Effect Of Electrical Double Layer Research Articles

Slip-driven electroosmotic transport through porous media.

We investigate the slip-driven transport of a Newtonian fluid through porous media under electrical double layer effect. We employ a semianalytical framework to obtain the underlying electrohydrodynamics for different configurations of porous media. We bring out an alteration in flow dynamics, stemming from interplay among the geometrical feature of the models and the interfacial slip as modulated by the electrical forcing. Further, we show the consequent effects of the underlying flow dynamics on the volumetric transport rate through different models. Also, we show the inception of reverse flow in the region close to the wall, resulting from the induced pressure gradient due to the convection of co-ions in the opposite direction to flow and pinpoint its effect of the flow rate variation under the influence of interfacial slip. We believe that the inferences obtained from the present analysis may improve the design of bio-MEMS (microelectromechanical systems) and microfluidic devices, which are used for in-situ bioremediation.

Joule heating, viscous dissipation and convective heat transfer of pressure-driven flow in a microchannel with surface charge-dependent slip

The present work develops closed form expressions of Joule heating, viscous dissipation and Nusselt number for steady, laminar, hydrodynamically and thermally fully developed pressure-driven flow (PDF) in a microchannel considering the combined effect of surface charge-induced electric double layer (EDL) and surface charge-dependent slip. On basis of these, the effects of zeta potential in a large range limited by the steric effect of free ions to valid the nonlinear Poisson-Boltzmann equation, surface charge-dependent slip, ionic concentration of the electrolyte and the microchannel height on the Joule heating, viscous dissipation and Nusselt number of the PDF are analyzed. The results reveal that the electric field strength generated from the motion of the net free charges within the EDL moving with the PDF shows a valley trend with the continually increasing absolute value of zeta potential, and it is a key factor to affect the thermal performances of the PDF. Both the Joule heating and viscous dissipation of the PDF show a non-monotonic variation with the continually increasing zeta potential because of the non-monotonic electric field strength. Additionally, the zeta potential can lead to the reduction of Nusselt number by reducing the velocity of PDF. However, when the magnitude of zeta potential increases to a high value, the Nusselt number shows a non-monotonic variation because of the non-monotonic velocity field of the PDF arising from the non-monotonic electric field strength. Furthermore, slip, ionic concentration and microchannel height can also significantly affect the Joule heating, viscous dissipation and the Nusselt number of the PDF in different manners. The underlying mechanisms of the corresponding results are analyzed. The subject of the paper is the theoretical results and the corresponding analysis.

Slip driven micro-pumping of binary system with a layer of non-conducting fluid under electrical double layer phenomenon

In this study, we report the consequences of the fluid/wall slippage on the micro-pumping of an immiscible binary system constituted by conducting (non-Newtonian) and non-conducting (Newtonian) fluid pair through a microchannel. We consider power-law model to represent the constitutive behaviour of the non-Newtonian fluid. We explore the effect of the combined influences of the applied pressure gradient and electrical forcing on micro-pumping by analytical calculations considering a flat interface between the fluids. We highlight the alteration in underlying dynamics, mainly attributable to the rheology driven modification in viscous resistance in the field as modulated by the interfacial slip and electrical double layer effect. Also, we establish a phenomenal amplification in micro-pumping effect, as realised through an enhanced volumetric flow rate though the channel under combined forcings environment. We believe that the inferences obtained from this analysis may improve the design of various bio-microfluidic devises/systems, which are often used for the transportation of binary layers including one non-Newtonain fluid in the system.

Heat and mass transfer of two-phase flow with Electric double layer effects induced due to peristaltic propulsion in the presence of transverse magnetic field

Abstract Biologically-inspired propulsion systems are currently receiving significant interest in the engineering applications. Motivated by these developments, in the present article analysed heat and mass transfer with the transverse magnetic field on peristaltic motion of two-phase flow (particle-fluid suspension) through a planar channel have been examined. The flow is observed under the influence of electric field and chemical reaction. The present flow problem is modeled using lubrication theory approximation and also with a combination of long wavelength and creeping flow regime assumptions. Moreover, the problem is further simplified using Debye linearization. Analytical solutions are obtained for the resulting coupled ordinary differential equations. The influence of various emerging parameters is discussed for velocity, temperature and concentration profile. Furthermore, the behavior of pressure rise is also discussed to analyse the pumping characteristics. Trapping mechanism is also presented with the help of streamlines. It is observed that velocity distribution tends to increase significantly due to the greater effect of electric field and electro-osmotic parameter, however, magnetic field and particle volume fraction provides a marked resistance to the flow. The influence of electric field and electro-osmotic parameter depicts converse behavior on temperature and concentration distribution. Furthermore, chemical reaction parameter causes a significant reduction in the concentration distribution. The main motivation of the present study is due to such fact that two-phase flow process is very important to analyse the peristaltic muscular expansion and contraction in propagating various biological fluids that act like a particle-fluid mixture. The present study has a wide range of applications in bio-medical engineering i.e. electromagnetic peristaltic micro pumps.

Ion selective redox cycling in zero-dimensional nanopore electrode arrays at low ionic strength

Surface charge characteristics and the electrical double layer (EDL) effect govern the transport of ions into and out of nanopores, producing a permselective concentration polarization, which dominates the electrochemical response of nanoelectrodes in solutions of low ionic strength. In this study, highly ordered, zero-dimensional nanopore electrode arrays (NEAs), with each nanopore presenting a pair of recessed electrodes, were fabricated to couple EDL effects with redox cycling, thereby achieving electrochemical detection with improved sensitivity and selectivity. These NEAs exhibit current amplification as high as 55-fold due to the redox cycling effect, which can be further increased by ∼500-fold upon the removal of the supporting electrolyte. The effect of nanopore geometry, which is a key factor determining the magnitude of the EDL effect, is fully characterized, as is the effect of the magnitude and sign of the charge of the redox-active species. The observed changes in limiting current with the concentration of the supporting electrolyte confirm the accumulation of cations and repulsion of anions in NEAs presenting negative surface charge. Exploiting this principle, dopamine was selectively determined in the presence of a 3000-fold excess of ascorbic acid within the NEA.

Electrokinetic aspects of water filtration by AlOOH-coated siliceous particles with nanoscale roughness

The vast majority of analytical and numerical models developed to explain pressure-driven electrokinetic phenomena assume that the local electrical double layer field over heterogenious surfaces is independent of the flow field and described by the Poison-Boltzman equation. However, for pressure-driven flow over a surface with heterogeneous patches with combined microscale and nanoscale structures the local electrical double layer fields are different above the patch and in the region between the patches. The nonuniform surface charge produces distortions in the equilibrium electrostatic field. The characteristic symptom of field distortion is the generation of flow velocities in all three coordinate directions, including a circulation pattern perpendicular to the main flow axis therefore severely distorting the Poisson-Boltzmann double layer.<em> </em>The result is an exceptionally high microbes and ions removal efficiencies from aqueous suspension by the alumina’s surfaces with combined microscale and nanoscale structures that strongly suggests existence of a coupling effect of the local electrical double layer (EDL) field with the local flow field.

Modeling electrical double-layer effects for microfluidic impedance spectroscopy from 100 kHz to 110 GHz.

Broadband microfluidic-based impedance spectroscopy can be used to characterize complex fluids, with applications in medical diagnostics and in chemical and pharmacological manufacturing. Many relevant fluids are ionic; during impedance measurements ions migrate to the electrodes, forming an electrical double-layer. Effects from the electrical double-layer dominate over, and reduce sensitivity to, the intrinsic impedance of the fluid below a characteristic frequency. Here we use calibrated measurements of saline solution in microfluidic coplanar waveguide devices at frequencies between 100 kHz and 110 GHz to directly measure the double-layer admittance for solutions of varying ionic conductivity. We successfully model the double-layer admittance using a combination of a Cole-Cole response with a constant phase element contribution. Our analysis yields a double-layer relaxation time that decreases linearly with solution conductivity, and allows for double-layer effects to be separated from the intrinsic fluid response and quantified for a wide range of conducting fluids.

Effect of electric double layer on electro-spreading dynamics of electrolyte drops

A theoretical model is proposed to study the spreading of electrolyte drops on charged surfaces. A nanoscopic-thick electric double layer (or EDL) is triggered when an electrolyte drop comes in contact with a charged solid – therefore, the focus of the present study is to theoretically probe a multiscale problem where a nanoscopic EDL affects the spreading of a macroscopic electrolytic drop. The genesis of this multiscale effect is the fact that this EDL induces an electrostatic wetting tension, WEDL which lowers the drop equilibrium contact angle analogous to the “electrowetting” effect. This lowering significantly affects the electro-spreading dynamics by enhancing the rate of change of the contact radius (R) and the dynamic contact angle during drop spreading. We further show that the prefactors and timescales dictating the scaling relationships between R and spreading time (t) for the initial and the final regimes are affected directly by WEDL, while the timescales for the attainment and termination of the regimes where R∼tα (α<1) are also influenced by WEDL. Finally, we explain that this multiscale EDL-mediated electrospreading is analogous to the applied electric driven induced electrowetting-driven spreading of drops with the role of the applied electric field being played by the EDL-induced electric field. While the EDL-mediated statics of drop wetting has been known for a while, this is for the first time we unravel the EDL effects on this dynamics of drop wetting providing a novel mechanism of controlling dynamics by merely controlling the charge of the substrate and the salt content of the drop.

Long-Term Synaptic Plasticity Emulated in Modified Graphene Oxide Electrolyte Gated IZO-Based Thin-Film Transistors.

Emulating neural behaviors at the synaptic level is of great significance for building neuromorphic computational systems and realizing artificial intelligence. Here, oxide-based electric double-layer (EDL) thin-film transistors were fabricated using 3-triethoxysilylpropylamine modified graphene oxide (KH550-GO) electrolyte as the gate dielectrics. Resulting from the EDL effect and electrochemical doping between mobile protons and the indium-zinc-oxide channel layer, long-term synaptic plasticity was emulated in our devices. Synaptic functions including long-term memory, synaptic temporal integration, and dynamic filters were successfully reproduced. In particular, spike rate-dependent plasticity (SRDP), one of the basic learning rules of long-term plasticity in the neural network where the synaptic weight changes according to the rate of presynaptic spikes, was emulated in our devices. Our results may facilitate the development of neuromorphic computational systems.

Photoelectrochemical Reduction of CO2 to Ethylene Glycol on IL-SAM-Modified Au Electrodes

As civilization continues to evolve, fossil resources are depleting and carbon dioxide (CO2) emissions are increasing. Recently, the conversion of CO2 to fuels has attracted much attention as one of the solution for resolving the above problems. Among them the conversion technologies powered by solar energy are expected to be clean methods, and some carbon compounds are generated with <10% conversion efficiencies. However, all these compounds have only one carbon atom, and compounds with a C-C bond have not been reported. Recently, our group found that the imidazolium ion-terminated self-assembled monolayer (IL-SAM)-modified electrodes promote electrochemical conversion of CO2 to ethylene glycol with high selectivity (the Faraday efficiency is >80%) in an aqueous electrolyte solution.[1] Here we report that the SAM-modified electrodes promote the solar energy conversion to ethylene glycol from CO2. The activity of photoelectrochemical CO2 reduction was investigated in a two-compartment cell with an anion exchange membrane. We utilized a two-wired photovoltaic (PV) cell system, wherein a four series-connected crystal silicon PV and platinum were used as the photoelectric conversion element and anode, respectively. The cathode was synthesized by a similar method as described previously.[1] The reaction was performed by irradiation of simulated solar light, and the light intensity and the acceptance surface area of the PV were regulated so as to keep the applied potential of cathode at -0.58 V (v.s. RHE). Potassium bicarbonate dissolved in an aqueous solution was used as an electrolyte. During the reaction, CO2 was bubbled through the cathode solution, and the catholyte was periodically drawn into a syringe and directly analyzed by gas chromatography. Initially, a high reaction current was obtained due to the electrical double layer and other effects, but several minutes later the value became approximately constant. The production rate of ethylene glycol at 3 h was 1.8 μmol and solar-to-ethylene glycol conversion efficiency was 0.16%. In this reaction, CO2 initially interacted with the C-2 ring position of the electrochemically deprotonated imidazolium, and produced a carboxylate species. The adjacent two imidazolium carboxylate species formed a dimer followed by the generation of oxalate, leading to further reduction to ethylene glycol on the imidazolium catalysts of the SAM. From this research, we achieved the photoelectrochemical conversion of CO2 to ethylene glycol with a C-C bond structure.[1] J. Tamura, A. Ono, Y. Sugano, C. Huang, H. Nishizawa, S. Mikoshiba, Phys. Chem. Chem. Phys. 2015, 17, 26072.

A single domain formulation on conjugate heat transfer in parallel plate microchannel with electrical double layer

In this study, the conjugate conduction-convection heat transfer in the thermally developing region of a parallel plate microchannel with the constant heat flux at the walls is investigated. The flow is forced to move by applying the pressure difference and is assumed to be fully developed. Moreover, the effects of electrical double layer (EDL) with low zeta potential on the fluid flow and on the temperature field are taken into account. To do so, an analytical/numerical method is proposed to solve the energy equation in a single domain formulation. The energy equation is split and a series solution based on the variational calculus is employed to determine the temperature field. The effects of various parameters such as Peclet number, non-dimensional wall thickness, thermal conductivity ratio, and the zeta potential are studied in details. The obtained results show that the increasing of zeta potential decreases the heat transfer rate. Moreover, the Nusselt number has a non-monotonic behavior when the wall thickness varies.

Study of the chloride transport in unsaturated concrete: Highlighting of electrical double layer, temperature and hysteresis effects

In this paper, we suggest a modelling of chloride ingress in unsaturated concretes. The originality of our model lies in its consideration of the electrical double layer (EDL), the temperature as well as the isotherm sorption-desorption hysteresis. In order to consider slag and fly ash effects, several concretes were considered and experimentally tested in tidal conditions. The chloride profile was simulated by a multispecies model. The simulations show that the chloride concentration increases with a negative EDL more than with a positive EDL and that this effect occurs mainly in the concrete containing slag where the overlapping is more important due to the pore size distribution. We also show that both temperature and hysteresis affect the chloride ingress at the surface more than in deeper layers of the concrete specimen most likely due to a drying effect. Finally, the results put forward a good agreement between measured and simulated chloride profiles mainly when the hysteresis is taken into account.

Surface charge-induced EDL interaction on the contact angle of surface nanobubbles.

The contact angle (CA) of surface nanobubbles is believed to affect the stability of nanobubbles and fluid drag in micro/nanofluidic systems. The CA of nanobubbles is dependent on size and is believed to be affected by the surface charge-induced electrical double layer (EDL). However, neither of these of attributes are well understood. In this paper, by introducing an EDL-induced electrostatic wetting tension, a theoretical model is first established to study the effect of EDLs formed near the solid-liquid interface and the liquid-nanobubble interface on the gas phase CA of nanobubbles. The size-dependence of this EDL interaction is studied as well. Next, by using atomic force microscopy (AFM), the effect of the EDL on nanobubbles' gas phase CA is studied with variable electrical potential at the solid-liquid interface, which is adjusted by an applied voltage. Both the theoretical and the experimental results show that the EDLs formed near the solid-liquid interface and the liquid-nanobubble interface lead to a reduction of gas phase CA of the surface nanobubbles because of an electrostatic wetting tension on the nanobubble due to the attractive electrostatic interaction between the liquid and nanobubble within the EDL, which is in the nanobubbles' outward direction. An EDL with a larger zeta potential magnitude leads to a larger gas phase CA reduction. Furthermore, the effect of EDL on the nanobubbles' gas phase CA shows a significant size-dependence considering the size dependence of the electrostatic wetting tension. The gas phase CA reduction due to the EDL decreases with increasing nanobubble height and increases with the nanobubble's increasing curvature radius, indicating that a surface charge-induced EDL could possibly explain the size dependence of the gas phase CA of nanobubbles.

Electroosmotic transport of immiscible binary system with a layer of non-conducting fluid under interfacial slip: The role applied pressure gradient.

We investigate the transport of immiscible binary fluid layers, constituted by one conducting (top layer fluid) and another non-conducting (bottom layer fluid) fluids in a microfluidic channel under the combined influences of an applied pressure gradient and imposed electric field. We solve the transport equation governing the flow dynamics analytically and obtain the closed-form expressions of the velocity fields. We bring out the alteration in the flow dynamics, mainly attributable to the non-linear interaction between interfacial slip and the electrical double layer effect over small scales as modulated by the applied pressure gradient. In particular, we show the augmentation in the net volume transport rate through the channel, emerging from an intricate competition among electrical forcing, applied pressure gradient and the viscous resistance as modulated by the interfacial slip. We believe that the results of this study may be of immense consequence for the design of various microfluidic devises, which are often used for the manipulation of two immiscible fluids in different biomedical/biochemical processes.

Controlled self-assembly of oligomers-grafted fibrous polyaniline/single zirconium phosphate nanosheet hybrids with potential-responsive ion exchange properties

A novel α-zirconium phosphate (α-ZrP) nanosheet-implanted fibrous polyaniline (PANI) hybrid film with potential-responsive ion exchange function is fabricated by a facile pulse electrochemical synthesis technique. The suppression effect of electrical double layer at the conducting substrate/colloidal solution interface for the formation of PANI/α-ZrP hybrid film is effectively improved through a layer of pre-deposited PANI oligomers, whose thickness plays an important role in the self-assembly of the hybrid film. During the pulse electrochemical synthesis process, the oligomers-grafted fibrous PANI is electropolymerized in the period of high potential; and the translucent exfoliated α-ZrP nanosheets are uniformly implanted into the fibrous PANI by electrostatic self-assembly in the following period of low potential. The amount ratio of α-ZrP nanosheet and fibrous PANI can be regulated by adjusting the pulse width based on their growth rate. The results show that the optimized pulse width for the fabrication of PANI/α-ZrP hybrid film with three-dimensional porous structure is 0.8s. Under this condition, the generated PANI/α-ZrP hybrid film shows fast uptake/release ability and high capacity for the separation of Pb2+ ions from aqueous solution based on its unique potential-induced ion exchange process. The adsorption equilibrium time is less than 30s, and the ion exchange capacity of porous PANI/α-ZrP hybrid film reaches 123.5mgg−1, which is almost 9 times higher than that of the compact PANI/α-ZrP hybrid film. Furthermore, the ion exchange capacity retains 85.2% of its initial value after 1000 cycles.

Diffuse electric double layer in planar nanostructures due to Fermi-Dirac statistics

A double nanocapacitor modelled by two equally charged planar surfaces that confine oppositely charged quanta subjected to Fermi-Dirac statistics is considered theoretically. A global thermodynamic equilibrium was found by minimization of the Helmholtz free energy satisfying constraints that require electroneutrality and fixed total number of confined quanta. The solution obtained by using the Euler–Lagrange method yields self–consistent quantities: distribution of quanta within the pore, electric potential, equilibrium free energy and differential capacitance. Within real values, a rigorous numerical solution and an approximate analytical solution for electrons in the low temperature limit was found. The Fermi–Dirac constraints on the wave functions in the nanopore induced an effect of a diffuse electrical double layer near both charged surfaces. This effect is comparable to the corresponding effect of entropy at finite temperatures and for classical particles, as described by the acknowledged Poisson–Boltzmann theory. At small distances and small surface charges, the electrons are almost evenly distributed within the pore, while at larger distances they condense to the charged surfaces, shielding the electric field. The force between the charged surfaces is repulsive and monotonously decreases with increasing distance between surfaces. The energies stored in the nanocapacitor are up to ≃ 50eV/nm2.

Ion Accumulation and Migration Effects on Redox Cycling in Nanopore Electrode Arrays at Low Ionic Strength.

Ion permselectivity can lead to accumulation in zero-dimensional nanopores, producing a significant increase in ion concentration, an effect which may be combined with unscreened ion migration to improve sensitivity in electrochemical measurements, as demonstrated by the enormous current amplification (∼2000-fold) previously observed in nanopore electrode arrays (NEA) in the absence of supporting electrolyte. Ionic strength is a key experimental factor that governs the magnitude of the additional current amplification (AFad) beyond simple redox cycling through both ion accumulation and ion migration effects. Separate contributions from ion accumulation and ion migration to the overall AFad were identified by studying NEAs with varying geometries, with larger AFad values being achieved in NEAs with smaller pores. In addition, larger AFad values were observed for Ru(NH3)6(3/2+) than for ferrocenium/ferrocene (Fc(+)/Fc) in aqueous solution, indicating that coupling efficiency in redox cycling can significantly affect AFad. While charged species are required to observe migration effects or ion accumulation, poising the top electrode at an oxidizing potential converts neutral species to cations, which can then exhibit current amplification similar to starting with the cation. The electrical double layer effect was also demonstrated for Fc/Fc(+) in acetonitrile and 1,2-dichloroethane, producing AFad up to 100× at low ionic strength. The pronounced AFad effects demonstrate the advantage of coupling redox cycling with ion accumulation and migration effects for ultrasensitive electrochemical measurements.

On the Factors Influencing the Liquidity of Slip Casting Al2O3 Slurry

Abstract: This paper use zeta potential test and dynamic viscosity test to research the influence of different factors such as pH value, mass fraction of solid phase and dispersant content on the liquidity of Al2O3 slurry. The test results show that, when the solid-phase mass fraction and particle size are given, the absolute value of zeta potential increases with the rise of pH value, raise at first then descrease with the adding of (NaPO3)6. This article analyzes the phenomena from a legal point of electric double layer effect. The results of liquidity test are consistent with zeta potential test. The optimum value of pH value, mass fraction of solid-phase and dispersant content are determined by formulation design. The study develop a Al2O3 slurry with high fluidity which has an important significant in slip casting process of Al2O3 ceramics.

Thickness-Dependent Flat Band Potential of Anatase TiO2(001) Epitaxial Films on Nb:SrTiO3(001) Investigated by UHV-Electrochemistry Approach

The semiconductor characteristics of anatase TiO2(001) films, which were pulsed laser-deposited on a 0.05 wt % Nb-doped SrTiO3(001) single crystal electrode, as well as the film/electrolyte heterointerfaces were investigated by the ultrahigh-vacuum electrochemistry approach. In addition to the current–voltage measurements, equivalent circuit modeling in electrochemical impedance spectroscopy at different electrode potentials reveals that the flat band potential of anatase TiO2 films shifts by +0.5 V when the film thickness increases with (1 × 4) surface reconstruction more visible in reflection high energy electron diffraction. The possible origins of the observed flat band potential shift will be discussed from the viewpoint of electric double layer effects, mainly of donor-type surface states and surface reconstruction, compared with the results of single crystal rutile TiO2(110).

Accounting for electric double layer and pressure gradient-induced dispersion effects in microfluidic current monitoring

Current monitoring (CM) is a common experimental method to determine the zeta potential of a microfluidic channel. Dissimilar solution concentrations and finite electric double layers (EDLs) cause deviations to typical CM curves, diminishing the utility of such a method. Here, we theoretically and experimentally investigate these effects, simulating the full time-dependent Navier–Stokes equation coupled with the Poisson–Boltzmann distribution for small ions and dilute systems and validating with experiments in both micro- and nanochannels. We find that current monitoring in finite EDL nanochannel systems and small concentration ratios can be approximated with a simple function, predicting ζ-potential to within 10 % even when the surface charge is unknown. However, as the concentrations used become more and more dissimilar, dispersion due to induced pressure gradients greatly affects the shapes of the current monitoring curves. We show that typical microchannel and nanochannel experiments can easily span a wide range of dispersion regimes, from pure axial diffusion to pure convective dispersion. We explain the shapes of the curves based on the regime of dispersion and give practical guidelines for CM with high-concentration ratios.