Junsuk Kim

Non-Negligible Diffusio-Osmosis Inside an Ion Concentration Polarization Layer.

The first experimental and theoretical evidence was provided for the non-negligible role of a diffusio-osmosis in the ion concentration polarization (ICP) layer, which had been reported to be in a high Peclet number regime. Under the assumption that the hydrated shells of cations were stripped out with the amplified electric field inside the ICP layer, its concentration profile possessed a steep concentration gradient at the stripped location. Since the concentration gradient drove a strong diffusio-osmosis, the combination of electro-osmotic and diffusio-osmotic slip velocity had a form of an anomalous nonmonotonic function with both a single- and multiple-cationic solution. A direct measurement of electrolytic concentrations around the layer quantitatively validated our new investigations. This non-negligible diffusio-osmotic contribution in a micro- and nanofluidic platform or porous medium would be essential for clarifying the fundamental insight of nanoscale electrokinetics as well as guiding the engineering of ICP-based electrochemical systems.

Ion Concentration Polarization by Bifurcated Current Path

Ion concentration polarization (ICP) is a fundamental electrokinetic process that occurs near a perm-selective membrane under dc bias. Overall process highly depends on the current transportation mechanisms such as electro-convection, surface conduction and diffusioosmosis and the fundamental characteristics can be significantly altered by external parameters, once the permselectivity was fixed. In this work, a new ICP device with a bifurcated current path as for the enhancement of the surface conduction was fabricated using a polymeric nanoporous material. It was protruded to the middle of a microchannel, while the material was exactly aligned at the interface between two microchannels in a conventional ICP device. Rigorous experiments revealed out that the propagation of ICP layer was initiated from the different locations of the protruded membrane according to the dominant current path which was determined by a bulk electrolyte concentration. Since the enhancement of surface conduction maintained the stability of ICP process, a strong electrokinetic flow associated with the amplified electric field inside ICP layer was significantly suppressed over the protruded membrane even at condensed limit. As a practical example of utilizing the protruded device, we successfully demonstrated a non-destructive micro/nanofluidic preconcentrator of fragile cellular species (i.e. red blood cells).

Pseudo 1-D Micro/Nanofluidic Device for Exact Electrokinetic Responses.

Conventionally, a 1-D micro/nanofluidic device, whose nanochannel bridged two microchannels, was widely chosen in the fundamental electrokinetic studies; however, the configuration had intrinsic limitations of the time-consuming and labor intensive tasks of filling and flushing the microchannel due to the high fluidic resistance of the nanochannel bridge. In this work, a pseudo 1-D micro/nanofluidic device incorporating air valves at each microchannel was proposed for mitigating these limitations. High Laplace pressure formed at liquid/air interface inside the microchannels played as a virtual valve only when the electrokinetic operations were conducted. The identical electrokinetic behaviors of the propagation of ion concentration polarization layer and current-voltage responses were obtained in comparison with the conventional 1-D micro/nanofluidic device by both experiments and numerical simulations. Therefore, the suggested pseudo 1-D micro/nanofluidic device owned not only experimental conveniences but also exact electrokinetic responses.

Experimental and theoretical analysis of spontaneous ion exchangeby diffusion-kinetic ion concentration polarization

In this presentation, we suggested a “diffusio-kinetic ion concentration polarization (diffusio-kinetic ICP)” for a spontaneous ion exchange system. The diffusio-kinetic ICP utilized the mechanisms of an ion diffusion, a spontaneous ion depletion and a spontaneous solute exclusion [1, 2] so that the ion exchange system would be accomplished without any external power source. nxa0When saline water was introduced into the microchannel of which side walls were composed of cation-selective membrane, the ion exchange process occurred and then the desalted diffuse layer was formed nearby the membrane. Because of the flow of saline water, the desalted diffuse layer would be deformed but retain finite thickness. If we gather the water adjacent to the membrane surface at outlet, we can get the desalted water through the spontaneous manner which means that any external electrical source is not required, whereas the ICP needs the electrically driven ionic flux through the membrane. nxa0Through the rigorous theoretical analysis, the diffusio-kinetic ICP was characterized by the Sherwood number which is the ratio of convective transfer and diffusion rate. When Sh > 1, convective transport was dominant. For each case, the desalted layer would be overlapped depending on Sh as shown in Fig. 2. nxa0Furthermore, the desalted layer was experimentally visualized in micro/nanofluidic platform. When the desalted layer was developed near the membrane surface, the concentration gradient was generated inside the desalted layer. Under a diffusiophoretic mechanism [3], the charged particles would be excluded from the nanoporous membrane as shown in Fig. 3. Thus, the spontaneous ion depletion was able to be verified by the direct visualization with various cases of Sh. Note that the thickness of the desalted layer was usually larger than the thickness of the particle exclusion zone because of the working mechanism of the diffusiophoresis. nxa0The fact that the diffusio-kinetic ICP platform can work without any external power source implied the merits in terms of the power consumption and the low-cost operation compared with the conventional technologies so that the diffusio-kinetic ICP would be an effective mean in remote and resource-limited settings.