Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain.
Langmuir. 2011 Sep 20;27(18):11710-21. doi: 10.1021/la201354s. Epub 2011 Aug 23.
Mismatches in electrokinetic properties between micro- and nanochannels give rise to superposition of electroosmotic and pressure-driven flows in the microchannels. Parabolic or similar flow profiles are known to cause the so-called hydrodynamic dispersion, which under certain conditions can be formally assimilated to an increase in the solute diffusivity (Taylor-Aris model). It is demonstrated theoretically that taking into account these phenomena modifies considerably the pattern of current-induced concentration polarization of micro/nanointerfaces as compared to the classical model of unstirred boundary layer. In particular, the hydrodynamic dispersion leads to disappearance of limiting current. At essentially "over-limiting" current densities, the time-dependent profiles of salt concentration in microchannels behave like sharp concentration "fronts" moving away from the interface until they reach the reservoir end of the microchannel. Under galvanostatic conditions postulated in this study, these "fronts" move with practically constant speed directly proportional to the current density. The sharp transition from a low-concentration to a high-concentration zone can be useful for the analyte preconcentration via stacking. The pattern of moving sharp concentration "fronts" has been predicted for the first time for relatively broad microchannels with negligible surface conductance. The Taylor-Aris approach to the description of hydrodynamic dispersion is quantitatively applicable only to the analysis of sufficiently "slow" processes (as compared to the characteristic time of diffusion relaxation in the transversal direction). A posteriori estimates reveal that the condition of "slow" processes is typically not satisfied close to current-polarized micro/nanointerfaces. Accordingly, to make the description quantitative, one needs to go beyond the Taylor-Aris approximation, which will be attempted in future studies. It is argued that doing so would make even stronger the dampening impact of hydrodynamic dispersion on the current-induced concentration polarization of micro/nanointerfaces.
微通道和纳米通道之间的电动特性不匹配导致微通道中电渗流和压力驱动流的叠加。众所周知，抛物线或类似的流动剖面会导致所谓的流体动力学弥散，在某些条件下，这种弥散可以正式等同于溶质扩散系数的增加（Taylor-Aris 模型）。理论上证明，与未搅动边界层的经典模型相比，考虑到这些现象会极大地改变微/纳米界面电流诱导浓度极化的模式。特别是，流体动力学弥散会导致极限电流的消失。在本质上“超极限”电流密度下，微通道中盐浓度的时变分布行为类似于从界面移开的尖锐浓度“前沿”，直到它们到达微通道的储液器端。在所假设的恒电流条件下，这些“前沿”以与电流密度成正比的实际恒定速度移动。从低浓度区到高浓度区的急剧转变可用于通过堆积进行分析物的预浓缩。对于具有可忽略的表面电导率的相对较宽的微通道，首次预测了移动尖锐浓度“前沿”的模式。Taylor-Aris 方法对流体动力学弥散的描述仅在定量上适用于分析足够“缓慢”的过程（与横向扩散弛豫的特征时间相比）。后验估计表明，靠近电流极化的微/纳米界面附近通常不满足“缓慢”过程的条件。因此，为了使描述具有定量意义，需要超越 Taylor-Aris 近似，这将在未来的研究中尝试。有人认为，这样做会使流体动力学弥散对微/纳米界面电流诱导浓度极化的抑制作用更加强烈。
