Effect Of Electrical Double Layer Research Articles

Numerical simulation of the transient heat and liquid moisture transfer through porous textiles with consideration of electric double layer

The liquid water transport coupled with moisture and heat transfer through porous textiles is a complicated process involving simultaneous, coupled heat and mass transfers. The flows in porous textiles are different from the traditional flows transfer in porous media due to the adsorption of moisture by fibers. Based on the Poisson-Boltzmann equation for electric double layers and Navier-Stokes equation for liquid flows, a mathematical model for describing resistance effects of electric double layer (EDL) on the coupled heat and liquid moisture transfer in porous textiles is developed. The resistance effect of the EDL in porous textiles can be measured by a dimensionless number, which is called electric resistance number. It is proportional to the square of the liquid dielectric constant, the solid surface zeta potential and inversely proportional to the liquid dynamic viscosity, electric conductivity and the square of the effective pore size. With specification of initial and boundary conditions, the distributions of the temperature, moisture concentration, and liquid water content in porous textiles have been obtained. The theoretical predictions are compared with experimental data, and good agreement is observed between the two, indicating that the heat and mass transfer process are influenced by the EDL in porous textiles.

Effect of an external force on the by-pass transition mechanism in internal flows electrical double layer effect in mini and micro channels

The effect of the electric double layer (EDL) on the bypass transition mechanism is explored through direct numerical simulations. The electrokinetic effects destabilize rapidly the flow when the local disturbance and/or the Reynolds number are respectively strong and large enough to overcome the transient growth regime. It is found that a weak perturbation quickly leads to the transition through bypass and non-linear interactions under the EDL effect, while an order of magnitude larger disturbance is incapable to destabilize the macro-scale flow. The EDL develops some new transitional wall structures during the bypass process. It is concluded that electrostatic effects can be efficiently used to enhance the mixing and heat transfer in microchannels, providing that the diffuse layer is large enough.

Interfacial pH-gradient induced micro-capillary filling with the aid of transverse electrodes arrays in presence of electrical double layer effects

In the present work, we outline the design and analysis of a micro-capillary filling mechanism through the aid of interfacial pH gradients (and hence interfacial tension gradients) generated by employing arrays of transverse electrodes inducing step changes in voltages, in a natural buffer system that requires low power and no synthetic ampholytes. The capillary transport is modulated by a dynamic and non-trivial coupling between the interfacial tension and viscous resistances, as a consequence of the underlying intermolecular interactions. The competing effects of the driving and the retarding forces effectively determine the displacement, velocity and acceleration characteristics of the capillary front, in a dynamically evolving manner. A comprehensive theoretical model of capillary dynamics is developed here to address these issues in details, thereby revealing the combined influence of the interfacial electrochemistry and the applied transverse voltages, as guided by the pertinent fundamental thermodynamic principles governed by free energy considerations and the physico-chemical phenomena over interfacial scales. Non-trivial implications of the pH-gradient driven micro-capillary transport are aptly emphasized, so as to offer significant physical insights on the adopted strategy as a guiding principle for facilitating capillary filling processes by inducing a modulation in the effective interfacial energy. Particular implications on the capillary filling time are also pinpointed, revealing the effectiveness of the adopted design strategy. Finally, a universal scaling relationship of the capillary filling time as a function of the pertinent operating parameters is derived, so as to provide a generalized guideline for implementing the design scheme. A non-dimensional parameter, depending simultaneously on the inter-electrode pitch and the transverse voltage, is identified, which may be kept to a minimal limit within the other operating constraints of the chosen system, towards minimizing the capillary filling time.

Bonding of a complicated polymer microchannel system for study of pressurized liquid flow characteristics with the electric double effect

Liquid flow characteristics with the electric double-layer (EDL) effect are studied in a microchannel made by polymer materials. The microchannel is integrated with arrays of a polymer pressure sensor for measurements of the local pressure distribution inside the channel. The bonding of this complicated microchannel requires special caution not only to ensure good contact between the channel walls and the cover plate leading to correct height of a channel but also to avoid liquid leakage at higher pressure. A bonding technique is developed in this work, which can be readily applied in a complicated lab-on-chip system for the study of micro-fluid behavior. The completed channel system with arrays of micro pressure sensors is used to study both the entry and fully developed flow of pure water. The accurate measurements of the pressure distribution along the channel clarify many of the conflicting issues reported in the microchannel flow system. It is found that the friction factor of liquid flow in the microchannel agrees well with the prediction accounting for the aspect ratio of the channel. The EDL effect on the local pressure drop, which may frequently occur in the liquid flow system, is also discussed and presented.

Low-voltage ZnO thin-film transistors operating at 2.0V gated with mesoporous SiO2 dielectric processed at room-temperature

Abstract Low-voltage thin-film transistors (TFTs) with ZnO nanocrystal channel layers and mesoporous SiO 2 gate dielectric are fabricated on glass substrates at room-temperature. The resulting n-type TFTs operate at a low voltage of 2.0 V. The equivalent field-effect electron mobility, current on/off ratio and subthreshold voltage swing is estimated to be 28.8 cm 2 V −1 s −1 , 3×10 6 and 84 mV/decade, respectively. The possible mechanism for low-voltage operation is discussed based on the electric double layer effect. Such room-temperature-processed low-voltage TFTs are very promising for low-power macroelectronics on temperature-sensitive substrates.

Derivation of Modified Reynolds Equation: A Porous Media Model With Effects of Electrokinetics

A lubrication theory that includes the effects of electrokinetics and surface microstructure is developed. A porous layer attached to the impermeable substrate is used to model the microstructure on a bearing surface. The Brinkman-extended Darcy equations and Stokes equations are modified by considering the electrical body force and utilized to model the flow in porous media and fluid film, respectively. The stress jump boundary conditions on the porous media/fluid film interface and the effects of viscous shear and electric double layer (EDL) are also considered when deriving the modified Reynolds equation. Under the usual assumptions of lubrication and Debye–Hückel approximation for low surface potential, the velocity distributions, the apparent viscosity, and the modified Reynolds equation are then derived. The apparent viscosity is expressed explicitly as functions of the Debye length, the electroviscosity, the charge density, the stress jump parameter, and the porous parameters (permeability, porosity, and porous film thickness). The considerations of EDL near the interface and the charge density of the flow in the porous media increase the apparent viscosity. The existence of porous film also increases the apparent viscosity as well. Both effects are important for flow within microspacing and lubrication problems. The apparent viscosity and the performance of 1D slider bearings are analyzed and discussed. The results show that the apparent viscosity and the load capacity increase as the permeability decreases, the stress jump parameter decreases, the charge density increases, the inverse Debye length decreases, or the porosity decreases.

On the Propagation of Concentration Polarization from Microchannel−Nanochannel Interfaces Part I: Analytical Model and Characteristic Analysis

We develop two models to describe ion transport in variable-height micro- and nanochannels. For the first model, we obtain a one-dimensional (unsteady) partial differential equation governing flow and charge transport through a shallow and wide electrokinetic channel. In this model, the effects of electric double layer (EDL) on axial transport are taken into account using exact solutions of the Poisson-Boltzmann equation. The second simpler model, which is approachable analytically, assumes that the EDLs are confined to near-wall regions. Using a characteristics analysis, we show that the latter model captures concentration polarization (CP) effects and provides useful insight into its dynamics. Two distinct CP regimes are identified: CP with propagation in which enrichment and depletion shocks propagate outward, and CP without propagation where polarization effects stay local to micro- nanochannel interfaces. The existence of each regime is found to depend on a nanochannel Dukhin number and mobility of the co-ion nondimensionalized by electroosmotic mobility. Interestingly, microchannel dimensions and axial diffusion are found to play an insignificant role in determining whether CP propagates. The steady state condition of propagating CP is shown to be controlled by channel heights, surface chemistry, and co-ion mobility instead of the reservoir condition. Both models are validated against experimental results in Part II of this two-paper series.

A study on the slip velocity on a pair of asymmetric electrodes for AC-electroosmosis in a microchannel

In numerical studies on microscale electroosmotic flows, the electric double layer (EDL) effect is usually predicted by solving the traditional Navier-Stokes equation subjected to the slip velocity induced by the electric-charged wall as a boundary condition. Recently, Suh and Kang (Physical Review E 77, 2008) presented the asymptotic solutions of the ion transport equations near a polarized electrode under the action of an AC field, and then proposed a new theoretical model of the slip velocity on the electrode considering the ion adsorption effect. In the present paper, we apply the model to a two-dimensional AC-electroosmotic flow in a microchannel to calculate the slip velocity on a pair of coplanar asymmetric electrodes embedded on the bottom wall, and then experimentally measure the slip velocity using the micro-PIV technique to validate the theoretical model. Comparison shows an excellent overall match between the theoretical and experimental results, except for on the narrow electrode at low frequencies. Next, we numerically perform parametric studies regarding the AC frequency, effective Stern-layer thickness and ion adsorption effect to further understand the characteristics of the AC electroosmotic flow. Results show that, as the frequency increases, the slip velocity also increases. In addition, the velocity decreases with increasing either Stern-layer thickness or ion adsorption effect.

Electroosmotic flow of non-Newtonian fluid in microchannels

Understanding electroosmotic flow of non-Newtonian fluid in microchannels is of both fundamental and practical significance for optimal design and operation of various microfluidic devices. A numerical study of electroosmotic flow in microchannels considering the non-Newtonian behavior has been carried out for the first time. One lattice Boltzmann equation is solved to obtain the electric potential distribution in the electrolyte, and another lattice Boltzmann equation which avoids the derivations of the velocity data to calculate the shear is applied to obtain the flow field for commonly used power-law non-Newtonian model. The simulation results show that the fluid rheological behavior is capable of changing the electroosmotic flow pattern significantly and the power-law exponent n plays an important role. For the shear thinning fluid of n < 1, the electrical double layer effect is confined to a smaller zone close to the wall surface and it is more inclined to develop into a plug-like flow whilst the shear thickening fluid of n > 1 is more difficult to grow into the plug-like flow compared to Newtonian fluid.

Particle AC electrokinetics in planar interdigitated microelectrode geometry

Electrokinetic movement of dielectric colloidal particles in an electric field has been previously studied by individually examining the effect of electro- and dielectrophoresis, and electroosmotic flow. In this paper, we use computer simulations and experiments to investigate the combined and frequency-dependent effect of dielectrophoretic and electroosmotic forces on the particle motion in nonuniform electric field inside a suspending, low-conductivity medium. The developed computational electrohydrodynamic model, which accounts for electric double-layer effects, is predictive. Specifically, for the case of a fringing field created by a potential difference applied to interfacial microelectrodes having a periodic, interdigitated configuration, it predicts the formation of two quasi-equilibrium zones where all forces are exactly or nearly balanced, resulting in a slow motion or trapping of particles. Existence of such zones depends on the frequency of the applied electrical excitation, particle size, and the dielectric permittivities of particles and the medium. The experimental visualization of the trajectories of fluorescently labeled latex particles qualitatively agrees with model predictions. The experiments also confirm the existence of one of the predicted quasi-equilibrium zones. Confirmation of the second zone requires different experimental conditions.

Physical modeling of the electrical double layer effects on multispecies ions transport in cement-based materials

The transport of ions through cement-based materials is described at a microscopic scale with a pore modeled by two infinitely large flat plates. The theory of the electrical double layer (EDL) shows that (i) the overlapping between the diffuse layers occurring in the pore is more important as the pore diameter will be small (low than the Debye length) and the pore walls will be strongly charged, (ii) the fluxes of coions and counterions will be respectively increased and attenuated in such pores. The gel pores of cement based materials have similar characteristics. As the capillary pores of the cement based materials with low porosity are connected between them by the pores gel, the transport of ions at a macroscopic scale could be greatly influenced by the overlapping effect of the diffuse layers.

Modeling of electrochemical micromachining: comparison to experiments

A detailed theoretical modeling of the electrochemical micromachining (EMM) process has been conducted here. The model takes into account a very small interelectrode gap thickness (order of a few micrometers) under which the electric field between the electrodes becomes a predominant factor in governing the machining process. The effect of electric double layer (EDL) also becomes significant under such conditions. The governing equations of the material removal rate are derived by considering a one-dimensional electric field, which is perpendicular to the cross section of the electrode. The model also incorporates the applied pulsed voltage used in EMM. A comparison between the theoretical and the experimental results indicate that, under high voltage and small interelectrode gap, the electric field and the formation of EDL have a significant effect on the material removal rate.

One-dimensional axial simulation of electric double layer overlap effects in devices combining micro- and nanochannels

This paper presents a numerical steady-state model of ion transport in micro- and nanofluidic devices with widely varying geometric scale, such as transitions between micro- and nanochannels. Finite element or finite volume simulation of such problems is challenging, due to the number of elements needed to produce a satisfactory mesh. Here, only the lengthwise channel dimension is meshed; standard analytical approximations are used to incorporate cross-channel properties. Singularly perturbed cases are built up by continuation. The method is shown to reproduce our previously reported measurements of electric double-layer effects on conductivity, ion concentration, and ion enhancement and depletion. Comparison with 2-D simulations reported in the literature shows that effects on accuracy due to the 1-D approximation are small. The model incorporates analytical models of surface charge density taken from the literature. This enables predictive simulation with reasonable accuracy using published parameter values, or these values may be tuned based on experiment to give improved results. Use of the model for iterative design and parameter estimation is demonstrated.

Shortcomings of geometrical approach in multi-species modelling of chloride migration in cement-based materials

Chloride migration tests and chronoamperometries were carried out on three cement mortars with different water/cement (w/c) ratios. The electrical current of the chloride migration is simulated with a multi-species model, which takes into account the geometrical properties of the pore structure throughout the diffusibility ratio. The comparison between the measured and the simulated currents shows a significant difference: systematically, simulated currents evolve faster than those measured. From these results, it appears that the purely geometrical approach alone is insufficient correctly to describe the transport of ionic species in cement-based materials. Finally, it is shown that the introduction of a ‘viscoelectric effect’, which conveys the electrical double-layer effect (EDL), improves the results of the model. As a first approach the EDL is taken into account empirically by considering a reducing factor 1/rD applied to the effective diffusion coefficients of the cations. This factor is deduced from Truc's work.

Effects of electrokinetic slip flow on lubrication theory

A lubrication theory that includes the coupled effects of electric double layer (EDL) and boundary slip is developed. The consideration of EDL (electro-viscous effect) and boundary slip near the solid surface increases and decreases apparent viscosity, respectively. Both effects are important for flow within microscales and lubrication problems. Under the usual assumptions of lubrication and Debye—Hückel approximation for low surface potential, the Navier—Stokes equation with body force due to the electrical potential, as well as the Navier slip boundary conditions, is utilized to derive the velocity distributions, apparent viscosity, and modified Reynolds equation. The apparent viscosity is expressed explicitly as functions of the Debye length, the electro-viscosity, and the slip length. The coupled effects of EDL and boundary slip on the apparent viscosity and one-dimensional slider-bearing performance are analysed and discussed. Results show that the load capacity increases as a decrease in inverse Debye length ( K), a decrease in slip lengths ( B), or an increase in electro-viscosity.

Modeling Ion Diffusion Current in Nanochannel Using Infinitesimal Distribution Resistor–Capacitor Circuits

A theoretical model that considers both the diffusion and electrical osmosis fluxes to estimate the diffusion current through a nanochannel is proposed. The equivalent impedance of a nanochannel is modeled as a series connection of finite resistor–capacitor (RC) circuits. The impedances of the electrical double layer and buffer solution in each infinitesimal RC circuit are represented using a capacitor and a resistor, respectively. The diffusion-induced current in the proposed model is position dependent due to the electrical double layer effect. Computer simulations and experiments using a nanoporous anodic aluminum oxide (AAO) thin film as the filter to separate electrolysis with an ion concentration gradient were conducted. Electrochemical impedance spectroscopy (EIS) was performed to measure the impedance of the porous AAO thin film in the frequency domain such that the capacitor and resistor can be estimated accordingly. A high degree of coincidence between the theoretical and experimental data is observed.

Electric double layer effect on observable characteristics of the tunnel current through a bridged electrochemical contact

Scanning tunneling microscopy and electrical conductivity of redox molecules in conducting media (aqueous or other media) acquire increasing importance both as novel single-molecule science and with a view on molecular scale functional elements. Such configurations require full and independent electrochemical potential control of both electrodes involved. We provide here a general formalism for the electric current through a redox group in an electrochemical tunnel contact. The formalism applies broadly in the limits of both weak and strong coupling of the redox group with the enclosing metal electrodes. Simple approximate expressions better suited for experimental data analysis are also derived. Particular attention is given to the effects of the Debye screening of the electric potential in the narrow tunneling gap based on the limit of the linearized Poisson-Boltzmann equation. The current/overpotential relation shows a maximum at a position which depends on the ionic strength. It is shown, in particular, that the dependence of the maximum position on the bias voltage may be nonmonotonous. Approximate expressions for the limiting value of the slope of the current/overpotential dependence and the width of the maximum on the bias voltage are also given and found to depend strongly on both the Debye screening and the position of the redox group in the tunnel gap, with diagnostic value in experimental data analysis.

Investigation of variable-property microchannel flows with electro-thermo-hydrodynamic interactions at constant pressure gradient or constant flow rate

In the present paper, a systematic investigation on numerical modeling of variable-property microchannel flows with electro-thermo-hydrodynamic interactions is presented. General variable-property electro-thermal flow model consisting of mass, momentum, energy, electric potential and ions transport equations in dimensionless form is formulated and employed to study transport phenomena in microchannel flows. The major concerns are: (1) evaluation of the model approximations such as equilibrium ion distribution, constant-property and/or small zeta potential for non-overlap electric double layer (EDL); (2) the electro-thermo-hydrodynamic interactions under conditions of strong EDL effects, and (3) differences in microchannel flow characteristics at driving conditions of constant pressure gradient (CPG) and constant flow rate (CFR). Variations of fluid, transport and electric properties with local temperature and/or ion concentration are considered. The present results demonstrate that the variable-property model with Poisson–Boltzmann potential is an appropriate approximation for transport phenomena in microchannel flows. The results also disclose that, compared to those in CPG flows based on the same Reynolds number, effects of temperature non-uniformity and variable-property are relatively more pronounced in the CFR flows. At either CPG or CFR condition, heat transfer performance changes little with the variations of electric and flow parameters in this low Reynolds number ( Re = 0.1) flow; while electric and flow fields can be significantly affected by variation of the governing parameters.

An effect of electrical double layer on MEG phantom with saline water

We examined the non-linear impedance of a saline water phantom using a prototype with 150-mm diameter sphere and with 5-mm dipole electrodes located at 20 mm from the center of the sphere. We found that the electrical impedance is a series of 0.84-μF capacitor and 250-Ω resistor, which requires unpractical source impedance and voltage, 1.9 MΩ and 760 V respectively, to drive the phantom without distortion. Also observed was a non-linearity due to the electrolysis, which can be avoided by constraining the frequency to be more than 1 kHz, in which frequency human brain signal is not properly emulated. Due to these difficulties, the phantom with saline water is not a good candidate for “the evaluation standard”.

Heat transfer in porous media

In this work, the effects of electrical double layer (EDL) near the solid/ liquid interface, on three dimensional heat transfer characteristic and pressure drop of water flow through a rectangular microchannel numerically are investigated. An additional body force originating from the existence of EDL is considered to modify the conventional Navier-stokes and energy equations. These modified equations are solved numerically for steady laminar flow on the basis of control volume approaches. Fluid velocity distribution and temperature with presence and absence of EDL effects are presented for various geometric cases and different boundary conditions. The results illustrate that, the liquid flow in rectangular microchannels is influenced significantly by the EDL, particularly in the high electric potentials, and hence deviates from flow characteristics described by classical fluid mechanics.