Asymmetric temporal variation of oscillating AC electroosmosis with a steady pressure-driven flow

Abstract

In most electrokinetic flows in micro-/nanofluidics, electroosmotic flow (EOF) is always a fundamental phenomenon. When an external AC electric field is applied, there will be a transient EOF velocity, which is crucial for understanding the mechanism of the unsteady AC EOF. However, the transient flow velocity cannot be experimentally measured with sufficiently and simultaneously temporal and spatial resolution. Here we report that under a strong AC electric field, an asymmetric temporal variation of oscillating EOF (OEOF) can be generated near the electric double layer (EDL) on the bottom wall of a microchannel. The flow is measured by laser-induced fluorescence photobleaching anemometer (LIFPA). Both the flow velocity of the OEOF and the linearly related internal electric field in the microchannel are theoretically governed by an inviscid Burgers’ equations with an external forcing term and controlled by two coefficients which represent the magnitudes of linear ( $${Z}_{l}$$ ) and nonlinear ( $${Z}_{nl}$$ ) terms. These two coefficients are separately determined by five dimensionless parameters, including Stokes number ( $${St}$$ ), aspect ratio ( $$\gamma$$ ), Schmidt number ( $${Sc}$$ ), Peclet number of mean flow velocity ( $${P}_{e, U}$$ ) and Peclet number of electrophoresis ( $${P}_{e, {EP}}$$ ). In this work, the influence of $${P}_{e, U}$$ and $${P}_{e, {EP}}$$ which are tightly related to the local mean flow velocity and the amplitude of AC electric field is investigated. When the electric field is low, $${P}_{e, U}\gg {P}_{e,{EP}}$$ and accordingly $${Z}_{l}\gg {Z}_{nl}$$ , a temporal symmetry of the time series of velocity is observed. However, when the electric field is high, $${P}_{e, U}\ll {P}_{e, {EP}}$$ and $${Z}_{l}\ll {Z}_{nl}$$ , then the periodicity and temporal symmetry of OEOF can be significantly broken and a temporal asymmetry of the time series of velocity is resulted in, especially under the square wave external electric field. We also find increased mean flow velocity and saturation of velocity fluctuations with the increasing electric field. This work provides a new methodology to study unsteady electrokinetic flows.