Electroconvective Instability Research Articles

Topological transition to rest in the electrohydrodynamics of nematics

ABSTRACT Some nematic liquid crystals undergo electro-convective instabilities if forced by a low-frequency oscillating electric field. When the forcing amplitude is increased, a series of bifurcations occurs, which correspond to increasing disorder and reduction of the size of the electro-convective rolls. At a threshold, a transition to an electrohydrodynamic state characterised by a high density of topological defects occurs. Here, we show that in some nematic mixtures this transition is replaced by a topological transition to rest, that is, the electrohydrodynamics are quenched by the nucleation of a state that is topologically non-equivalent to the initial texture of the nematic.

Electroconvective instability at the surface of one-dimensionally patterned ion exchange membranes

The development of an electroconvective vortex explains the induction of overlimiting current in high voltage applications. However, an in-depth understanding of the dynamics remains challenging because the electrohydrodynamic behavior is unstable. Here, we show how the vortex develops and actively control this using a one-dimensional (1D) grid-patterned ion exchange membrane (IEM). We employed ultraviolet nanosecond laser ablation to fabricate micro-scale (100 μm) non-conductive 1D grid patterns on a commercial IEM. Vortex growth and the electrical responses were simultaneously monitored to derive correlations between the vortex dynamics and ion transport. The rate of electric potential fall increased rapidly during vortex development but decreased more gradually on transverse merging of the vortices. When shear flow was present, the modified scaling law was redefined via dimensionless number analysis of the electroconvective flow by the length of the impermeable 1D grid pattern and the applied voltage. The overlimiting currents and the desalination performances of 1D grid-patterned IEMs were successfully demonstrated. Based on these insights into the electroconvective vortex, we show that the electrical heterogeneity of the IEM actively controls the vortex and enhances desalination performance.

Electroconvective instability near an ion-selective surface: A mesoscopic lattice Boltzmann study.

Direct numerical simulations of electroconvection instability near an ion-selective surface are conducted using a mesoscopic lattice Boltzmann method (LBM). An electrohydrodynamic model of ion transport and fluid flow is presented. We numerically solve the Poisson-Nernst-Planck equations for the electric field and the Navier-Stokes equations for the flow field. The results cover Ohmic, limiting, and overlimiting current regimes, and they are in good agreement with the asymptotic analytical solution for the relationship between current and voltage. The influences of different ion transport mechanisms on the voltage-current relationship are discussed. The results reveal that the electroconvection mechanism is as important as other ion transport mechanisms in electrohydrodynamic flow. By comparing the contribution of different regions in the numerical domain, we find that the flow in the extended space charge layer is dominated by electroconvection. We also study the influences of multiple driving parameters, and the electrohydrodynamic coupling constant plays a dominant role in triggering convective instability. The flow pattern and ion concentration distribution are described in detail. Moreover, the route of flow from steady state to periodic oscillation and then to chaos is discussed.

Nonsymmetric ionic transport in a nonbinary electrolyte at high voltage

A phenomenological study focusing on investigation of ionic transport within one- and two-dimensional electrolytic cells operating at high voltage and confined by two open electrodes is presented. The cell is filled with a nonsymmetric electrolyte consisting of one positively charged and two negatively charged species. The study offers a physical interpretation and detailed discussion of the origin and development of electroconvective instability typical of configurations relevant to digital printing technologies.

Electroconvection near an ion-selective surface with Butler–Volmer kinetics

We study the effects of interfacial kinetics on the electro-hydrodynamics of ion transport near an ion-selective surface using a combination of linear stability analysis and numerical simulation. The finite kinetics of the electrolyte–electrode interface affects the ion transfer and electroconvection in many ways. On a surface of fixed topography, such as a metal surface of slow and stable ion deposition or covered by a polymer membrane, the finite kinetics reduces the current in one-dimensional ion diffusion/migration, increases the critical voltage for the onset of the electroconvective instability, changes the dynamics of the electroconvection and the overlimiting current, and enhances the lateral ion diffusion within the interfacial layer. The first three effects are indirectly caused by the reaction kinetics and can be characterized by an effective voltage difference across the liquid electrolyte. In comparison, the last effect is controlled by a direct interplay between kinetics and nonlinear electroconvection. Scaling laws for ion transport and features of electroconvection are proposed. We also analyse the linear stability of a surface which evolves under ion deposition and find that the finite kinetics decreases the growth rate of both electroconvective and morphological instabilities and therefore modifies the wavenumber of the most unstable mode.

Electroconvective Instability in Water Electrolysis: An Evaluation of Electroconvective Patterns and Their Onset Features

In electrochemical systems, an understanding of the underlying transport processes is required to aid in their better design. This includes knowledge of possible near-electrode convective mixing that can enhance measured currents. Here, for a binary acidic electrolyte in contact with a platinum electrode, we provide evidence of electroconvective instability during electrocatalytic proton reduction. The current-voltage characteristics indicate that electroconvection, visualized with a fluorescent dye, drives current densities larger than the diffusion transport limit. The onset and transition times of the instability do not follow the expected inverse-square dependence on the current density, but, above a bulk-reaction-limited current density are delayed by the water dissociation reaction. The dominant size of the electroconvective patterns is also measured and found to vary as the diffusion length scale, confirming previous predictions on the size of electroconvective vortices.

Sheltering electroconvective instability in a weak electrolyte

Sheltering vortices by shear flow is a common method in plasma and neutral fluids and has recently been successfully applied to control ionic fluids. This work proposes a new chemical sheltering vortex method for electroconvective instability (ECI) based on the Onsager chemical effect. We reveal unique ECI behaviors in a weak electrolyte with the Onsager effect, including the vortex height selection, overlimiting transport, and vortex structure. Due to the strong electric field strength in the electric double layer, the Onsager effect in a weak electrolyte causes neutral molecules to generate additional free ions, which weakens the thickness of the extended space charge layer and causes the fluid to transition from a chaotic ECI to a steady ECI. Consequently, the Onsager effect shelters ECI without an oblique vortex, which is significantly different from the shear flow effect [Kwak et al., “Sheltering the perturbed vortical layer of electroconvection under shear flow,” J. Fluid Mech. 813, 799 (2017)]. We believe that the proposed chemical control strategy can be an alternative candidate for ionic fluids.

Suppression of electroconvective and morphological instabilities by an imposed cross flow of the electrolyte

Electroconvection and its coupling with a morphological instability are important in many applications, including electrodialysis, batteries and fuel cells. In this work, we study the effects of a two-dimensional channel flow on the electroconvective and morphological instabilities using two approaches. In the bulk analysis, we consider the instability of the electroneutral bulk region driven by a second kind electroosmosis slip velocity boundary condition and derive the asymptotic solutions for small and large wavenumbers. In the full analysis, we consider the entire region of the liquid electrolyte and use the ultraspherical spectral method to numerically solve the eigenvalue problems. Both studies show that the imposed flow significantly affects the electroconvective instability. The imposed flow generates a shielding effect by deforming the perturbed ion concentration field and hinders the ion transfer from low- to high- concentration regions which causes the instability. It fully suppresses the electroconvective instability at small wavenumbers and reduces the growth rate of the perturbations at large wavenumbers. The direct effect of the flow on the morphological instability is minor, while the suppression of the electroconvective instability may change the wavenumber of the most unstable mode of the coupled instabilities. For the electroconvective instability, the bulk analysis is qualitatively different from the full analysis at high wavenumbers. For the morphological instability, good agreement is found between the two studies at both small and large wavenumbers.

Mechanisms of hydrodynamic instability in concentration polarization

The authors use experimental and theoretical results to study ionic transport through a charge-selective membrane system and show the nonequilibrium, short-wave, nature of electro-convective instability.

Scaling relations in shear electroconvective vortices

This paper is devoted to the quantitative understanding of the electroosmotic slip velocity, which is the most essential physical quantity of shear electroconvective (SEC) microfluidics. It is well known that SEC instability caused by the electroosmotic slip velocity is triggered near the permselective membranes. Here, we present for the first time the unifying scaling relations of the electroosmotic slip velocity and overlimiting transport in SEC flow under the moderate voltage. The interplaying effects of the salt flux gradient and voltage result in a slip velocity that loses the pressure flow effect. Determined by both the applied potential and the electrolyte physical properties, the slip velocity is shown to scale as V4/3κ2/3, which deviates significantly from the relation of V2 reported in classical theory [I. Rubinstein and B. Zaltzman, “Equilibrium electroconvective instability,” Phys. Rev. Lett. 114(11), 114502 (2015)]. Since the convection flux and the electromigration flux reached an asymptotic equilibrium, a universal scaling κVPe1/3 was obtained for the overlimiting transport. Detailed direct numerical simulations in conjunction with existing experimental data [R. Kwak et al., “Shear flow of an electrically charged fluid by ion concentration polarization: Scaling laws for electroconvective vortices,” Phys. Rev. Lett. 110, 114501 (2013)] corroborate this novel scaling. Our theory provides a unified view and a perfect interpretation of the existing SEC microfluidics.

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.

Effect of the resin content in cation-exchange membranes on development of electroconvection

Comparative analysis of the effect of ion-exchanger content in heterogeneous sulfonated ion-exchange membranes Ralex CM Pes produced by MEGA a.s. (Czech Republic) on their microstructure, current-voltage characteristics and intensity of electroconvective instability was carried out. Using the SEM method, it has been found that with an increase in resin content from 45 to 70 wt%, fraction of the ion-exchanger on the surface of swollen membranes increases 1.8 times, and the distance between them is halved. At the cross-section of membranes, the growth of fraction of ion-exchange regions is 46%. An increase in the resin loading into the membrane causes a 1.7-fold and a twofold increase in the fraction of macropore on the surface and on the cross-section, respectively.With an increase in the ion-exchanger content in membranes, a decrease in the potential of the onset of the overlimiting state and a decrease in the plateau length of the limiting current on the current-voltage curve were revealed. It is shown that a change in resin/inert binder ratio determines the occurrence and development of heteroelectroconvection. Using laser interferometry and flicker noise spectroscopy, evidences of more intense electroconvective mixing of the solution at the boundary with the Ralex cation-exchange membrane with maximum resin loading were obtained.

Surface conduction and electroosmotic flow around charged dielectric pillar arrays in microchannels.

Dielectric microstructures have been reported to have a negative influence on permselective ion transportation because ions do not migrate in areas where the structures are located. However, the structure can promote the transportation if the membrane is confined to a microscopic scale. In such a scale where the area to volume ratio is significantly large, the primary driving mechanisms of the ion transportation transition from electro-convective instability (EOI) to surface conduction (SC) and electroosmotic flow (EOF). Here, we provide rigorous evidence on how the SC and EOF around the dielectric microstructures can accelerate the ion transportation by multi-physics simulations and experimental visualizations. The microstructures further polarize the ion distribution by SC and EOF so that ion carriers can travel to the membrane more efficiently. Furthermore, we verified, for the first time, that the arrangements of microstructures have a critical impact on the ion transportation. While convective flows are isolated in the crystal pillar configuration, the flows show an elongated pattern and create an additional path for ion current in the aligned pillar configuration. Therefore, the fundamental findings of the electrokinetic effects on the dielectric microstructures suggest an innovative application in micro/nanofluidic devices with high mass transport efficiency.

Microscale electrodeionization: In situ concentration profiling and flow visualization

Electrodeionization (EDI) is membrane-based desalination utilizing ion exchange membranes and ion exchange resins. By combining Electrodialysis and Ion exchanger, EDI can produce ultrapure water in a continuous-flow manner. Although its theoretical mechanisms are well documented, there is no experimental platform that can provide microscopic details inside of the system. In this paper, we present microscale EDI that can visualize in situ ion concentration, pH, and fluid flows. The platform was fabricated by filling ion exchange resins as a monolayer in a transparent polydimethylsiloxane channel between cation and anion exchange membranes. According to operating voltages (0–15V), distinct behaviors of ion concentration profile, pH shift, and fluid flows were observed in Ohmic, limiting, and overlimiting regimes. It is noteworthy that overlimiting regimes can be sub-categorized as water-splitting and electroconvection regimes. In the early stage (4–8V), water-splitting is dominant with pH change near the membranes and resins; under a higher voltage (8–15V), electroconvection starts to occur even water-splitting tries to suppress the development of the extended space charge layer and corresponding electroconvective instability. Accelerated ionic migration by electroconvection can improve current efficiency up to 80%. This is a clear departure from overlimiting dynamics in electrodialysis (with electroconvection only), ion exchanger (with no distinct regime), and even from that in previous EDI experiments (with water splitting only).

Signature of electroconvective instability in transient galvanostatic and potentiostatic modes in a microchannel-nanoslot device

For a sufficiently deep microchannel, both experiments and simulations show a distinct transient nonmonotonic behavior of the system chronopotentiometric and chronoamperometric responses, resulting from the emergence of electroconvective instability in the overlimiting conductance regime.

Characterization of Chaotic Electroconvection near Flat Inert Electrodes under Oscillatory Voltages.

The onset of electroconvective instability in an aqueous binary electrolyte under external oscillatory electric fields at a single constant frequency is investigated in a 2D parallel flat electrode setup. Direct numerical simulations (DNS) of the Poisson–Nernst–Planck equations coupled with the Navier–Stokes equations at a low Reynolds number are carried out. Previous studies show that direct current (DC) electric field can create electroconvection near ion-selecting membranes in microfluidic devices. In this study, we show that electroconvection can be generated near flat inert electrodes when the applied electric field is oscillatory in time. A range of applied voltage, the oscillation frequency and the ratio of ionic diffusivities is examined to characterize the regime in which electroconvection takes place. Similar to electroconvection under DC voltages, AC electroconvection occurs at sufficiently high applied voltages in units of thermal volts and is characterized by transverse instabilities, physically manifested by an array of counter-rotating vortices near the electrode surfaces. The oscillating external electric field periodically generate and destroy such unsteady vortical structures. As the oscillation frequency is reduced to of the intrinsic resistor–capacitor (RC) frequency of electrolyte, electroconvective instability is considerably amplified. This is accompanied by severe depletion of ionic species outside the thin electric double layer and by vigorous convective transport involving a wide range of scales including those comparable to the distance L between the parallel electrodes. The underlying mechanisms are distinctly nonlinear and multi-dimensional. However, at higher frequencies of order of the RC frequency, the electrolyte response becomes linear, and the present DNS prediction closely resembles those explained by 1D asymptotic studies. Electroconvective instability supports increased electric current across the system. Increasing anion diffusivity results in stronger amplification of electroconvection over all oscillation frequencies examined in this study. Such asymmetry in ionic diffusivity, however, does not yield consistent changes in statistics and energy spectrum at all wall-normal locations and frequencies, implying more complex dynamics and different scaling for electrolytes with unequal diffusivities. Electric current is substantially amplified beyond the ohmic current at high oscillation frequencies. Also, it is found that anion diffusivity higher than cation has stronger impact on smaller-scale motions ().

Electroconvective Instability on Undulated Ion-selective Surface

Electroconvective Instability on Undulated Ion-selective Surface

Спектральные свойства флуктуаций концентрационного поля в электромембранных системах с сульфокатионообменными мембранами с разной дисперсностью ионообменника

Методом Фурье-анализа определен спектральный состав флуктуаций концентрационного поля в стратифицированных системах с экспериментальными гетерогенными сульфокатионообменными мембранами Ralex CM Pes («MEGA» a.s., Чехия), содержащими ионообменник различной дисперсности. С увеличением времени измельчения ионообменных частиц, соответственно степени их дисперсности, выявлено уменьшение размеров проводящих участков и расстояния между ними на поверхности мембран. Высокая шумовая составляющая колебаний концентрационного поля в растворе вследствие развития электроконвективных вихрей установлена на границе с мембраной, характеризующейся более однородной поверхностью.

Length-dependent instability of shear electroconvective flow: From electroconvective instability to Rayleigh-Bénard instability

Ion and water transport by electroconvection continually finds new applications, arousing considerable research interest. This paper is devoted to the important issue of the effects caused by shear flow, as this flow always occurs in various electrochemical applications, such as electrodeposition, electroplating, and electrodialysis. In this paper, the dimensionless Poiseuille-Navier-Stokes and Poisson-Nernst-Planck model is proposed, which contains the buoyancy force induced by ion concentration polarization. The numerical results show that in the existing literature, the Rayleigh-Bénard convection is neglected and the Debye layer effect is overestimated, leading to a large difference between the simulation results and the experimental data. In addition, the chaotic phenomenon of shear flow is discussed in detail based on the proposed model. The main contributions are as follows: (i) There are two distinct instability phenomena, namely, electroconvective instability, caused by the electric force, and Rayleigh-Bénard instability, caused by the buoyancy force. (ii) For electroconvective instability, the fully overlapping vortex structures in the microchannel are obtained numerically for the first time. In addition, the shear sheltering effect is verified numerically. (iii) The effects of the characteristic length and electrohydrodynamic coupling constant on the Rayleigh-Bénard instability are studied. (iv) The transition condition from electroconvective instability to Rayleigh-Bénard instability is investigated. The analysis shows that choosing a characteristic length consistent with the actual structure is a necessary condition for achieving high-precision analysis of fluid behaviors such as the flow pattern. This conclusion provides important guidance for the design and optimization of the concentration microfluidic chip.

Effect of the sulfocation-exchanger dispersity on the surface morphology, microrelief of heterogeneous membranes and development of electroconvection in intense current modes

Comparative analysis of the effect of electrical and geometric heterogeneity of the surface of heterogeneous sulfocation-exchanger membranes Ralex CM Pes produced by MEGA a.s. (Czech Republic) and the Russian membranes MK-40 manufactured by JSC “Shchekinoazot” on current voltage characteristics (CVC), conditions of occurrence and intensity of electroconvective instability at the membrane/solution interface in intense current modes was carried out. Membranes MK-40 were prepared by hot-pressing technology (https://www.n-azot.ru), membranes Ralex CM Pes by hot-rolling technology (https://www.mega.cz). In the manufacture of the membranes Ralex CM Pes, the milling time for sulfocation-exchanger on a ball mill varied from 5 to 80 min, the volume ratio of cation-exchanger to polyethylene was kept the same. It was found that in the swollen state of the membranes with the increase in the milling time of the ion-exchanger, the ratio between the conducting (ion-exchanger grain) and inert (polyethylene) sections on the surface of the membranes remained constant, and the size of the conducting sections and the distances between them decreased, while the surface microrelief became smoother. Despite the decrease in roughness of the membrane surface, as the degree of dispersity of the initial sulfocation-exchanger increases, the CVC of the membranes show an increase in the limiting diffusion current, a reduction of the plateau section of the limiting current, and an increase in the size of the electroconvective instability region in the solution at the interphase boundary. These facts indicate that when the conditions of the membrane-preparing technology change, a more significant factor determining the conditions for the occurrence and intensity of the development of heteroelectroconvection is a decrease in the electrical heterogeneity of the membrane surface.