High-frequency flow reversal of AC electro-osmosis due to steric effects

TL;DR

This paper explains high-frequency flow reversal in AC electro-osmosis by incorporating steric effects of finite-sized ions, revealing limitations of classical models and proposing a more accurate theoretical framework.

Contribution

It introduces mean-field models accounting for ion size effects to explain flow reversal, highlighting the breakdown of classical Poisson-Boltzmann assumptions.

Findings

Steric effects cause flow reversal at high frequencies.

Classical Poisson-Boltzmann model fails to predict experimental results.

Large effective ion sizes are needed for quantitative agreement.

Abstract

The current theory of alternating-current electro-osmosis (ACEO) is unable to explain the experimentally observed flow reversal of planar ACEO pumps at high frequency (above the peak, typically 10-100 kHz), low salt concentration (1-1000 $\mu$M), and moderate voltage (2-6 V), even if taking into account Faradaic surface reactions, nonlinear double-layer capacitance and bulk electrothermal flows. We attribute this failure to the breakdown of the classical Poisson-Boltzmann model of the diffuse double layer, which assumes a dilute solution of point-like ions. In spite of low bulk salt concentration, the large voltage induced across the double layer leads to crowding of the ions and a related decrease in surface capacitance. Using several mean-field models for finite-sized ions, we show that steric effects generally lead to high frequency flow reversal of ACEO pumps, similar to experiments. For quantitative agreement, however, an unrealistically large effective ion size (several nm) must be used, which we attribute to neglected correlation effects.