Experimental Investigation of the Nonlinear Electrokinetic Responses inside an Ion Depletion Zone

Abstract

During ion concentration polarization (ICP) phenomenon, the electro-osmotic instability (EOI) was the core mechanism of electrokinetic transportation from aqueous electrolyte solutions to ion-selective membranes. The lab-on-a-chip (LOC) device based on the micro-/nano-fluidic channel enabled one to identify such flow field as well as the associated electric field and oncentration profile. Recently, electrokinetic analysis considering microchannel wall charge effect has shown that surface conduction (SC) or electro-osmotic flow (EOF) played a decisive role for ion transportation while the EOI was negligible owing to the micro-size channel dimension. Hence, it is necessary for experimental verification of such mechanisms in detail. Thus, the deeply understanding of the ICP phenomenon is crucial regarding the electric field, concentration distribution and flow-field in LOC device. In this thesis, we experimentally investigate the transporting role of surface conduction and of electro-osmotic flow during ICP by effectively controlling the cross-sectional area of the PDMS microchannel structure. From the basis of our verification, we extracted the valuable parameter (the ratio of the perimeter to the cross-sectional area of the microchannel) in the region where surface conduction dominates. The result has shown that the surface contacting the electrolyte solution becomes higher, the ion transport through the surface charge extremely increases. Thus, it is expected to be a strategy for developing the effective ion rectification in ICP platform. In the second section of this thesis, we conceived a device design that can measure the concentration field and electric field utilizing the localized EOF inside the ion depletion zone. The fabricated groove microchannel captured the EOF of the entire ion depletion layer. Interestingly, the non-negligible diffusio-osmotic contribution were observed as well as the EOF one, which has never been predicted before. In general, diffusio-osmotic flow (DOF) was observed in either cases where the gradient of the concentration field is abrupt or the concentration is very low. The ion depletion zone generally maintains a low concentration field while causing instability of the flow field inside the ICP, which proves to be the effect of DOF. Furthermore, it is confirmed that the DOF phenomenon occurs depending on the kind of the cation by tracking the micro-oil droplet and the mass spectrometer. In the final section of this thesis, we presented the issue on the electrical power of the ICP system and the solution with an engineering skill. Some experiments has suggested the permanent structures (i.e. pillar arrays or spacer) or the heterogeneous membrane system. Those system have in common to control the size of the ion depletion zone with the structures. Here, we proposed the ICP system incorporated with the injection of the hydrodynamic control. This system enabled one to shrink the ion depletion zone, while the size can be determined both by the electric field and the pressure driven flows. Furthermore, the voltage-current characteristics in the ICP-pressure system was significantly different from that in the conventional ICP system, exhibiting the limination of the limiting current. Thus, such system offered the higher electrical power conversion efficiency. We revolutionize the LOC device not only for Investigating the underlying ICP phenomenon by tracking the flows, but also for demonstrating the possible transport mechanisms and relative physics by measuring the electrical responses in detail.