Oscillating electroosmotic flow in channels and capillaries with modulated wall charge distribution

TL;DR

This paper investigates oscillating electroosmotic flows in channels and capillaries with modulated wall charge under AC electric fields, revealing complex vortex structures, non-zero charge flux, and hysteresis effects influenced by frequency and fluid properties.

Contribution

It introduces the analysis of flow structures and charge transport in electrolytes under AC fields, highlighting the impact of oscillation period on flow degeneracies and conductance behaviors.

Findings

Flow structures include vortices with circulation direction depending on oscillation period.

Charge flux remains nonzero despite zero mass flux in oscillating conditions.

Flow exhibits hysteresis and diverging conductance related to frequency and fluid parameters.

Abstract

Electrolyte-filled channels with modulated wall charge distribution subjected to an applied DC electric field, form time-independent vortices whose sense of circulation is determined by the field direction [Physical Review Letters $ \mathbf{75}, 755, (1995)$]. In this paper we show that an electrolyte in a channel or cylindrical capillary subjected to an external \emph{alternating} (AC) electric field gives rise to various laminar flow structures, including vortices whose sense of circulation changes with the period of oscillation of the applied AC field. The introduction of a period of oscillation lifts certain degeneracies associated with its time-independent counterpart. Although, in general, the mass flux vanishes, the charge flux is nonzero. The flow is accompanied by a longitudinal (oscillating) advective current that displays hysteresis accompanied by a diverging and negative self-similar conductance that depends on the applied voltage [Nano Letters $\mathbf{10}, 2674, (2010)$]. We show that this behavior can be interpreted with respect to a ``memory retention time'', that depends on frequency, viscosity and the Debye length and could thus form the impetus for investigating control protocols of signal carriers.