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Considering the influence of the streaming potential and electroviscous effects, the analytical solutions for electromagnetohydrodynamic (EMHD) flows in parallel plate microchannels are obtained. The electrolyte solutions in the microchannels are taken as generalized Maxwell fluids, and slip boundary conditions are adopted. To accurately analyze the EMHD flow characteristics, the variation trends of the electroviscous effects with the corresponding parameters must be understood. The results show that the electroviscous effects increase with the increase in the relaxation time De, the slip coefficient, and the wall zeta potential ψ̅0. However, the increase in the inverse of the electrical double-layer (EDL) thickness K, the electrical oscillating Reynolds number Re, and the ionic Péeclet number Pe can decrease the electroviscous effects. We also demonstrate that the electroviscous effect on the EMHD flows of generalized Maxwell fluids is larger than that of Newtonian fluids. This work will be useful in designing EMHD flows in parallel plate microchannels. [ABSTRACT FROM AUTHOR]

Highlights • Electroviscous effect on pressure driven flow and heat transfer is studied. • The surface chemical reaction in microchannels is considered. • The boundary slip is considered. • Three different electrical boundary conditions are compared. Abstract This study investigates the fluidic flow and related heat transfer in a microchannel formed by two parallel plates. Uniform heat flux is applied at the walls with considering the surface chemical reaction, which determines the surface charge on the wall of the microchannel. Our results show that the pH and ionic concentration apparently affect the surface chemical reaction and the subsequent fluidic behavior and heat transfer in the microchannel, especially with the boundary slip. The dimensionless velocity and temperature increase with the ionic concentration. The difference of the dimensionless flow rate and the Nusselt number is obvious under three different electrical boundary conditions: constant surface charge density, constant zeta potential and MI model. Under the boundary slip, the dimensionless flow rate and Nusselt number decrease by 50% and 12%, respectively, with increasing pH under the MI model, but keep unchanged under constant zeta potential or surface charge model. The dimensionless flow rate and Nusselt number also increase with the ionic concentration due to weakened electroviscous effect. Meanwhile, the slip increases the influence of ionic concentration on the dimensionless flow rate and Nusselt number. The results of this work may guide the design and optimization of microfluidic devices. [ABSTRACT FROM AUTHOR]

In this paper, we aim to account for the partitioning of finite sized ions and electric double layer (EDL) overlapping effects on the electrostatics and hydrodynamics of soft nanofluidics by stablishing a modified Poisson-Boltzmann (MPB) equation enjoying mean field approach. The application of the present MPB equation enables us to describe the interaction between the steric effect and electrostatic repulsion of EDL ions due to permittivity difference of polyelectrolyte layer (PEL) and electrolyte solution. Utilizing the Debye-Hückel approximation pertinent to low surface potentials, we analytically derive the solutions of electric potential and velocity profiles of mixed electroosmotic and pressure driven flows in polyelectrolyte grafted nanotubes. As well, we theoretically predict the electrophoretic mobility of soft colloidal particles immersed in aqueous solutions of NaCl, KCl, NaMnO 4 and LiIO 3 in the presence of ion partitioning effect (IPE). For the case of larger surface potentials, the nonlinear governing equations of the problem are analyzed by finite difference method, and the accuracy of the numerical solutions are examined by verifying the results with existing experimental data. Moreover, we calculate three electrokinetically associated phenomena namely streaming potential, electroviscous effect and electro-chemo-mechanical energy conversion (ECMEC) efficiency through soft nanotube having constant surface potential/charge density. Then, we compare our results under the purview of the present MPB equation with those predicted by point-like charge modeling of ions. The essential findings of our investigation highlight the importance of accounting for IPE arising from the permittivity difference of PEL and bulk salt solution when high degrees of softness, charge density and thickness of the PEL are encountered in transport mechanism in soft nanofluidics. We also show that one of the most important consequences of IPE is several fold increase of electrophoretic mobility of soft particle and ECMEC efficiency of soft nanotube due to increasing the PEL-electrolyte permittivity difference. [ABSTRACT FROM AUTHOR]

As a significant principle to guide the optimization design of fractal tree-like network, Murray’s law is obtained under the assumption of neglecting surface charge at the solid-liquid interface. However, surface charge should be considered due to the surface charge-induced electroviscous effect on the fluid flow over the micro/nanoscale. The present work carries out the optimization analysis of fractal tree-like microchannel network for electroviscous flow to realize minimum hydraulic resistance under the constraint of constant channel volume by considering the surface charge. It is found that the surface charge significantly modifies the conventional Murray’s law. Both zeta potential to manifest surface charge and microchannel radius make non-monotonic effects on the optimal radius ratio between the daughter channel and the parent channel for the electroviscous flow in the fractal tree-like microchannel network to achieve a minimum hydraulic resistance. These non–monotonic effects are related to the apparent electro-viscosity in the microchannel with different channel radius and different zeta potential. Additionally, the optimal radius ratio is found to be linearly dependent on both the ratio of apparent electro-viscosities of electroviscous flow in the microchannels at the two successive branching levels and the length ratio of the microchannels at two successive branching levels. [ABSTRACT FROM AUTHOR]

On the basis of the analytical solutions of electrical potential and ion distributions within the surface charge-induced overlapping electric double layer (EDL), this present work studies theoretically the overlapping EDL induced-electroviscous effect (EVE) on the pressure-driven flow in a parallel-plate microchannel with both the no-slip condition and the surface charge-dependent slip condition. The results show that the overlapping EDL induced-EVE shows a non-monotonic trend from enhancement to reduction with the gradually increasing absolute value of zeta potential. When the absolute value of zeta potential is large enough, the EVE and the relevant fluid drag will finally show a decreasing trend with the increasing absolute value of zeta potential. In addition and more interesting, the non-monotonicity of overlapping EDL induced-EVE shows a characteristic of size dependence. The critical zeta potential beginning to exhibit the decreasing trend of EVE is related to the size of the microchannel. The overlapping EDL-induced EVE in a microchannel with a smaller height needs a larger critical zeta potential to exhibit the decreasing trend. The mechanisms of these phenomena are analyzed. The subject of the paper is the theoretical results and the corresponding analysis. [ABSTRACT FROM AUTHOR]

Purpose: To determine the intrinsic viscosity of several monoclonal antibodies (mAbs) under varying pH and ionic strength solution conditions. Methods: An online viscosity detector attached to HPLC (Viscotek®) was used to determine the intrinsic viscosity of mAbs. The Ross and Minton equation was used for viscosity prediction at high protein concentrations. Bulk viscosity was determined by a Cambridge viscometer. Results: At 15 mM ionic strength, intrinsic viscosity of the mAbs determined by the single-point approach varied from 5.6 to 6.4 mL/g with changes in pH. High ionic strength did not significantly alter intrinsic viscosity, while a significant increase (up to 24.0 mL/g) was observed near zero mM. No difference in bulk viscosity of mAb3 was observed around pH 6 as a function of ionic strength. Data analysis revealed that near zero mM ionic strength limitations of the single-point technique result in erroneously high intrinsic viscosity. Conclusions: Intrinsic viscosity is a valuable tool that can be used to model baseline viscosity at higher protein concentrations. However, it is not predictive of solution non-ideality at higher protein concentrations. Furthermore, breakdown of numerous assumptions limits the applicability of experimental techniques near zero mM ionic strength conditions. For molecules and conditions studied, the single-point approach produced reliable intrinsic viscosity results at 15 mM. However, this approach must be used with caution near zero mM ionic strength. Data analysis can be used to reveal whether determined intrinsic viscosity is reliable or erroneously high. [ABSTRACT FROM AUTHOR]

This study shows how the membrane morphology can be optimized to improve the trade-off between membrane permselectivity and ion conductivity which are the primary parameters determining the goodness of a membrane for electrochemical applications. In particular the attention is here focused on the electrochemical transport properties at highly concentrated solutions (up to 8 M LiCl) which are particularly challenging for the membranes' performances. To this end sulfonated poly(arylene thioether sulfone) (SPTES) membranes with different characteristics of the nanoscopic pore network have been synthetized by varying the copolymer composition. It is shown how the relatively high-charged SPTES membranes with low swelling degree have overlapping electrical double layers even at electrolyte concentrations above 2 M. This ultimately results in permselectivities close to the maximum theoretical ones predicted by the modelling of highly charged nanopores. Yet these membranes retain high ion conductivity similar to the state-of-art cation exchange membranes with considerably lower permselectivity. Chemical stability in acidic (4 M) and vanadium oxidative environments show no degradation over time and together these results show that SPTES membranes with optimized morphology are promising for several electrochemical applications such as flow batteries. [ABSTRACT FROM AUTHOR]

Although electroviscous effect and convective heat transfer of pressure-driven flow in a microtube have been widely studied, the effect of surface charge on the boundary slip is less considered in previous studies in these fields. This present work develops closed form expressions of velocity distribution, temperature distribution and Nusselt number for hydrodynamically and thermally fully developed pressure-driven flow in a microtube considering the dependence of slip on the surface charge. On basis of these, the combined effect of surface charge and surface charge-dependent slip on the electroviscous effect and convective heat transfer of the pressure-driven flow are studied. The results show that slip length decreases with increasing magnitude of zeta potential of the solid–liquid interface, and this dependence of slip on surface charge inevitably affect the fluidic behavior and convective heat transfer. The slip can increase the Nusselt number by increasing the velocity of the pressure-driven flow, however, the zeta potential leads to a decrease in the Nusselt number by decreasing the velocity. When considering the dependence of slip on surface charge, there is a further reduction on the Nusselt number due to a further decreasing velocity induced by the decreasing slip length. Both a larger magnitude of zeta potential and a larger slip length lead to a larger misestimate of the flow rate and Nusselt number without considering the surface charge-dependent slip. The underlying mechanisms of these phenomena are analyzed. [ABSTRACT FROM AUTHOR]

Hydraulic characteristics of non-Newtonian fluid flow in smooth fused silica microtubes with diameter ranging from 75 to 320 μm are experimentally investigated. Hydraulic characteristics of deionized water (DI water), anion polyacrylamide (APAM) and nonionic polyacrylamide (NPAM) solution with sodium chloride (NaCl), calcium chloride (CaCl 2 ) and magnesium chloride (MgCl 2 ) over a wide range of flow rates are presented. The results show that the three types of electrolytes have important effect on the hydraulic characteristics of APAM solution flow in microchannels. However, the NPAM solution is hardly affected by the present electrolytes with different concentrations. Similar to the NPAM solution, no obvious electrolyte-effect is found for the DI water flow in smooth fused silica microchannels, and the experimentally obtained flow rate and friction factor agree well with conventional theory predictions. [ABSTRACT FROM AUTHOR]

In this paper we model the streaming potential (SP) and the electroviscous effects (EVE) in a soft nanochannel for system parameters that forbid the use of linearized Debye–Hückel (DH) treatment. This linear treatment, which is an essential ingredient in electrokinetics modeling, necessitates that the system parameters must be such that the electrostatic potential in the entire system must be much smaller than k B T / ez (where k B T is the thermal energy, z is the ion valence and e is the electronic charge). In our previous paper we provided analytical results for the SP and the EVE in a soft nanochannel considering the DH treatment. Therefore, in this paper we extend this previous calculation; our numerical approach allows studying much wider ranges of parameters. This numerical treatment is based on solution of an integro-differential equation governing the flow velocity; to the best of our knowledge such an approach has never been previously used for SP modeling. In our previous study, we have witnessed an enhanced SP and an enhanced EVE for the soft nanochannel for the case of small electrostatic potential. Here our numerical results help us predict a breakdown of these trends for the case of larger potentials. These findings are important to probe the problem of SP and EVE in soft nanochannels across a sufficiently large range of electrostatic potentials, with implications in issues such as electrochemomechanical energy conversion. [ABSTRACT FROM AUTHOR]

The water and ion transport through a mesoporous γ-alumina membrane and a microporous organosilica membrane was simulated using the extended Nernst–Planck equation combined with models for Donnan, steric and dielectric interfacial exclusion mechanisms. Due to the surface charge within the pore, the electroviscous effect was introduced in the model. The modified model fits well the rejection and permeability data for both membranes. The organosilica membrane shows a higher selectivity compared to the γ-alumina membrane, but the permeate flux is lower. At low ionic strength the electroviscous effect lowers the water flux through the γ-alumina membrane. The electroviscous effect is negligible for the organosilica membrane because its absolute surface potential (~20 mV) is low compared to the γ-alumina membrane (~60 mV). The simulation shows that the electroviscous effect should be included for the membranes with high surface potential (>20 mV) and a pore size below 2–5 times of the electroviscous double layer thickness. [ABSTRACT FROM AUTHOR]

The steady-state electroviscous (pressure-driven) flow of an aqueous solution of potassium chloride through a nanofluidic borosilicate glass junction is modelled using a finite volume approach. Electric-field propagation through the glass section of the junction is included in the analysis. Channel half-widths of 20-70nm and salt concentrations of 10 - 6 - 10 1 M are considered. Two types of common nano/microfluidic flow through the junction are analysed: a separating flow and a mixing flow. The hydrodynamic (pressure) and electrokinetic (electric potential) resistances of the junction are quantified in terms of equivalent lengths. For both flows, the hydrodynamic equivalent length of a 90 ° junction is found to be between ~0.85 and ~1.05 channel half-widths, while the electrokinetic equivalent length varies from ~0.3 to ~0.6 channel half-widths. Decreasing the junction angle decreases the equivalent lengths of the junction. [ABSTRACT FROM AUTHOR]

Many industrial applications of oil-in-water emulsions involve salts containing ions of different valence. The properties of the oil-water interface (e.g., interfacial tension, zeta potential and interfacial shear viscosity) are strongly influenced by the presence of these salts. This work investigates the role of NaCl, CaCl2and AlCl3on these properties of the hexane-water interface in presence of a cationic surfactant, viz., hexadecyltrimethylammonium bromide. Addition of salt enhanced the adsorption of surfactant molecules at the hexane-water interface, which increased the interfacial charge density, and consequently, the zeta potential. Interfacial shear viscosity significantly decreased in the presence of salt. The effectiveness of salt at a given concentration was in the sequence: AlCl3 > CaCl2 > NaCl. The hexane-in-water emulsions coarsened with time due to the coalescence of hexane droplets. The increase in droplet size with time was analyzed by a model based on the frequency of rupture of the thin aqueous film. The rate constants for coalescence were determined. The rate of coalescence increased in presence of salt. [ABSTRACT FROM PUBLISHER]

Highlights: [•] Theoretical modelling of concentrated “realistic salt-free suspensions”. [•] Influence of CO2 and H2O derived ionic species. [•] Added non-equilibrium dissociation–association kinetic equations. [•] Augmented electroviscous coefficient and DC electrophoretic mobility. [ABSTRACT FROM AUTHOR]

• The ion transport, fluidic flow and heat transfer in nanochannels were investigated in depth. • Strong height-dependent surface charge was found in overlapping EDL nanochannels. • Larger deviation on ions transport and fluidic flow were found when considering the height effect or not. • The results provide meaning information for the design and performance optimization of nanofluidic devices. The electroviscous effect has been extensively investigated in micro/nano scale ions transport, fluidic flow and heat transfer, etc. Previous researches often used constant surface charge in theoretic calculation and numerical simulations. However, for thick overlapping electric double layer (EDL) nanochannels, the channel height size effect on surface charge cannot be neglected. In this work, we consider the channel height size effect on surface charge to investigate the related ions transport, fluidic flow and heat transfer in nanochannels. The corresponding results show that the deviation considering the height effect or not is significant, especially in the situation of boundary slip. The relative errors of local ionic concentration and potential can reach 50% and 8%, respectively. The maximum relative errors of induced electrical field strength and the ratio of apparent viscosity to fluidic viscosity reach 10% and 9%, respectively, indicating strong height size effect on the fluidic flow. The deviation of Nu is relatively small and almost not affected by the surface charge. These deviations will decrease to zero when the channel height is large enough or the solution concentration is high enough to reach a non-overlapping EDL situation. The results of this work are meaningful to further understand the electroviscous effect in nanochannels, which may provide useful guidelines for the design and performance optimization of nanofluidic devices. [Display omitted] [ABSTRACT FROM AUTHOR]

We have studied the rheological behavior of dilute suspensions of electrostatically dispersed Na-montmorillonite to elucidate the importance of the electroviscous effect under the condition of zero stress limits. The viscosities of the suspensions were measured using an original spiral-type viscometer that consisted of two measuring cylinders connected with a one-meter-long capillary tube. Extremely dilute suspensions, whose volume fractions ranged from 2.0×10−4 to 2.0×10−3, were used for the experiments. The ionic strength of the suspensions was controlled to be less than 0.001M. Thus, a fully developed electrical double layer was formed. It was confirmed that the measured viscosities of the suspensions increased with a decrease in their ionic strength, in accordance with a manifestation of the electroviscous effect. The viscosity increased markedly owing to the secondary electroviscous effect under the limit of the salt-free condition. To analyze this effect, we focused on the effective radii of the suspended montmorillonite particles. We estimated their effective radii by fitting the Dougherty–Krieger equation to the viscosity data. We found that the obtained effective radii depended significantly on the ionic strength in the low Peclet number regime, which was similar to the theoretical dependence of the effective radii determined from the interaction balance between the particles governed by hydrodynamic, electrostatic, and diffusive energies. Thus, the increase in the effective radius with a decrease in the ionic strength gives rise to the expansion of the electric repulsive force, resulting in an increase of the viscosity of this clay suspension. In addition, the magnitudes of the effective radii were greater than the Debye length. This indicates that even when the distance between the montmorillonite particles is greater than the Debye length, the electrical repulsive force affects the interaction between the particles. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a controllable sliding bearing system, in which a liquid crystal was applied as a lubricant, was constructed, and experiments were conducted to study its controllability and dynamic properties. In the present system, a sinusoidal load was applied to the bearing pad by a magnetic exciter and the equivalent spring and damping constants were identified under various electric fields, sliding velocities, and film thicknesses. Results show that the applied electric field strength had a great effect on the damping properties of the liquid crystal film, whereas it had less effect on the equivalent spring constant. [Copyright &y& Elsevier]

Abstract: Several authors observed in the past a larger than twofold increase in viscosity of organic liquids under the influence of an electric field of the order of 106 V/m. This was called electro viscous effect (EVE). Significantly higher electric fields, of up to 108–109 V/m, arise in the electric double layer in solutions close to an electrode. Therefore, the viscosity can be expected to increase at strongly charged liquid–solid interfaces. In more recent years, it was also observed that even in the absence of an externally controlled electric field the viscosity of water can be up to 107 times higher close to a hydrophilic surface than in the bulk (“hydrophilic forces”). Here, we present electrochemical atomic force microscopy (EC-AFM) measurements by which we can overcome the critical threshold of the electric field H =106 V/m by the control of the potentials applied to both a conducting sample and a conducting tip immersed in solution. Using the EC-AFM, we have investigated for the first time the EVE in an aqueous electrolyte. We can show that by controlling the applied potential, we can control the viscosity and the thickness of the super viscous liquid layer close to the solid interface. Using this technique, we are further able to separate effects on viscosity induced by the hydrophilicity of the surfaces, by the strong nanoconfinement of the liquid between tip and surface, and by the applied electric field. [Copyright &y& Elsevier]

Abstract: This work presents a detailed study on the pH sensitivity of polyethersulfone (PES) hollow fiber membrane (HFM) modified by blending an amphiphilic terpolymer poly(styrene–acrylic acid–N-vinylpyrrolidone) (P(St–AA–NVP)), the HFM exhibits a pH-responsive behavior with very large water flux change as the pH value change, from nearly 38.64 to 0.64ml/(m2 hmmHg). Meanwhile, the influence of the occurred macro- and micro-phase separation on water contact angle, pH sensitivity, and anti-fouling property was investigated. X-ray photoelectron spectroscopy analysis revealed the enrichment of the hydrophilic segments of the terpolymer at the membrane surface. A rough surface resulted from the poor miscibility of the blend was observed, and it was proved that the phase separation would increase the pH sensitivity. Furthermore, the ultrafiltration experiments indicated that the phase separation had great effect on the pure water flux but little influence on the protein anti-fouling property after the blending of the terpolymer; and the modified membrane showed good protein anti-fouling property. [Copyright &y& Elsevier]

Abstract: The competition between streaming potential effect and hydrodynamic slip effect in pressure-driven microchannel flows is elucidated. The treatment is performed analytically in the framework of the Debye–Hückel linear approximation and the Navier slip boundary condition. As compared with pressure-driven flows in nonslip microchannels, pressure-driven flows in microchannels with hydrodynamic slippage are found to experience severer flow reduction (streaming potential effect). An apparent viscosity ratio is also defined to identify the competition between streaming potential effect and hydrodynamic slip effect in pressure-driven microchannel flows. It is shown that the streaming potential effect predominates when this ratio is larger than unity and the slip effect predominates when this ratio is smaller than unity. More importantly, we derive a formula for the critical slip length which provides a guideline for designing hydrophobic surfaces over which the effect of hydrodynamic slip exactly counteracts that of streaming potential. [Copyright &y& Elsevier]

Abstract: Single phase non-Newtonian microporous flow combined with the electroviscous effect is investigated in the pore-scale under conditions of various rheological properties and electrokinetic parameters. The lattice Boltzmann method is employed to solve both the electric potential field and flow velocity field. The simulation of commonly used power-law non-Newtonian flow shows that the electroviscous effect on the flow depends on both the fluid rheological behavior and pore surface area ratio significantly. For the shear thinning fluid with power-law exponent n <1, the fluid viscosity near the wall is smaller and the electrovicous effect plays a more important role compared to the Newtonian fluid and shear thickening fluid. The high pore surface area ratio in the porous structure increases the electroviscous force and the induced flow resistance becomes important even to the flow of Newtonian and shear thickening fluids. [Copyright &y& Elsevier]

Abstract: An electrokinetic transport based approach for quantification of reversible flux decline due to the concentration polarization of an electrolyte solution in presence of charged colloids is presented. The model envisions the electrolyte transport across a charged cake or gel layer as transport of ions through charged cylindrical capillaries. This model is coupled with the standard theory of concentration polarization during cross flow membrane filtration. The analysis is carried out entirely in terms of generalized, non-dimensional variables. A dimensionless group termed as the scaled gel layer resistance evolves from the analysis, which accounts for the electrical properties of the charged nano-colloids and the electrolyte solution. A parametric study is performed to elucidate the coupled influence of mass transfer, membrane resistance, gel resistance, and electrical properties of the gel-electrolyte polarized layer. The effects of these parameters are examined on the filtration performance through the model equations. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the functional terpolymer of poly (methyl methacrylate–acrylic acid–vinyl pyrrolidone) was synthesized via free radical solution polymerization using dimethylacetamide (DMAC) as the solvent. The terpolymer can be directly blended with polyethersulfone (PES) using DMAC as the solvent to prepare PES hollow fiber membrane by using a dry-wet spinning technique based on a liquid–liquid phase separation technique. The blended PES hollow fiber membranes showed evident pH sensitivity; and the pH-valve effect was observed at the pH values between 7.0 and 10.0. The fluxes under acid conditions were over 10 times larger than those under basic conditions, and with the increase of the terpolymer blended in the membranes, the flux change increased. The fluxes also showed pH reversibility. Both the pore size change and the electroviscous effect had great effect on the pH sensitivity. Furthermore, the hydrophilicity of the blended membranes increased, and the membranes showed good protein antifouling property. [Copyright &y& Elsevier]

Abstract: The electroviscous effect of realistic salt-free colloidal suspensions is analyzed theoretically. We study the influence on the electroviscous coefficient of the surface charge density and the particle volume fraction. By realistic salt-free colloidal suspensions we mean aqueous suspensions which have been deionized as far as possible without any electrolyte added during the preparation, in which the only ions present can be (i) the so-called added counterions, coming from the ionization of surface groups and thus counterbalancing the surface charge, (ii) the H+ and OH− ions from water dissociation, and (iii) the ions produced by the atmospheric CO2 contamination. Our model is elaborated in the framework of a classical mean-field theory, using the spherical cell model approach and the appropriate local equilibrium reactions. It is valid for arbitrary surface charge density and particle concentrations. We have also made a new interpretation of the electroviscous coefficient: the electroviscous coefficient p of the suspension is the ratio between the electrohydrodynamic and the pure hydrodynamic contributions to the specific viscosity of the suspension. The numerical results show that it is necessary to consider the water dissociation influence for volume fractions lower than approximately 10−3, whereas the atmospheric contamination, if the suspensions are open to the atmosphere, is important in the region of volume fractions ϕ <0.03. [Copyright &y& Elsevier]

Abstract: Understanding non-Newtonian flow in microchannels is of both fundamental and practical significance for various microfluidic devices. A numerical study of non-Newtonian flow in microchannels combined with electroviscous effect has been conducted. The electric potential in the electroviscous force term is calculated by solving a lattice Boltzmann equation. And another lattice Boltzmann equation without derivations of the velocity when calculating the shear is employed to obtain flow field. The simulation of commonly used power-law non-Newtonian flow shows that the electroviscous effect on the flow depends significantly on the fluid rheological behavior. For the shear thinning fluid of the power-law exponent n <1, the fluid viscosity near the wall is smaller and the electroviscous effect plays a more important role. And its effect on the flow increases as the ratio of the Debye length to the channel height increases and the exponent n decreases. While the shear thickening fluid of n >1 is less affected by the electroviscous force, it can be neglected in practical applications. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: In this study, a simplified method to prepare pH-sensitive polyethersulfone (PES) hollow fiber membranes was provided by blending with a copolymer of acrylonitrile and acrylic acid (PANAA). The copolymer PANAA was synthesized by free radical solution polymerization using N-methyl pyrrolidone (NMP) as the solvent. The PANAA copolymer solution can be directly blended with PES solution (using NMP as the solvent) to prepare hollow fibers by using a dry–wet spinning technique based on the liquid–liquid phase separation technique. The blended PES hollow fiber membranes showed evident pH-sensitivity and pH reversibility; and the pH-valve effect was observed at the pH between 4.5 and 11.0. We also proved that the electroviscous effects played the most important role on the pH-sensitivity and flux control for the blended membranes. [Copyright &y& Elsevier]

Abstract: The influence of pH, KCl and temperature on the performance of an Alfa Laval NFT-50 nanofiltration membrane was evaluated by glucose and KCl retention measurements at constant flux. It was found that at constant temperature an increase in pH in the absence of KCl reduced the water permeability, and this was correlated to the electroviscous effect. The selectivity decreased with increasing KCl concentration and the decrease became more evident with increasing pH. The decrease in selectivity could be related to membrane swelling, and it appears that pH had no effect on swelling other than enhancing the effect of KCl. The influence of KCl and pH on membrane performance was evaluated as a function of temperature after reaching steady state at 50°C. The degree of swelling was higher at 50°C than at 20°C, due to an increase in polymer flexibility with increasing temperature. The decrease in selectivity with increasing temperature was less for KCl than for glucose, showing that the charge effect was influenced less by temperature than diffusion through the membrane. [Copyright &y& Elsevier]

Abstract: The aim of this investigation was to develop a basic understanding of the emulsification process by considering simple systems such as n-hexane, n-heptane, n-decane, and kerosene oil in water. The technique employed for the purpose was ultrasonification. The effect of ultrasonification time, chain length, viscosity, surface tension, oil content, and ionic strength of the media on the quality of emulsion has been studied. The emulsions were viewed through microscope to measure the number, size, and size distribution of droplets. Quantification of turbidity and viscosity was also used to characterize the emulsions. It has been found that the number and size of the droplets vary with the time of ultrasonification, contents of oils, molecular mass of the oils, and ionic strength of the media, and hence the quality of the emulsion is influenced by these parameters. The droplet size decreases, whereas the number of drops increases with the time of emulsification, approaching an optimum distribution at about 15 min of ultrasonification. Further, the increase in the molecular mass of the oil increases the size of the droplets and hence decreases the stability of the emulsion. The addition of electrolytes encourages coalescence and enhances the instability in the system. The results are in accord with the equations proposed by us. [Copyright &y& Elsevier]

We have investigated the hydrodynamic drag force between charged particles in electrolyte solutions, specifically the electroviscous force that arises from the distortion of the electrical double layers by the flow field. We report an improvement on the thin-double-layer theory (S.G. Bike, D.C. Prieve, J. Colloid Interface Sci. 136 (1990) 95–112), using a more accurate boundary condition for the radial charge current. The differences become important when the double layers start to overlap. We have found that nonlinear hydrodynamic effects are small, whereas nonlinear electric effects can be significant, in some instances leading to qualitatively different behavior. If the ion diffusivities are highly asymmetric, the electroviscous force can be reduced by an order of magnitude when there is an excess of the mobile ions in the double layer. The common supposition that there are substantial differences in the electroviscous force predicted by constant-charge and constant-potential boundary conditions is incorrect; our calculations show that it is an artifact introduced by the Debye–Hückel approximation. [Copyright &y& Elsevier]

A fundamental understanding of the flow characteristics of electrolyte solutions in microchannels is critical to the design and control of microfluidic devices. Experimental studies have shown that the electroviscous effect is appreciable for a dilute solution in a small microchannel. However, the experimentally observed electroviscous effects cannot be predicted by the traditional theoretical model, which involves the use of the Boltzmann distribution for the ionic concentration field. It has been found that the Boltzmann distribution is not applicable to systems with dilute electrolyte solutions in small microchannels because it violates the ion number conservation condition. A new theoretical model is developed in this paper using the Nernst equation and the ion number conservation, instead of the Boltzmann distribution, to obtain the ionic concentration field. The ionic concentration field, electrical potential field, and flow field in small microchannels are studied using the model developed here. In order to verify this model, the model-predicted dP/dx (applied pressure gradient) ∼Re (Reynolds number) relationship is compared with the experimentally determined dP/dx∼Re relationship. Strong agreement between the model predictions and the experimental results supports this model. [Copyright &y& Elsevier]

Streaming potential, electroviscous effect, pore conductivity and membrane potential were measured for a ceramic ultrafiltration membrane at various KCl concentrations. A space charge model was used to calculate the surface potentials from the experimental data. Surface potentials determined from the four experimental methods are in relatively good agreement although some discrepancies occur at low ionic concentrations. Pore conductivity and membrane potential methods lead to similar surface potentials on the whole range of concentrations studied but these latter are smaller than those obtained for both streaming potential and electroviscous effect measurements. [Copyright &y& Elsevier]

The electrokinetic flow of an electrolyte solution through an elliptical microchannel is studied theoretically. The system under consideration simulates the flow of a fluid, for example, in vein. We show that, for a constant cross-sectional area, both the electroosmotic volumetric flow rate and the streaming potential increase monotonically with an increase in the aspect ratio, and both the total electric current and the electroviscous effect may exhibit a local minimum as the aspect ratio varies. Also, for a constant average potential on the channel wall, the difference between the results based on three kinds of boundary conditions, which include constant surface charge, constant surface potential, and charge-regulated surface, is inappreciable if the hydraulic diameter is larger than 1 μm. [Copyright &y& Elsevier]

• The electroviscous effect in nanochannels with overlapping EDL was comprehensively investigated. • The solution concentration-dependent surface charge and boundary slip apparently affect the fluidic behavior and heat transfer in nanochannels. • The slip boundary significantly improves the dimensionless flow rate and Nu in small nanochannels with thick overlapping EDL. • The results provide useful guidelines for the design and performance optimization of micro/nano fluidic devices. The electroviscous effect and convective heat transfer of pressure-driven flow in micro/nanochannels have been widely studied in the past decade. However, previous studies often used constant physical parameters in calculations and overlooked the situation of probable overlapping electric double layers (EDL). In this work, we report the combined effect of concentration-dependent surface charge and surface charge-dependent boundary slip on the flow and related heat transfer in nanochannels with overlapping EDL. The results show that the solution concentration-dependent surface charge and boundary slip apparently affect the electric conductivity and velocity distribution in the nanochannel, subsequently affect the fluidic behavior and heat transfer. The dimensionless flow rate and the Nusselt number (Nu) decrease first and then increase with the increasing solution concentration. Besides, the slip significantly improves the dimensionless flow rate and Nu , especially in small nanochannels with thick overlapping EDL. These results provide useful guidelines for the design and performance optimization of micro/nano fluidic devices. [ABSTRACT FROM AUTHOR]

• Rheology modulated enhancement in electrochemomechanical energy conversion. • Electrostatics in Polyelectrolyte layer enhances the conversion efficiency. • Increased electroviscous effect due to the higher streaming potential results in the conversion efficiency. We study the influence of viscoelastic fluid on the enhancement of the electrochemomechanical energy conversion efficiency in the polyelectrolyte layer grafted narrow-fluidic channels. We obtain the velocity distribution using the simplified Phan-Thien-Tanner model and calculate the conversion efficiency in the purview of both with and without Debye-Hückel approximations. The analysis unveils that an increment in the fluid velocity obtained with an increment in the shear-thinning nature of the viscoelastic fluid increases the streaming of the counter-ions, which, in turn, increases the conversion efficiency. Also, it is shown that the increased electroviscous effect due to higher rate of streaming potential being developed results in a reduction in the predicted enhancement of the conversion efficiency. This phenomenon is observed for a window of parameters viz. higher polyelectrolyte layer thickness, higher viscoelastic parameter, and lower Debye-Hückel parameter of the electrolyte. We also report that the relatively higher electrostatic potential stemming from the overlapping electrical double layers and the higher wall potential (greater than 25 mV), lead to non-intuitive variation in the conversion efficiency following the pronounced influence of the electroviscous effect. While targeting an increment in the conversion efficiency using viscoelastic fluid and grafted polyelectrolyte layer, we also examine the influence of slip at the walls of the channel and the spatial variation of electrical conductance between the stern layer conductance and that of the walls of the channel. However, the rheology modulated enhancement in the efficiency is found to be effective in case of variation in the stern layer conductance and interfacial slip at the walls of the channel. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Investigation of electroviscous effects is of interest to technologies that exploit transport of ions through ion exchange membranes, charged capillaries, and porous media. When ions move through such media due to a hydrostatic pressure difference, they interact with the fixed charges, leading to an increased hydraulic resistance. Experimentally this is observed as an apparent increase in the viscosity of the solution. Electroviscous effects are present in all electrochemical membrane-based processes ranging from nanofiltration to fuel-cells and redox flow batteries. Direct measurements of electroviscous effects varying the applied ionic current through Nafion membranes have, to the best of the authors' knowledge, not yet been reported in literature. In the current study, electroviscous phenomena in different Nafion ion exchange membranes are measured directly with a method where the volume permeation is measured under constant trans-membrane pressure difference while varying the ion current density in the membrane. The direct measurement of the electroviscous effect is compared to the one calculated from the phenomenological transport equations and measured transport coefficients. Within the experimental uncertainty, there is a good agreement between the two values for all membranes tested. We report here an electroviscous effect for all Nafion membranes tested to be κ H κ H − 1 = 1.15 − 0.052 + 0.035 . [ABSTRACT FROM AUTHOR]

• Autonomous filling of viscoelastic fluid in a capillary. • sPTT model is used to describe the rheology of viscoelastic fluid. • Effect of streaming potential on the filling dynamics is investigated. • Oscillations are seen in the temporal evolution of the filling dynamics. • A unique scale (x ∼ t) at a later stage of filling in the capillary is established. We study the streaming potential modulated autonomous filling dynamics of a viscoelastic fluid in a microfluidic channel. To describe the rheology of viscoelastic fluid, we consider a simplified Phan-Thien-Tanner (sPTT) model. Considering the electroviscous effect, along with the effects of surface tension force and viscous resistance, we derive a reduced-order model for obtaining the variations in the filling dynamics for different physical parameters of the system. We observe that a complex competition between the viscoelasticity and the electroviscous effect leads to the non-trivial behaviour in the temporal evolution of the filling dynamics. Moreover, our analysis unveils that an enhancement in the electroviscous effect due to increasing viscoelasticity of the fluid leads to a non-linear reduction in the filling length of the fluid in the channel. Consequently, the notion of having a higher filling length for more elastic nature of the fluid is contradicted here due to the presence of the electroviscous effect. Finally, we establish a regime (x ∼ t) at the later stage of filling in the channel. We show that this regime is unique to the surface tension driven filling of viscoelastic fluids in a charged fluidic pathways under the influence of electroviscous effect. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Understanding the behavior of bentonite dispersions is of remarkable importance in many industrial and engineering applications. Dispersion behavior depends on interparticle interactions. These mechanisms are influenced by the volumetric content of solid particles, ionic concentration in suspending fluid, ion valence, and pH. In this research, natural sodium bentonite dispersions were analyzed. Tests were performed in dispersions prepared with different volumes of particles in solutions of sodium chloride, calcium chloride, and magnesium sulphate with different concentrations. The rheology of dispersions was analyzed by fitting three rheological models, namely the power law, Bingham plastic, and Herschel-Bulkley models. Changes in shear stress and model parameters with ionic strength were analyzed. The observed behavior was interpreted in terms of particle-particle association and was later confirmed by computing the Derjaguin, Landau, Verwey and Oberbeek (DLVO) interparticle interaction energy. Finally, electroviscous effects were analyzed by computing the importance of the electroviscous parameter considering Einstein's equation and the Dougherty-Krieger equation. The electroviscous parameter was found to decrease with the increase in ionic concentration. The influence of ion type had a negligible effect on the electroviscous parameter. Unlabelled Image • Rheology of sodium bentonite dispersion in NaCl, CaCl 2 and MgSO 4 solutions was analyzed. • Rheology model's were calibrated to the experimental results. • Influence of ionic strength on shear stress and model's parameters was quantified. • Rheology behavior was explained in terms of particle to particle interactions. • Electroviscous coefficients were determined from calibrated rheology models. [ABSTRACT FROM AUTHOR]