Equilibrium electroconvective instability

Nanofluidic Ionic Memristors

Living organisms use ions and small molecules as information carriers to communicate with the external environment at ultralow power consumption. Inspired by biological systems, artificial ion-based devices have emerged in recent years to try to realize efficient information-processing paradigms. Nanofluidic ionic memristors, memory resistors based on confined fluidic systems whose internal ionic conductance states depend on the historical voltage, have attracted broad attention and are used as neuromorphic devices for computing. Despite their high exposure, nanofluidic ionic memristors are still in the initial stage. Therefore, systematic guidance for developing and reasonably designing ionic memristors is necessary. This review systematically summarizes the history, mechanisms, and potential applications of nanofluidic ionic memristors. The essential challenges in the field and the outlook for the future potential applications of nanofluidic ionic memristors are also discussed.

Differential Impact of Surface Conduction and Electroosmotic Flow on Ion Transport Enhancement by Microscale Auxiliary Structures

Our research investigates the impact of auxiliary structures on ion transport in electrochemical systems such as batteries and microscale desalination units, whose importance for sustainable development has increased dramatically in recent decades. The electrochemical systems typically feature ion-selective surfaces, such as electrodes and ion exchange membranes, where ion depletion can cause performance issues including metal dendrite formation and flow instability. Recent research has shown that auxiliary structures in these electrochemical systems can enhance ion transfer near ion-selective surfaces, thereby resolving the instability problem and improving the energy conversion efficiency of the system. Our study leverages recent advancements in nanoscale electrokinetics to model these auxiliary structures as pillar arrays near an ion exchange membrane in a microchannel. We examine how these structures enhance ion transports relative to the characteristic length scale of microchannel depth and pillars' proximity to the ion-selective surface. Results show that the effect of the pillars varies significantly with their placement. Specifically, in deeper microchannels, where electrokinetic convection is stronger, the closer the auxiliary structure is to the ion-selective membrane, the better the ion transfer. However, in the thinner microchannel, the proximity of the auxiliary structure to the ion selective membrane has a less significant correlation with the ion transfer. Therefore, this finding highlights the importance of spatial arrangement of the auxiliary structures in improving the performance of electrochemical devices. Conclusively, this study can help to better understand energy conversion systems such as fuel cells, salinity gradient power generation systems, and electrochemical desalination systems, where auxiliary structures can be used in the vicinity of ion-selective surfaces. Especially, our fundamental electrokinetic study provides an effective means for designing the efficient electrochemical platforms utilizing micro/nanofluidics.

On-Chip Electrochemical Sensing with an Enhanced Detecting Signal Due to Concentration Polarization-Based Analyte Preconcentration

Here, we integrated two key technologies within a microfluidic system, an electrokinetic preconcentration of analytes by ion Concentration Polarization (CP) and local electrochemical sensors to detect the analytes, which can synergistically act to significantly enhance the detection signal. This synergistic combination, offering both decoupled and coupled operation modes for continuous monitoring, was validated by the intensified fluorescent intensities of CP-preconcentrated analytes and the associated enhanced electrochemical response using differential pulse voltammetry and chronoamperometry. The system performance was evaluated by varying the location of the active electrochemical sensor, target analyte concentrations, and electrolyte concentration using fluorescein molecules as the model analyte and Homovanillic acid (HVA) as the target bioanalyte within both phosphate-buffered saline (PBS) and artificial sweat solution. The combination of on-chip electrochemical sensing with CP-based preconcentration renders this generic approach adaptable to various analytes. This advanced system shows remarkable promise for enhancing biosensing detection in practical applications while bridging the gap between fundamental research and practical implementation.

Influence of Concentration Gradients on Electroconvection at a Cation-Exchange Membrane Surface

Membrane-based systems, such as electrodialysis, play an important role in desalination and industrial separation processes. Electrodialysis uses alternating anion- and cation-exchange membranes with a perpendicular electric field to generate concentrated and diluate streams from a feed solution. It is known that under overlimiting current conditions, reduced charge and mass transfer at the membrane interface leads to regions of high ion depletion generating instability and vortices termed electroconvection. While electroconvective mixing is known to directly impact the separation efficiency of electrodialysis, the influence of ion concentration gradients across the membrane experienced in a functional electrodialysis system is not known. Here, we report the influence of ion concentration gradients across a cation exchange membrane (Nafion) that is both aligned with and opposed to the applied electric field. Experiments were conducted by coflowing NaCl solutions of different concentrations (0.1-100 mM) on each side of the membrane, and electroconvection was visualized with a fluorescence dye (Rhodamine 6G). We obtained concentration profiles from fluorescence image data and systematically measured the thickness of the depletion boundary layer dBL under different conditions. We found smaller dBL values at a higher flow rate both with and without concentration gradients. Our results show that electroconvection is enhanced when the electric field is opposite to the direction of the concentration gradient.

Phosphates Transfer in Pristine and Modified CJMA-2 Membrane during Electrodialysis Processing of NaxH(3-x)PO4 Solutions with pH from 4.5 to 9.9

Phosphate recovery from different second streams using electrodialysis (ED) is a promising step to a nutrients circular economy. However, the relatively low ED performance hinders the widespread adoption of this environmentally sound method. The formation of "bonded species" between phosphates and the weakly basic fixed groups (primary and secondary amines) of the anion exchange membrane can be the cause of decrease in current efficiency and increase in energy consumption. ED processing of Na_xH_(3-x)PO₄ alkaline solutions and the use of intense current modes promote the formation of a bipolar junction from negatively charged bound species and positively charged fixed groups. This phenomenon causes a change in the shape of current-voltage curves, increase in resistance, and an enhancement in proton generation during long-term operation of anion-exchange membrane with weakly basic fixed groups. Shielding of primary and secondary amines with a modifier containing quaternary ammonium bases significantly improves ED performance in the recovery of phosphates from Na_xH_(3-x)PO₄ solution with pH 4.5. Indeed, in the limiting and underlimiting current modes, 40% of phosphates are recovered 1.3 times faster, and energy consumption is reduced by 1.9 times in the case of the modified membrane compared to the pristine one. Studies were performed using a new commercial anion exchange membrane CJMA-2.

Electrodialysis Tartrate Stabilization of Wine Materials: Fouling and a New Approach to the Cleaning of Aliphatic Anion-Exchange Membranes

Electrodialysis (ED) is an attractive method of tartrate stabilization of wine due to its rapidity and reagentlessness. At the same time, fouling of ion-exchange membranes by the components of wine materials is still an unsolved problem. The effect of ethanol, polyphenols (mainly anthocyanins and proanthocyanidins) and saccharides (fructose) on the fouling of aliphatic ion-exchange membranes CJMA-6 and CJMC-5 (manufactured by Hefei Chemjoy Polymer Materials Co. Ltd., Hefei, China) was analyzed using model solutions. It was shown that the mechanism and consequences of fouling are different in the absence of an electric field and during electrodialysis. In particular, a layer of colloidal particles is deposited on the surface of the CJMA-6 anion-exchange membrane in underlimiting current modes. Its thickness increases with increasing current density, apparently due to the implementation of a trap mechanism involving tartaric acid anions, as well as protons, which are products of water splitting and "acid dissociation". A successful attempt was made to clean CJMA-6 in operando by pumping a water-alcohol solution of KCl through the desalination compartment and changing electric field direction. It has been established that such a cleaning process suppresses the subsequent biofouling of ion-exchange membranes. In addition, selective recovery of polyphenols with high antioxidant activity is possible.

Efficient Anion-Exchange Membranes with Anti-Scaling Properties Obtained by Surface Modification of Commercial Membranes Using a Polyquaternium-22

Anion-exchange membranes modified with a polyquaternium-22 (PQ-22) polymer were studied for their use in electrodialysis. The use of PQ-22 for modification makes it possible to "replace" weakly basic amino groups on the membrane surface with quaternary amino groups. It was found that the content of quaternary amino groups in PQ-22 is higher than the content of carboxyl groups, which is the reason for the effectiveness of this polymer even when modifying Ralex AHM-PES membranes that initially contain only quaternary amino groups. In the case of membranes containing weakly basic amino groups, the PQ-22 polymer modification efficiency is even higher. The surface charge of the modified MA-41P membrane increased, while the limiting current density on the current-voltage curves increased by more than 1.5 times and the plateau length decreased by 2.5 times. These and other characteristics indicate that the rate of water splitting decreased and the electroconvective mixing at the membrane surface intensified, which was confirmed by direct visualization of vortex structures. Increasing the surface charge of the commercial MA-41P anion-exchange membrane, reducing the rate of water splitting, and enhancing electroconvection leads to mitigated scaling on its surface during electrodialysis.

Profiled Ion-Exchange Membranes for Reverse and Conventional Electrodialysis

Profiled ion-exchange membranes are promising for improving the parameters of reverse electrodialysis due to the reduction of pumping power and electrical resistance. The smooth commercial heterogeneous cation-exchange MK-40 and anion-exchange MA-41 membranes were chosen as the initial membranes. Profiled membranes with three different types of surface profiles were obtained by hot pressing the initial membranes. The bilayer membranes were made on the basis of single-layer profiled membranes by casting MF-4SK film on the profiled surfaces. The diffusion permeability of all types of single-layer and bilayer profiled membranes was higher than of the initial ones due to the appearance of large defects on their surface during pressing. The conductivity of the profiled membrane was lower in the diluted solution and higher in the concentrated solution than of the initial one for all samples except for the bilayer anion-exchange membrane. The conductivity of that sample was lower than that of the initial anion-exchange MA-41 membrane over the entire range of studied concentrations. The counter-ion transport numbers for all studied membranes were calculated based on the concentration dependences of conductivity and diffusion permeability of the membrane by the microheterogeneous model. The selectivity of single layer and bilayer profiled membranes became lower after their profiling due to the increase of the solution phases of membranes. The asymmetry of the current-voltage curves for all single-layer and bilayer profiled membranes was found. The application of the single layer and bilayer profiled membranes in reverse electrodialysis did not lead to an increase in power density.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Dependence of Electrochemical Properties of MK-40 Heterogeneous Membrane on Number of Adsorbed Layers of Polymers

The creation of monovalent selective ion exchange membranes benefits the desalination of surface waters by removing interfering monovalent ions while preserving polyvalent ionic nutrients. Studies of a promising method of layer-by-layer adsorption of polymers for the creation of monovalent selective coatings note a significant effect of the number of formed layers and of the nature of the external layer on the properties of the resulting membranes. This article reports the changes in properties of layer-by-layer coated heterogeneous membranes occurring at increasing numbers of layers that are attributed to the supposed intermixing of polymers between the layers, namely dependence of limiting current densities determined from i-V curve, enhanced electroconvection that was attributed to the appearing electrical heterogeneity of the surface, and the decreasing monovalent selectivity in electrodialysis of mixed NaCl + CaCl2 solution (from 1.33 to about 1) between the samples with five and six to eight layers of polymers.

Influence of Electroconvection on Chronopotentiograms of an Anion-Exchange Membrane in Solutions of Weak Polybasic Acid Salts

Visualization of electroconvective (EC) vortices at the undulated surface of an AMX anion-exchange membrane (Astom, Osaka, Japan) was carried out in parallel with the measurement of chronopotentiograms. Weak polybasic acid salts, including 0.02 M solutions of tartaric (NaHT), phosphoric (NaH₂PO₄), and citric (NaH₂Cit) acids salts, and NaCl were investigated. It was shown that, for a given current density normalized to the theoretical limiting current calculated by the Leveque equation (i/i_lim^theor), EC vortex zone thickness, d_EC, decreases in the order NaCl > NaHT > NaH₂PO₄ > NaH₂Cit. This order is inverse to the increase in the intensity of proton generation in the membrane systems under study. The higher the intensity of proton generation, the lower the electroconvection. This is due to the fact that protons released into the depleted solution reduce the space charge density, which is the driver of EC. In all studied systems, a region in chronopotentiograms between the rapid growth of the potential drop and the attainment of its stationary values corresponds to the appearance of EC vortex clusters. The amplitude of the potential drop oscillations in the chronopotentiograms is proportional to the size of the observed vortex clusters.

A Review on Ion-Exchange Membranes Fouling during Electrodialysis Process in Food Industry, Part 2: Influence on Transport Properties and Electrochemical Characteristics, Cleaning and Its Consequences

Ion-exchange membranes (IEMs) are increasingly used in dialysis and electrodialysis processes for the extraction, fractionation and concentration of valuable components, as well as reagent-free control of liquid media pH in the food industry. Fouling of IEMs is specific compared to that observed in the case of reverse or direct osmosis, ultrafiltration, microfiltration, and other membrane processes. This specificity is determined by the high concentration of fixed groups in IEMs, as well as by the phenomena inherent only in electromembrane processes, i.e., induced by an electric field. This review analyzes modern scientific publications on the effect of foulants (mainly typical for the dairy, wine and fruit juice industries) on the structural, transport, mass transfer, and electrochemical characteristics of cation-exchange and anion-exchange membranes. The relationship between the nature of the foulant and the structure, physicochemical, transport properties and behavior of ion-exchange membranes in an electric field is analyzed using experimental data (ion exchange capacity, water content, conductivity, diffusion permeability, limiting current density, water splitting, electroconvection, etc.) and modern mathematical models. The implications of traditional chemical cleaning are taken into account in this analysis and modern non-destructive membrane cleaning methods are discussed. Finally, challenges for the near future were identified.

Numerical Simulation of Continuous Extraction of Li+ from High Mg2+/Li+ Ratio Brines Based on Free Flow Ion Concentration Polarization Microfluidic System

Ion concentration polarization (ICP) is a promising mechanism for concentrating and/or separating charged molecules. This work simulates the extraction of Li⁺ ions in a diluted high Mg²⁺/Li⁺ ratio salt lake brines based on free flow ICP focusing (FF-ICPF). The model solution of diluted brine continuously flows through the system with Li⁺ slightly concentrated and Mg²⁺ significantly removed by ICP driven by external pressure and perpendicular electric field. In a typical case, our results showed that this system could focus Li⁺ concentration by ~1.28 times while decreasing the Mg²⁺/Li⁺ ratio by about 85% (from 40 to 5.85). Although Li⁺ and Mg²⁺ ions are not separated as an end product, which is preferably required by the lithium industry, this method is capable of decreasing the Mg²⁺/Li⁺ ratio significantly and has great potential as a preprocessing technology for lithium extraction from salt lake brines.

Sparse nonlinear models of chaotic electroconvection

Convection is a fundamental fluid transport phenomenon, where the large-scale motion of a fluid is driven, for example, by a thermal gradient or an electric potential. Modelling convection has given rise to the development of chaos theory and the reduced-order modelling of multiphysics systems; however, these models have been limited to relatively simple thermal convection phenomena. In this work, we develop a reduced-order model for chaotic electroconvection at high electric Rayleigh number. The chaos in this system is related to the standard Lorenz model obtained from Rayleigh-Benard convection, although our system is driven by a more complex three-way coupling between the fluid, the charge density, and the electric field. Coherent structures are extracted from temporally and spatially resolved charge density fields via proper orthogonal decomposition (POD). A nonlinear model is then developed for the chaotic time evolution of these coherent structures using the sparse identification of nonlinear dynamics (SINDy) algorithm, constrained to preserve the symmetries observed in the original system. The resulting model exhibits the dominant chaotic dynamics of the original high-dimensional system, capturing the essential nonlinear interactions with a simple reduced-order model.

Bipolar Nanochannels: A Systematic Approach to Asymmetric Problems

Nanofluidic diodes are capable of rectifying the electrical current by several orders of magnitude. In the current state of affairs, determining the rectification factor is not possible as it depends on many system parameters. In this work, we systematically scan the effects of geometry and excess counterion concentrations (i.e., surface charge effects). We show that the current-voltage response varies between the two extreme behaviors of unipolar and bipolar responses. The exact behavior depends on the geometry and surface charge properties of the system. Here, we have gone beyond the typical setup that only considers the dynamics within the nanochannel itself and we have included the effects of the adjoining microchannels. Systems that include both nanochannels and microchannels exhibit the classical signatures of concentration polarization, such as ionic depletion and enrichment. Here, where we have scanned a wide range of parameters, we show that bipolar and semi-bipolar systems exhibit a wider range of phenomena that are intrinsically more complicated. Our system characterization is for both, the much more investigated case of steady state and the less investigated, but equally interesting, time-transient case. For example, it is common to characterize the system by its steady-state result (current-voltage response, rectification factor, and transport number). Here, we demonstrate that the time-transient behavior of the fluxes can also be used to characterize the system, and that the time-dependent rectification factors and transport numbers are meaningful. The systematic approach taken in this work, and the results presented herein, can be used to further elucidate the complicated behavior of the current-voltage response of nanofluidic diodes and to rationalize experimental results. The insights of this work can be used to enhance and improve the design of all nanofluidic diodes.

Pulsed electric field-assisted overlimiting current enhancement through a perm-selective membrane

Overlimiting current through a perm-selective membrane has been actively researched not only for the fundamental advancement of electrokinetics but also for energy/environmental applications such as electrodialysis, fuel cells, etc. In particular, various strategies were reported for the enhancement of overlimiting current because these applications demand efficient mass transport through the membrane. In this work, we presented in operando visualization and rigorous numerical study for the overlimiting current density enhancement using a pulsed electric field which is one of the most cost-effective parameters to be externally controlled. We clearly demonstrated that the current density had a peak value as a function of the pulse frequency and would suggest its correlation to a concentration profile and diffusion relaxation time ([small tau, Greek, tilde]diff). As the pulse frequency was chosen which is similar to ([small tau, Greek, tilde]diff)-1, the concentration profiles (i.e. established current paths) were maintained even in off-state due to remnant current paths helping the fast ion transportation. The fundamental evidence presented in this work would provide a strategical design of a perm-selective membrane system for a higher mass transportation efficiency.

Ion transport in nanopores with highly overlapping electric double layers

Investigation of ion transport through nanopores with highly overlapping electric double layers is extremely challenging. This can be attributed to the non-linear Poisson-Boltzmann equation that governs the behavior of the electrical potential distribution as well as other characteristics of ion transport. In this work, we leverage the approach of Schnitzer and Yariv [Phys. Rev. E 87, 054301 (2013)] to reduce the complexity of the governing equation. An asymptotic solution is derived, which shows remarkable correspondence to simulations of the non-approximated equations. This new solution is leveraged to address a number of highly debated issues. We derive the equivalent of the Gouy-Chapman equation for systems with highly overlapping electric double layers. This new relationship between the surface charge density and the surface potential is then utilized to determine the power-law scaling of nanopore conductances as a function of the bulk concentrations. We derive the coefficients of transport for the case of overlapping electric double layers and compare it to the renowned uniform potential model. We show that the uniform potential model is only an approximation for the exact solution for small surface charges. The findings of this work can be leveraged to uncover additional hidden attributes of ion transport through nanopores.

Eco friendly nanofluidic platforms using biodegradable nanoporous materials

Splendid advancement of micro/nanofluidic researches in the field of bio- and chemical-analysis enables various ubiquitous applications such as bio-medical diagnostics and environmental monitoring, etc. In such devices, nanostructures are the essential elements so that the nanofabrication methods have been major issues since the last couple of decades. However, most of nanofabrication methods are sophisticated and expensive due to the requirement of high-class cleanroom facilities, while low-cost and biocompatible materials have been already introduced in the microfluidic platforms. Thus, an off-the-shelf and biodegradable material for those nanostructures can complete the concept of an eco-friendly micro/nanofluidic platform. In this work, biodegradable materials originated from well-known organisms such as human nail plate and denatured hen egg (albumen and yolk) were rigorously investigated as a perm-selective nanoporous membrane. A simple micro/nanofluidic device integrated with such materials was fabricated to demonstrate nanofluidic phenomena. These distinctive evidences (the visualization of ion concentration polarization phenomenon, ohmic/limiting/over-limiting current behavior and surface charge-governed conductance) can fulfill the requirements of functional nanostructures for the nanofluidic applications. Therefore, while these materials were less robust than nano-lithographically fabricated structures, bio-oriented perm-selective materials would be utilized as a one of key elements of the biodegradable and eco friendly micro/nanofluidic applications.

Controlling electrochemical growth of metallic zinc electrodes: Toward affordable rechargeable energy storage systems

Scalable approaches for precisely manipulating the growth of crystals are of broad-based science and technological interest. New research interests have reemerged in a subgroup of these phenomena-electrochemical growth of metals in battery anodes. In this Review, the geometry of the building blocks and their mode of assembly are defined as key descriptors to categorize deposition morphologies. To control Zn electrodeposit morphology, we consider fundamental electrokinetic principles and the associated critical issues. It is found that the solid-electrolyte interphase (SEI) formed on Zn has a similarly strong influence as for alkali metals at low current regimes, characterized by a moss-like morphology. Another key conclusion is that the unique crystal structure of Zn, featuring high anisotropy facets resulting from the hexagonal close-packed lattice with a c/a ratio of 1.85, imposes predominant influences on its growth. In our view, precisely regulating the SEI and the crystallographic features of the Zn offers exciting opportunities that will drive transformative progress.

Continuous and spontaneous nanoparticle separation by diffusiophoresis

The separation of nanoparticles has drawn critical attention in various microfluidic applications including chemical analysis, diagnostics and environmental monitoring. Thus, a number of nanoparticle separation methods have been extensively proposed. However, most of the conventional methods require complicated structured devices, expensive manufacturing processes, and external power sources. While a spontaneous diffusiophoretic separation device based on an ion exchange mechanism could overcome such drawbacks, the recovery of separated particles and the inevitable development of an acidic environment due to the release of H+ from the cation exchange membrane limit its practical applicability. Therefore, in this work, we present a simple but robust nanoparticle separation method based on spontaneously induced diffusiophoresis, which is operated in a continuous manner to overcome the limitations of conventional methods. First, we confirmed that the particle exclusion distance followed the previously developed scaling law of diffusiophoresis. Consequently, we demonstrated the separation of nanoparticles of 40 nm, 200 nm and 2 μm diameter by utilizing the fact that the exclusion distances of various particles were proportional to their diffusiophoretic mobility. Furthermore, the use of Tris buffer increased the diffusiophoretic migration of nanoparticles due to the enhanced concentration gradient, and enabled the produced solution to be compatible with pH-sensitive bio-samples. Therefore, we expect this continuous and spontaneous diffusiophoretic separation platform to be useful in practical applications for analyzing various nano-meter scale bio-particles.

Transitions and Instabilities in Imperfect Ion-Selective Membranes

Numerical investigation of the underlimiting, limiting, and overlimiting current modes and their transitions in imperfect ion-selective membranes with fluid flow through permitted through the membrane is presented. The system is treated as a three layer composite system of electrolyte-porous membrane-electrolyte where the Nernst–Planck–Poisson–Stokes system of equations is used in the electrolyte, and the Darcy–Brinkman approach is employed in the nanoporous membrane. In order to resolve thin Debye and Darcy layers, quasi-spectral methods are applied using Chebyshev polynomials for their accumulation of zeros and, hence, best resolution in the layers. The boundary between underlimiting and overlimiting current regimes is subject of linear stability analysis, where the transition to overlimiting current is assumed due to the electrokinetic instability of the one-dimensional quiescent state. However, the well-developed overlimiting current is inherently a problem of nonlinear stability and is subject of the direct numerical simulation of the full system of equations. Both high and low fixed charge density membranes (low- and high concentration electrolyte solutions), acting respectively as (nearly) perfect or imperfect membranes, are considered. The perfect membrane is adequately described by a one-layer model while the imperfect membrane has a more sophisticated response. In particular, the direct transition from underlimiting to overlimiting currents, bypassing the limiting currents, is found to be possible for imperfect membranes (high-concentration electrolyte). The transition to the overlimiting currents for the low-concentration electrolyte solutions is monotonic, while for the high-concentration solutions it is oscillatory. Despite the fact that velocities in the porous membrane are much smaller than in the electrolyte region, it is further demonstrated that they can dramatically influence the nature and transition to the overlimiting regimes. A map of the bifurcations, transitions, and regimes is constructed in coordinates of the fixed membrane charge and the Darcy number.

Scaling laws of electroconvective flow with finite vortex height near permselective membranes

In a steady state, the linear scaling laws are confirmed between the intensity characteristics of electroconvective (EC) vortex (including the vortex height and electroosmotic slip velocity) and the applied voltage for the nonshear EC flow with finite vortex height near permselective membranes. This finding in the nonshear EC flow is different from the shear EC flow [Kwak et al., Phys. Rev. Lett. 110, 114501 (2013)10.1103/PhysRevLett.110.114501] and indicates that the local concentration gradient has a significant improvement in the analysis of slip velocity. Further, our study reveals that the EC vortex is mainly driven by the second peak effect of the Coulomb thrust in the extended space-charge layer, and the linear scaling law exhibited by the Coulomb thrust is an essential reason for the linear scaling laws of vortex intensity. The scaling laws proposed in this paper are supported by our direct numerical simulation data and previous experimental observations [Rubinstein et al., Phys. Rev. Lett. 101, 236101 (2008)10.1103/PhysRevLett.101.236101].

Transport and Electrochemical Characteristics of CJMCED Homogeneous Cation Exchange Membranes in Sodium Chloride, Calcium Chloride, and Sodium Sulfate Solutions

Recently developed and produced by Hefei Chemjoy Polymer Material Co. Ltd., homogeneous CJMC-3 and CJMC-5 cation-exchange membranes (CJMCED) are characterized. The membrane conductivity in NaCl, Na2SO4, and CaCl2 solutions, permeability in respect to the NaCl and CaCl2 diffusion, transport numbers, current-voltage curves (CVC), and the difference in the pH (DpH) of the NaCl solution at the desalination compartment output and input are examined for these membranes in comparison with a well-studied commercial Neosepta CMX cation-exchange membrane produced by Astom Corporation, Japan. It is found that the conductivity, CVC (at relatively low voltages), and water splitting rate (characterized by DpH) for both CJMCED membranes are rather close to these characteristics for the CMX membrane. However, the diffusion permeability of the CJMCED membranes is significantly higher than that of the CMX membrane. This is due to the essentially more porous structure of the CJMCED membranes; the latter reduces the counterion permselectivity of these membranes, while allowing much easier transport of large ions, such as anthocyanins present in natural dyes of fruit and berry juices. The new membranes are promising for use in electrodialysis demineralization of brackish water and natural food solutions.

The Development of Electroconvection at the Surface of a Heterogeneous Cation-Exchange Membrane Modified with Perfluorosulfonic Acid Polymer Film Containing Titanium Oxide

One way to enhance mass transfer and reduce fouling in wastewater electrodialysis is stimulation of electroconvective mixing of the solution adjoining membranes by modifying their surfaces. Several samples were prepared by casting the perfluorosulfonic acid (PFSA) polymer film doped with TiO₂ nanoparticles onto the surface of the heterogeneous cation-exchange membrane MK-40. It is found that changes in surface characteristics conditioned by such modification lead to an increase in the limiting current density due to the stimulation of electroconvection, which develops according to the mechanism of electroosmosis of the first kind. The greatest increase in the current compared to the pristine membrane can be obtained by modification with the film being 20 μm thick and containing 3 wt% of TiO₂. The sample containing 6 wt% of TiO₂ provides higher mass transfer in overlimiting current modes due to the development of nonequilibrium electroconvection. A 1.5-fold increase in the thickness of the modifying film reduces the positive effect of introducing TiO₂ nanoparticles due to (1) partial shielding of the nanoparticles on the surface of the modified membrane; (2) a decrease in the tangential component of the electric force, which affects the development of electroconvection.

Dynamic analysis of the extended space charge layer using chronopotentiometric measurements

In this paper, we experimentally verified the length (LESC) and the concentration (cESC) of the extended space charge (ESC) layer in front of the electrical double layer (EDL) using the chronopotentiometric measurement and the equivalent circuit model analysis. From the experimentation, the coupled-response of the EDL and the ESC layer was discriminated from the contribution of electro-osmotic flow (EOF). In addition, we derived the potential differences across the ESC (VESC) layer using the circuit model of the ICP layer under rigorous consideration of ESC and EDL. As a result, we obtained that VESC was linearly proportional to the square of the applied current (iapplied). Hence, LESC and cESC were quantitatively provided, where LESC is linear to the iapplied and cESC is constant regardless of iapplied. Thus, this experimentation could not only clarify an essential ICP theory but also guide in ESC-based applications.

Shear electroconvective instability in electrodialysis channel under extreme depletion and its scaling laws

The electroconvective instability (ECI) in an electrodialysis channel under a strong electric field is studied here. The phenomenon of ECI with extreme depletion (ECI-HD) is reported; that is, the overlapping vortices cause the extreme depletion zone to propagate in the horizontal direction. Using scaling theory and direct numerical simulation, we indicate a series of features under the ECI-HD. The decrease in ion transport rate with voltage in ECI-HD is different from the enhancement in the ECI with moderate depletion (ECI-MD), which results in a unique peak in the voltage-current curve. More importantly, we reveal that the ECI is regulated by a scaling factor consisting of the electric field, hydrodynamic coupling coefficient, and Péclet number. For the ECI-HD, the scaling factor has an opposite effect on the vortex size and overlimiting current as that on the ECI-MD. The extreme depletion zone of the ECI-HD also has an uncommon diffusion self-similar dynamics. These unique scaling laws allow one to establish the quantitative bridge between the ion concentration, electric field, and vortex size by the overlimiting current.

Pattern Formation of Three-Dimensional Electroconvection on a Charge Selective Surface

When a charge selective surface consumes or transports only cations or anions in the electrolyte, biased ion rejection initiates hydrodynamic instability, resulting in vortical fluid motions called electroconvection. In this Letter, we describe the first laboratory observation of three-dimensional electroconvection on a charge selective surface. Combining experiment and scaling analysis, we successfully categorized three distinct patterns of 3D electroconvection according to [(Ra_{E})/(Re^{2}Sc)] [electric Rayleigh number (Ra_{E}), Reynolds number (Re), Schmidt number (Sc)] as (i) polygonal, (ii) transverse, or (iii) longitudinal rolls. If Re increases or Ra_{E} decreases, pure longitudinal rolls are presented. On the other hand, transverse rolls are formed between longitudinal rolls, and two rolls are transformed as polygonal one at higher Ra_{E} or lower Re. In this pattern selection scenario, Sc determines the critical electric Rayleigh number (Ra_{E}^{*}) for the onset of each roll, resulting in Ra_{E}^{*}∼Re^{2}Sc. We also verify that convective ion flux by electroconvection (represented by an electric Nusselt number Nu_{E}) is fitted to a power law, Nu_{E}∼[(Ra_{E}-Ra_{E}^{*})/(Re^{2}Sc)]^{α_{1}}Re^{α_{2}}Pe^{α_{3}} [Péclet number (Pe)], where each term represents the characteristics of electroconvection, shear flow, and ion transport.

Controllable pH Manipulations in Micro/Nanofluidic Device Using Nanoscale Electrokinetics

Recently introduced nanoscale electrokinetic phenomenon called ion concentration polarization (ICP) has been suffered from serious pH changes to the sample fluid. A number of studies have focused on the origin of pH changes and strategies for regulating it. Instead of avoiding pH changes, in this work, we tried to demonstrate new ways to utilize this inevitable pH change. First, one can obtain a well-defined pH gradient in proton-received microchannel by applying a fixed electric current through a proton exchange membrane. Furthermore, one can tune the pH gradient on demand by adjusting the proton mass transportation (i.e., adjusting electric current). Secondly, we demonstrated that the occurrence of ICP can be examined by sensing a surrounding pH of electrolyte solution. When pH > threshold pH, patterned pH-responsive hydrogel inside a straight microchannel acted as a nanojunction to block the microchannel, while it did as a microjunction when pH < threshold pH. In case of forming a nanojunction, electrical current significantly dropped compared to the case of a microjunction. The strategies that presented in this work would be a basis for useful engineering applications such as a localized pH stimulation to biomolecules using tunable pH gradient generation and portable pH sensor with pH-sensitive hydrogel.

Three-dimensional electroconvective vortices in cross flow

This study focuses on the three-dimensional (3D) electrohydrodynamic flow instability between two parallel electrodes driven by unipolar charge injection with and without cross flow. Lattice Boltzmann method with a two-relaxation time model is used to compute flow patterns. In the absence of cross flow, the base-state solution is hydrostatic, and the electric field is one-dimensional. With strong charge injection and high electrical Rayleigh number, the system exhibits electroconvective vortices. Disturbed by perturbation patterns, such as rolling pattern, square pattern, and hexagon pattern, the flow develops corresponding to the most unstable mode. The growth rate and pattern transitions are studied using dynamic mode decomposition of the transient numerical solutions. The interactions between cross flow and electroconvective vortices lead to suppression and disappearance of structures with velocity components in the direction of cross flow, while the other components are not affected. Surprisingly, the transition from a 3D to a 2D flow pattern enhances the convective charge transport, marked by an increase in the electric Nusselt number. Hysteresis in the 3D to 2D transition is characterized by the nondimensional parameter Y, a ratio of the electrical force term to the viscous term in the momentum equation.

Hierarchical micro/nanoporous ion-exchangeable sponge

Inspired by the versatile properties of a sponge, we propose an ion-exchangeable sponge composed of hierarchical micropores and nanopores for water treatment. Sodium ions in brackish water (10-300 mM) absorbed in the sponge were exchanged for hydrogen ions in a short incubation time and the desalted water was released by squeezing the sponge with a single hand grip. This simple desalination process was attributed to the multi-scale porous structures in the sponge. A number of nanoporous thin films were formed like bubbles surrounded by the closed backbone of the microporous sponge. The hierarchical micro/nanopores maximized the contact area of the ion-exchanging surface with the saline solution so that scaled-up desalination was achieved. Furthermore, the growth of wheat shoots in the desalted water was demonstrated in vivo after using this micro/nanofluidic based water-treatment with the sponges. Wheat shoots grown in NaHCO₃/Na₂CO₃ solutions treated by the sponges were 110 to 226.45% longer than those grown in the original salty water. This implies that the ion-exchangeable sponge could serve as an appropriate technology for the treatment of ground water affected by acid rain and weathered alkaline rocks.

How Electrical Heterogeneity Parameters of Ion-Exchange Membrane Surface Affect the Mass Transfer and Water Splitting Rate in Electrodialysis

Electrodialysis (ED) has been demonstrated as an effective membrane method for desalination, concentration, and separation. Electroconvection (EC) is a phenomenon which can essentially increase the mass transfer rate and reduce the undesirable water splitting effect. Efforts by a number of researchers are ongoing to create conditions for developing EC, in particular, through the formation of electrical heterogeneity on the membrane surface. We attempt, for the first time, to optimize the parameters of surface electrical heterogeneity for ion-exchange membranes used in a laboratory ED cell. Thirteen different patterns on the surface of two Neosepta anion-exchange membranes, AMX and AMX-Sb, were tested. Low-conductive fluoropolymer spots were formed on the membrane surface using the electrospinning technique. Spots in the form of squares, rectangles, and circles with different sizes and distances between them were applied. We found that the spots' shape did not have a visible effect. The best effect, i.e., the maximum mass transfer rate and the minimum water splitting rate, was found when the spots' size was close to that of the diffusion layer thickness, δ (about 250 μm in the experimental conditions), and the distance between the spots was slightly larger than δ, such that the fraction of the screened surface was about 20%.