Electroosmotic flow in small-scale channels induced by surface-acoustic waves
Electroosmotic flow in small-scale channels induced by surface-acoustic waves
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DOI：
10.1103/physrevfluids.5.123702
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发表时间：
2020-06
影响因子：
2.7
通讯作者：
M. Dietzel;S. Hardt
中科院分区：
文献类型：
--
作者：
M. Dietzel;S. Hardt
数值模拟的Navier-Stokes，Nernst-Planck，和泊松方程来描述在电解质水溶液中的平行板纳米通道，其中表面声波（SAW）是站在或行进沿着（压电活性）通道壁的传输过程。研究发现，除了传统的声流之外，还诱导了时间平均电渗流。采用的流函数涡制定它示出的麦克斯韦应力项导致电渗推进，是定性相同的交流（AC）电渗（电）的上下文中所讨论的。差异的产生主要是由于SAW的高致动频率，这是在MHz范围内，而不是在kHz制度典型的ACWs。此外，在SAW的行进方向上的瞬时空间周期性的色散关系，而不是一个自由的几何参数的SAW固有地联系在一起。在低频端，双电层（EDL）厚度和SAW波长之间的比值变得非常小，因此导致非零时间平均流的净力变得同样小。在高频端，EDL的RC时间远大于SAW频率的倒数，导致EDL的有效电荷密度消失。对于平行板通道，可以通过在两个通道壁上使用两个具有相同频率但相移180 ^\circ $的SAW来最大化。看来，SAW-泵浦是这种情况下的主要泵浦机制。所提出的致动可能是一种可行的替代方案，用于驱动液体电解质通过狭窄的管道和通道，而不需要电互连和电极。
Numerical simulations of the Navier-Stokes, Nernst-Planck, and the Poisson equations are employed to describe the transport processes in an aqueous electrolyte in a parallel-plate nanochannel, where surface-acoustic waves (SAWs) are standing or traveling along (piezo-active) channel walls. It is found that -- in addition to the conventional acoustic streaming flow -- a time-averaged electroosmotic flow is induced. Employing the streamfunction-vorticity formulation it is shown that the Maxwell stress term causes an electroosmotic propulsion that is qualitatively identical to the one discussed in the context of alternating current (AC) electroosmosis (EOF). Differences arise mainly due to the high actuation frequencies of SAWs, which are in the MHz range rather than in the kHz regime typical for ACEOF. Moreover, the instantaneous spatial periodicity of the EOF in the travel direction of the SAW is intrinsically linked to the dispersion relation of the latter rather than a free geometric parameter. This leads to a specific frequency band where an EOF of sizable magnitude can be found. On the low frequency end, the ratio between the electric double layer (EDL) thickness and the SAW wavelength becomes extremely small so that the net force leading to a non-vanishing time-averaged flow becomes equally small. On the high frequency end, the RC time of the EDL is much larger than the inverse of the SAW frequency leading to a vanishing effective charge density of the EDL. For a parallel-plate channel the EOF can be maximized by using two SAWs on both channel walls that have the same frequency but are phase-shifted by $180^\circ$. It appears that the SAW-EOF is the dominant pumping mechanism for such a scenario. The proposed actuation might be a viable alternative for driving liquid electrolytes through narrow ducts and channels, without the need for electric interconnects and electrodes.