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.

中文翻译：

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具有稳定压力驱动流的振荡交流电渗的不对称时间变化

在微/纳米流体中的大多数电动流动中，电渗流 (EOF) 始终是一种基本现象。当施加外部交流电场时，会出现瞬态 EOF 速度，这对于理解非定常 AC EOF 的机制至关重要。然而，瞬态流速不能以足够且同时的时间和空间分辨率进行实验测量。在这里我们报告说，在强交流电场下，可以在微通道底壁上的双电层 (EDL) 附近产生振荡 EOF (OEOF) 的不对称时间变化。通过激光诱导荧光光漂白风速计 (LIFPA) 测量流量。OEOF 的流速和微通道中线性相关的内部电场在理论上都由具有外力项的无粘性 Burgers 方程控制，并由两个代表线性大小的系数控制 ($${Z}_{ l}$$ ) 和非线性 ( $${Z}_{nl}$$ ) 项。这两个系数分别由五个无量纲参数确定，包括斯托克斯数（ $${St}$$ ）、纵横比（ $$\gamma$$ ）、施密特数（ $${Sc}$$ ）、佩克莱数平均流速 ( $${P}_{e, U}$$ ) 和电泳的 Peclet 数 ( $${P}_{e, {EP}}$$ )。在这项工作中，$${P}_{e, U}$$ 和 $${P}_{e, {EP}}$$ 的影响与局部平均流速和振幅密切相关。研究交流电场。当电场低时，$${P}_{e, U}\gg {P}_{e, {EP}}$$ 和相应的 $${Z}_{l}\gg {Z}_{nl}$$ ，观察到速度时间序列的时间对称性。然而，当电场较高时，$${P}_{e, U}\ll {P}_{e, {EP}}$$ 和 $${Z}_{l}\ll {Z} _{nl}$$ ，那么 OEOF 的周期性和时间对称性会被显着破坏，导致速度时间序列的时间不对称性，特别是在方波外电场下。我们还发现随着电场的增加，平均流速增加和速度波动饱和。这项工作为研究非定常电动流动提供了一种新方法。那么OEOF的周期性和时间对称性会被显着破坏，导致速度时间序列的时间不对称性，特别是在方波外电场下。我们还发现随着电场的增加，平均流速增加和速度波动饱和。这项工作为研究非定常电动流动提供了一种新方法。那么OEOF的周期性和时间对称性会被显着破坏，导致速度时间序列的时间不对称性，特别是在方波外电场下。我们还发现随着电场的增加，平均流速增加和速度波动饱和。这项工作为研究非定常电动流动提供了一种新方法。