School of Mechanical and Aerospace Engineering, Nanyang Technological University , Nanyang Avenue 50, Singapore 639798.
Biomicrofluidics. 2015 Apr 9;9(2):024113. doi: 10.1063/1.4917386. eCollection 2015 Mar.
Electroosmotic flow (EOF) with two or more fluids is commonly encountered in various microfluidics applications. However, no investigation has hitherto been conducted to investigate the hysteretic or flow direction-dependent behavior during the displacement flow of solutions with dissimilar ionic species. In this investigation, electroosmotic displacement flow involving dissimilar ionic solutions was studied experimentally through a current monitoring method and numerically through finite element simulations. The flow hysteresis can be characterized by the turning and displacement times; turning time refers to the abrupt gradient change of current-time curve while displacement time is the time for one solution to completely displace the other solution. Both experimental and simulation results illustrate that the turning and displacement times for a particular solution pair can be directional-dependent, indicating that the flow conditions in the microchannel are not the same in the two different flow directions. The mechanics of EOF hysteresis was elucidated through the theoretical model which includes the ionic mobility of each species, a major governing parameter. Two distinct mechanics have been identified as the causes for the EOF hysteresis involving dissimilar ionic solutions: the widening/sharpening effect of interfacial region between the two solutions and the difference in ion concentration distributions (and thus average zeta potentials) in different flow directions. The outcome of this investigation contributes to the fundamental understanding of flow behavior in microfluidic systems involving solution pair with dissimilar ionic species.
在各种微流控应用中，通常会遇到两种或多种流体的电渗流 (EOF)。然而，迄今为止，还没有研究针对具有不同离子种类的溶液在置换流过程中滞后或流向相关行为。在这项研究中，通过电流监测方法进行了涉及不同离子溶液的电渗置换流的实验研究，并通过有限元模拟进行了数值研究。通过电流-时间曲线的急剧梯度变化来描述流动滞后的转折时间，而通过一种溶液完全置换另一种溶液所需的时间来描述流动滞后的置换时间。实验和模拟结果都表明，对于特定的溶液对，转折时间和置换时间可能具有方向性依赖性，这表明在两个不同的流动方向上，微通道中的流动条件并不相同。通过包括每个物种的离子迁移率在内的主要控制参数的理论模型，阐明了EOF 滞后的力学。已经确定了两种不同的机制是导致涉及不同离子溶液的 EOF 滞后的原因：两种溶液之间界面区域的变宽/变锐效应以及不同流动方向上离子浓度分布（以及因此平均 ζ 电势）的差异。这项研究的结果有助于对涉及具有不同离子种类的溶液对的微流控系统中流动行为的基础理解。
