While EO has been studied extensively at liquid-solid interfaces, the case of liquid-vapor interfaces, found e.g. in foam films and bubbles, remains to be explored. Here we perform molecular dynamics (MD) simulations of EO in a film of aqueous electrolyte covered with fluid layers of ionic surfactants and surrounded by gas. Following the experimental procedure, we compute the zeta potential from the EO velocity, defined as the velocity difference between the middle of the liquid film and the surrounding gas. We show that the zeta potential can be smaller or larger than existing predictions depending on the surfactant coverage. We explain the failure of previous descriptions by the fact that surfactants and bound ions move as rigid bodies and do not transmit the electric driving force to the liquid locally. Considering the reciprocal streaming current effect, we then develop an extended model, which can be used to predict the experimental zeta potential of surfactant-laden liquid-gas interfaces.