Sung Jae Kim

Direct seawater desalination by ion concentration polarization

A shortage of fresh water is one of the acute challenges facing the world today. An energy-efficient approach to converting sea water into fresh water could be of substantial benefit, but current desalination methods require high power consumption and operating costs or large-scale infrastructures, which make them difficult to implement in resource-limited settings or in disaster scenarios. Here, we report a process for converting sea water (salinity approximately 500 mM or approximately 30,000 mg l(-1)) to fresh water (salinity <10 mM or <600 mg l(-1)) in which a continuous stream of sea water is divided into desalted and concentrated streams by ion concentration polarization, a phenomenon that occurs when an ion current is passed through ion-selective membranes. During operation, both salts and larger particles (cells, viruses and microorganisms) are pushed away from the membrane (a nanochannel or nanoporous membrane), which significantly reduces the possibility of membrane fouling and salt accumulation, thus avoiding two problems that plague other membrane filtration methods. To implement this approach, a simple microfluidic device was fabricated and shown to be capable of continuous desalination of sea water (approximately 99% salt rejection at 50% recovery rate) at a power consumption of less than 3.5 Wh l(-1), which is comparable to current state-of-the-art systems. Rather than competing with larger desalination plants, the method could be used to make small- or medium-scale systems, with the possibility of battery-powered operation.

Nanofluidic concentration devices for biomolecules utilizing ion concentration polarization: theory, fabrication, and applications

Recently, a new type of electrokinetic concentration devices has been developed in a microfluidic chip format, which allows efficient trapping and concentration of biomolecules by utilizing ion concentration polarization near nanofluidic structures. These devices have drawn much attention not only due to their potential application in biomolecule sensing, but also due to the rich scientific content related to ion concentration polarization, the underlying physical phenomenon for the operation of these electrokinetic concentration devices. This tutorial review provides an introduction to the scientific and engineering advances achieved, in-depth discussion about several interesting applications of these unique concentration devices, and their current limitations and challenges.

Amplified electrokinetic response by concentration polarization near nanofluidic channel.

Ion concentration polarization is the fundamental transport phenomenon that occurs near ion-selective membranes, but this important membrane phenomenon has been poorly understood due to theoretical and experimental challenges. Here, we report the first direct measurements of detailed flow and electric potential profiles within and near the depletion region. This work is an important step toward a full characterization of this coupled transport problem. Using microfabricated electrodes integrated with the microfluidic device, we measured and confirmed that the electric field inside an ion depletion region is amplified more than 30-fold compared to outside of the depletion zone due to the highly nonuniform ion concentration distribution along the microchannel. As a result, the electrokinetic motion of both fluid (electroosmosis) and particle (electrophoresis) was significantly amplified. The detailed flow profile within the depletion zone was also measured for the first time by optically tracking photobleached neutral dye molecules. We further showed that the amplified electrokinetic flows generated in this device may be used as a field-controlled, microfluidic fluid pump and switch.

Self-sealed vertical polymeric nanoporous-junctions for high-throughput nanofluidic applications.

We developed a reliable but simple integration method of polymeric nanostructure in a poly(dimethylsiloxane) (PDMS)-based microfluidic channel, for nanofluidic applications. The Nafion polymer junction was creased by infiltrating polymer solution between the gaps created by mechanical cutting, without any photolithography or etching processes. The PDMS can seal itself with the heterogeneous polymeric nanoporous material between the PDMS/PDMS gap due to its flexibility without any (covalent) bonding between PDMS and the polymer materials. Thus, one can easily integrate the nanoporous-junction into a PDMS microchip in a leak-free manner with excellent repeatability. We demonstrated nanofluidic preconcentration of proteins (beta-phycoerythrin) using the device. Because the polymeric junction spans across the entire microchannel height, the preconcentration was achieved with high-pressure field or even in large channels, with the dimensions of 1000 microm width x 100 microm depth.

Continuous-flow biomolecule and cell concentrator by ion concentration polarization.

We present a novel continuous-flow nanofluidic biomolecule/cell concentrator, utilizing the ion concentration polarization (ICP) phenomenon. The device has one main microchannel which bifurcates into two channels, one for a narrow, concentrated stream and the other for a wider but target-free stream. A nanojunction [cation-selective material (Nafion)] is patterned along the tilted concentrated channel. Application of an electric field generates the ICP zone near the nanojunction so that biomolecules and cells are guided into the narrow, concentrated channel by hydrodynamic force. Once biomolecules from the main channel are continuously streamed out to the concentrated channel, one can achieve a continuous flow of the same sample solution but with higher concentrations up to 100-fold. By controlling hydrodynamic resistance of the main and concentrated channel, the concentration factors can be adjusted. We demonstrated the continuous-flow concentration with various targets, such as bacteria [fluorescein sodium salt, recombinant green fluorescence protein (rGFP), red blood cells (RBCs), and Escherichia coli ( E. coli )]. Specially, fluorescein isothiocyanate (FITC)-conjugated lectin from Lens culinaris (lentil) (FITC-lectin) was tested on the different buffer conditions to clarify the effect of polarities of the target sample. This system is ideally suited for a generic concentration front-end for a wide variety of biosensors, with minimal integration-related complications.

Experimental verification of overlimiting current by surface conduction and electro-osmotic flow in microchannels.

Direct evidence is provided for the transition from surface conduction (SC) to electro-osmotic flow (EOF) above a critical channel depth (d) of a nanofluidic device. The dependence of the overlimiting conductance (OLC) on d is consistent with theoretical predictions, scaling as d(-1) for SC and d(4/5) for EOF with a minimum around d=8  μm. The propagation of transient deionization shocks is also visualized, revealing complex patterns of EOF vortices and unstable convection with increasing d. This unified picture of surface-driven OLC can guide further advances in electrokinetic theory, as well as engineering applications of ion concentration polarization in microfluidics and porous media.

Multi-vortical flow inducing electrokinetic instability in ion concentration polarization layer

In this work, we investigated multiple vortical flows inside the ion concentration polarization (ICP) layer that forms due to a coupling of applied electric fields and the semipermeable nanoporous junction between microchannels. While only a primary vortex near perm-selective membrane is traditionally known to lead to electrokinetic instability, multiple vortexes induced by the primary vortex were found to play a major role in the electrokinetic instability. The existence of multiple vortexes was directly confirmed by experiments using particle tracers and interdigitated electrodes were used to measure the local concentration profile inside the ICP layer. At larger applied electric fields, we observed aperiodic fluid motion due to electrokinetic instabilities which develop from a coupling of applied electric fields and electrical conductivity gradients induced by the ICP. The electrokinetic instability at micro-nanofluidic interfaces is important in the development of various electro-chemical-mechanical applications such as fuel cells, bio-analytical preconcentration methods, water purification/desalination and the fundamental study of ion electromigration through nanochannels and nonporous perm-selective membranes.

Overlimiting current through ion concentration polarization layer: hydrodynamic convection effects.

In this work, we experimentally investigated an effect of the hydrodynamic convective flow on ion transport through a nanoporous membrane in a micro/nanofluidic modeled system. The convective motion of ions in an ion concentration polarization (ICP) layer was controlled by external hydrodynamic inflows adjacent to the nanoporous membrane. The ion depletion region, which is regarded as a high electrical resistance, was spatially confined to a triangular shape with the additional hydrodynamic convective flow, resulting in a significant alteration in the classical ohmic-limiting-overlimiting current characteristics. Furthermore, the extreme spatial confinement can completely eliminate the limiting current region at a higher flow rate, while the ICP layer still exists. The presented results enable one to obtain a high current value which turns out to be a high electrical power efficiency. Therefore, this mechanism could be utilized as an optimizing power consumption strategy for various electrochemical membrane systems such as fuel-cells, electro-desalination systems and nanofluidic preconcentrators, etc.

Morphology‐Directed Selective Production of Ethylene or Ethane from CO2 on a Cu Mesopore Electrode

The electrocatalytic conversion of CO2 to value-added hydrocarbons is receiving significant attention as a promising way to close the broken carbon-cycle. While most metal catalysts produce C1 species, such as carbon monoxide and formate, the production of various hydrocarbons and alcohols comprising more than two carbons has been achieved using copper (Cu)-based catalysts only. Methods for producing specific C2 reduction outcomes with high selectivity, however, are not available thus far. Herein, the morphological effect of a Cu mesopore electrode on the selective production of C2 products, ethylene or ethane, is presented. Cu mesopore electrodes with precisely controlled pore widths and depths were prepared by using a thermal deposition process on anodized aluminum oxide. With this simple synthesis method, we demonstrated that C2 chemical selectivity can be tuned by systematically altering the morphology. Supported by computational simulations, we proved that nanomorphology can change the local pH and, additionally, retention time of key intermediates by confining the chemicals inside the pores.

Selective preconcentration and online collection of charged molecules using ion concentration polarization

A multilayer micro/nanofluidic device was presented for the selective preconcentration and online collection of charged molecules with different physicochemical properties based on ion concentration polarization phenomena. With a balance of electroosmotic drag force and electrophoretic force on the molecules, a sample mixture of sulforhodamine B and Alexa Fluor 488 could be highly preconcentrated and separated simultaneously. A repeated microchamber structure was employed to capture each dye at a desirable position. For subsequent on-chip or off-chip application, pneumatic microvalves were integrated and selectively collected the target dyes with cyclic valve operations. Using the integrated system, Alexa Fluor 488 was solely collected (with a separation resolution of 1.75) out of the mixture at a 30-fold preconcentration ratio. This integrated device would be a key component for lab on a chip applications.