Electrokinetic flow velocity in charged slit-like microfluidic channels with linearized Poisson-Boltzmann field

In cases of the microfluidic channel, where the thickness of electric double layer is often comparable with the characteristic size of flow channels, the electrokinetic influence on the flow behavior can be found. The externally applied body force originating from the electrostatic interaction between the linearized Poisson-Boltzmann field and the flow-induced electrical field is applied in the equation of motion. An analytical solution to this Navier-Stokes equation of motion for well-defined geometry of slit-like microchannel is obtained by employing Green’s function. Also, an explicit analytical expression for the induced electrokinetic potential is successfully derived as functions of relevant physicochemical parameters. The effects of the ionic concentration of the fluid, the zeta potential of the solid surface, and the width of microchannels on the velocity profile as well as the streaming potential are examined. The electric double layer effect on the velocity profile becomes stronger as the channel width decreases, where the average fluid velocity is entirely reduced with the decrease in ionic concentration. The induced electrokinetic potential increases with an increase in pressure gradient, while it decreases as the ionic concentration increases.     

Electrochemical and electrokinetic characterization of cellulose acetate polymeric membranes

The electrochemical and electrokinetic aspects of cellulose acetate membranes of varying pore structure and desalting abilities have been investigated. The electrochemical studies included measurement of conductance and membrane potential for various membrane electrolyte systems. The electrokinetic characterization was made from streaming potential measurements. The data obtained are explained in terms of interfacial double layer phenomena prevalent in porous permselective barrier systems. The average pore diameter evaluated independently is also presented and an attempt has been made to understand the solute–water transport in terms of weak ionic character of membrane surface.     

Relation between salt rejection and electrokinetic properties on Shirasu porous glass (SPG) membranes with nano-order uniform pores

The salt rejection by Shirasu porous glass (SPG) membranes having nano-order uniform pores was investigated for understanding the electrokinetic mechanism resulting from the surface charge developed on the membrane when in contact with salt solutions. Due to the dissociation of the hydroxyl groups such as silanol groups on the membrane surface, the membrane was negatively charged over a pH range of 3–10 from electrophoretic measurements. Cross-flow filtration experiments showed that up to 63% of NaCl was rejected by an SPG membrane having a mean pore diameters of 33 nm in a 1 mol m⁻³ NaCl solution at pH 7 under a transmembrane pressure of 74 kPa, even though the pore diameter is much larger than the ion diameter. This is a consequence of the electrostatic repulsive interaction between the co-ions (Cl⁻ ions) and the membrane surface. At the same pH, the rejection factor of NaCl decreased with increasing salt concentration due to an increase in the ionic strength. More negative charge on the membrane surface at higher pH resulted in higher rejection factors of NaCl for a fixed salt concentration. Higher rejection factors of NaCl by SPG membranes with smaller pore sizes for a fixed concentration are due to the higher ratio of the thickness of the electric double layer (Debye length) to the pore radius. The SPG membrane showed a salt rejection sequence: Na₂SO₄, NaCl and CaCl₂ at the same pH. This is because divalent anions (SO₄²⁻) are more strongly repelled by the negatively charged membrane, while divalent cations (Ca²⁺) adsorb specifically onto the membrane surface than monovalent cations (Na⁺). The salt rejection factor increased with increasing permeate volume flux. Due to the stronger acidity of the membrane materials, SPG membranes had a higher rejection factor and a lower isoelectric point (IEP < 3) than ceramic membranes.     

Reverse osmosis performance of organosilica membranes and comparison with the pervaporation and gas permeation properties

Hybrid organosilica membranes were successfully prepared using bis(triethoxysilyl)ethane (BTESE) and applied to reverse osmosis (RO) desalination. The organosilica membrane calcined at 300^°C almost completely rejected salts and neutral solutes with low‐molecular‐weight. Increasing the operating pressure led to an increase in water flux and salt rejection, while the flux and rejection decreased as salt concentration increased. The water permeation mechanism differed from the viscous flow mechanism. Observed activation energies for permeation were larger for membranes with a smaller pore size, and were considerably larger than the activation energy for water viscosity. The organosilica membranes exhibited exceptional hydrothermal stability in temperature cycles up to 90^°C. The applicability of the generalized solution‐diffusion (SD) model to RO and pervaporation (PV) desalination processes were examined, and the quantitative differences in water permeance were accurately predicted by the application of generalized transport equations. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1298–1307, 2013     

Hyperfiltration in porous fixed charge membranes

The hyperfiltration properties of fixed charge membranes containing pores of various widths have been examined under conditions where flows are large and restricted to the pores. The profiles for the ionic concentrations, electrostatic potential and solution velocity across the pore widths have been determined. A generalised expression for the salt rejection coefficient has been derived.It is found that the relation between salt rejection and the salt distribution coefficient simplifies to be of the same form as that which applies for the fixed charge membrane described by Teorell, Meyer and Sievers. The absolute values of the rejection coefficient are smaller however, for porous membranes, since a diminished salt exclusion obtains.     

波度和锥度对液体润滑机械密封空化特性影响

为进一步探究机械密封液膜中空穴初生、演变等影响因素,基于质量守恒空化模型,建立考虑微观波度和锥度的液体润滑机械密封数学模型,采用有限控制体积法对控制方程进行离散,综合分析不同波幅、波数及锥度对空化特性的影响。结果表明:波度产生流体动压润滑效应,锥度产生流体静压润滑效应,两者共同影响空穴的初生和演变,而又影响空穴沿周向和径向的变化;空穴初生时,液膜破裂位置位于膜厚沿周向发散的低压区,且在空穴演变时其沿某一半径方向保持不变;量纲1波幅不超过0.5且波数不低于8或量纲1负锥度大于0.5的工况均可加快空化率,促进空化发生;而相同波幅且波数小于8或锥度趋向于正锥度的工况均可减缓空化率,有效抑制空化发生。     

考虑锥度及波度的螺旋槽液膜密封动态特性分析

密封端面因热力变形或机械加工产生的形貌变化显著影响密封性能。建立考虑径向锥度和周向波度的螺旋槽液膜密封数学模型,利用偏导数法求解动态雷诺方程,并采用有限元法计算液膜密封开启力、刚度、动态刚度及阻尼系数,进而分析了径向锥度及周向波度对液膜密封稳、动态特性的影响。结果表明：液膜密封开启力随着锥度的增加逐渐减小,随着波幅的增加逐渐变大,同一波幅下,波数增多,开启力减小;液膜刚度随着锥度的增加逐渐减小,且波数不同时随波幅的变化趋势不同;液膜动态刚度系数绝对值随锥度的增加逐渐减小,轴向刚度系数和角向刚度系数受波度的影响比较明显;液膜动态阻尼系数随锥度的增加逐渐减小,随波幅的增加逐渐变大。     

Ionic liquid recovery alternatives in ionic liquid‐based three‐phase partitioning (ILTPP)

Ionic liquid‐based three‐phase partitioning (ILTPP) is a promising technique to recover high‐added value proteins at the liquid–liquid interface. Its economic and environmental performance highly depends on the net ionic liquid consumption. Alternatives to maximize the fraction of ionic liquid that can be recycled are studied. It is demonstrated that the addition of extra salt, previously proposed in literature, has a very limited effect on ionic liquid recovery for relatively high protein concentrations in the feed stream, and that it may even lead to an increase of the ionic liquid losses under certain conditions. However, small additions of salt are shown to be effective and profitable from an economic point of view. Vacuum evaporation is shown to allow for the complete ionic liquid and salt recovery, reinforcing the sustainability and viability of ILTPP processes. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3577–3586, 2014     

Modeling the Electrokinetic Decontamination of Concrete

Abstract

The decontamination of concrete is a major concern in many Department of Energy (DOE) facilities. Numerous techniques (abrasive methods, manual methods, ultrasonics, concrete surface layer removal, chemical extraction methods, etc.) have been used to remove radioactive contamination from the surface of concrete. Recently, processes that are based on electrokinetic phenomena have been developed to decontaminate concrete. Electrokinetic decontamination has been shown to remove from 70 to over 90% of the surface radioactivity. To evaluate and improve the electrokinetic processes, a model has been developed to simulate the transport of ionic radionuclei constituents through the pores of concrete and into the anolyte and catholyte. The model takes into account the adsorption and desorption kinetics of the radionuclei from the pore walls, and ion transport by electro-osmosis, electromigration, and diffusion. A numerical technique, orthogonal collocation, is used to simultaneously solve the governing convective diffusion equations for a porous concrete slab and the current density equation.

This paper presents the theoretical framework of the model and the results from the computation of the dynamics of ion transport during electrokinetic treatment of concrete. The simulation results are in good agreement with experimental data.

     

Arsenic removal from drinking water by a “loose” nanofiltration membrane

The removal of arsenic from water by a “loose” nanofiltration (NF) membrane was investigated. Prior to the arsenic removal studies, the loose NF membrane was characterized for molecular weight cut-off and pore size by saccharide retention measurements, and electrokinetic charge by streaming potential measurements. In addition, separation of both single salt and mixed salt electrolyte solutions was studied to investigate the ion transport properties of the membrane. Arsenic rejection experiments included variation of pH, arsenic feed concentration, and presence of background electrolyte. In general, arsenic rejection increased with increasing pH and arsenic feed concentration, and was enhanced in the presence of 0.01 M NaCl. Arsenic was removed 60–90% from synthetic feed waters containing 10, 32, 100, and 316 μg/L As(V), resulting in permeate arsenic concentrations of 4, 6, 10, and 25μg/L, respectively. The behavior of the membrane is consistent with the extended Nernst-Planck equation model predictions for an uncharged membrane where size exclusion controls ion retention. However, separation of Arsenic species was a due to a combination of size exclusion, preferential passage of more mobile ions, and charge exclusion.     

Non-equilibrium molecular dynamics simulation of electrokinetic effects on heterogeneous ionic transport in nano-channel

The presence of the electric double layer (EDL) near the solid/liquid interface has a great impact on the liquid flowing through nano channels. In this paper, a non-equilibrium molecular dynamics method (NEMD) model is developed to investigate the transport characteristics of the heterogeneous ionic fluid flowing in a 4.672 nm-depth channel due to the electrokinetic effect induced by the EDL. An external perturbing velocity is introduced on the liquid instead of a constant force to study the electrokinetic effects on the flow. The effect of charges adsorbed on channel surfaces on flow resistance is especially considered. Two different charged surfaces with discrete or uniform charge distributions are compared, respectively. Besides Lennard-Jones (L-J) potential energy and Coulomb force, the ion–dipole interaction between the charged particle and the water molecule in the liquid is especially taken into account. According to the simulation results, the liquid density reaches the peak value in the EDL. However, unlike that predicted by the Poisson–Boltzmann theory, the liquid density profile is not exactly an anti-parabola curve, which has a significant fall before the density reaching the peak value in the EDL. It is shown that the charges absorbed on the channel surface have little influence on the liquid density distribution, but they play an important role in the fluid velocity distribution. Meanwhile, the liquid velocity slippage in the EDL emerges, which is well in agreement with the prior experimental data available. Moreover, though the non-dimensional velocities are nearly the same under different external velocity fields, the velocity slippage for the uniformly charged wall is much larger than that for the discretely charged wall. All of these results provide a probable reason why the flow behavior in a nano-channel is particularly different from that in a macro duct.     

A Three‐Dimensional Two‐Phase Model for a Membraneless Fuel Cell Using Decomposition of Hydrogen Peroxide with Y‐Shaped Microchannel

A three‐dimensional, two‐phase, multi‐component mixture model in conjunction with a finite‐volume‐based computational fluid dynamics (CFD) technique is applied to simulate the operation of membraneless fuel cell with Y‐shape channel. Hydrogen peroxide is employed both as fuel and oxidant, which are dissolved in diluted sodium hydroxide and sulfuric acid solutions, respectively. Almost all transport phenomena occurring in the fuel cell such as fluid flows, mass transport, electrochemical kinetics, and charge transport are accounted in this model. The oxygen O₂ gas, which is a product on the anode electrode, is assumed to be insoluble. The presence of gas phase acts to prevent the processes of reactant supply and product removal. Thus, the cell performance is hindered, while it is operated at the normal current density situation. On the other hand, the capillary action is found to enhance the electrolyte transport in the anode porous electrode, which may slightly improve the cell performance at the high‐current density situation. Besides, a secondary vortex flow is induced due to the transportation of the gas phase, which drifts from the bottom to the top of the channel. The mixing zone is then inclined, which may result in serious fuel crossing phenomenon.     

Performance of cellulose acetate butyrate membranes in hyperfiltration of sodium chloride and urea feed solution

Cellulose acetate butyrate (CAB) membranes gave high salt and urea rejection with a water flux of about 3 gfd (gallons/ft² · day) during hyperfiltration at 600 psig. Evidence was obtained which indicated that the CAB membranes used in this work were asymmetric. Membrane heat treatment increased urea rejection significantly while salt rejection was invariant, and water flux decreased. An increase in feed solution temperature caused a significant increase in water flux and a small decrease in urea and salt rejection. Increasing the pressure increased water flux and urea and salt rejection. During a 400-hr life test, the water flux decreased by about 25% while urea rejection increased and salt rejection was invariant. The influence of pressure, membrane heat treatment, and compaction during CAB membranes life testing on urea and salt rejection provided evidence that these two solutes were rejected by somewhat different mechanisms. Salt rejection was consistent with a solution–diffusion mechanism for membrane transport and uncoupled flow while changes in urea rejection with pressure, membrane heat treatment, and compaction during life testing suggested that urea was at least partially rejected by membrane exclusion resulting from geometric factors.     

Modeling of the Area‐specific Resistance of SOFC Cathodes by Application of a Square Grain Model Involving Grain Boundaries

A square grain model is proposed for the calculation of the area‐specific resistance (ASR) of porous cathodes for solid oxide fuel cells (SOFCs) by means of the finite element approach. The grains and pores are represented by squares of equal side length. The grain boundaries are assumed to be thin slabs of uniform thickness. Both blocking conditions for the ionic current and fast transport of oxide ions along the grain boundaries have been taken into account. The results of the simulation suggest that highly active cathode materials could be developed by increasing the grain boundary ionic conductivity. In the case of an average grain size of 0.1 μm, a remarkable decrease of the ASR is predicted, if the ionic conductivity of the grain boundaries exceeds that of the bulk by a factor of 100. The model has been applied to simulate the increase of the ASR due to degradation of La_0.6Sr_0.4CoO_3–δ in dry and humid atmospheres at 600 °C. A rapid increase of the ASR is predicted in H₂O‐containing atmospheres. The effect of Cr‐poisoning on the ASR has been modeled for dry and humid atmospheres at 600 °C. The degradation owing to Cr‐poisoning is most pronounced in atmospheres containing water vapor.     

Observations and analysis of electrokinetically driven particle trapping in planar microelectrode arrays

In situ observations of submicron fluorescent tracers suspended in high ionic strength media sealed in a confined geometry are combined with 3‐D simulations in order to provide a better understanding of the synergism between dielectrophoresis and electrothermal flows that cause rapid particle transport and trapping on the surface of planar quadrupolar microelectrodes. The influence of electrode design on the microfluidic patterns and observed particle collection is examined by employing two different types of microelectrodes in the experiments. The potential use of quadrupolar microelectrodes as means for achieving accelerated sampling and signal amplification in future surface based biosensor devices is illustrated with an experiment involving stable capture of antigen‐coated polystyrene particles on the surface of an antibody‐functionalized microelectrode array.     

Shear dispersion in combined pressure‐driven and electro‐osmotic flows in a capillary tube with a porous wall

An analytical expression is derived for the shear dispersion during transport of a neutral nonreacting solute within a coupled system comprised of a capillary tube and a porous medium under the combined effects of pressure‐driven and electro‐osmotic flows. We use the Reynolds decomposition technique to obtain a dispersion coefficient by considering a sufficiently low wall or zeta potential that accounts for the combined flows. The coupled dispersion coefficient depends on the Debye–Hückel parameter, Poiseuille contribution fraction, and Péclet number. The developed model also provides a shear dispersion coefficient for an impervious capillary tube (noncoupled system). The ratio of the coupled (porous wall) and noncoupled (impervious) dispersion coefficients reveals that it is essential to include the transport of chemical species from the tube to the porous medium in several important physical situations. These findings have implications for design of chemical species transport in porous microfluidic networks and separation of emulsions in microchannel‐membrane systems. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3981–3995, 2015     

PEMFC带沟槽气体扩散层内传输特性孔隙网络模拟

采用三维孔隙网络模型计算了不同沟槽参数下气体扩散层(GDL)的液态水突破压力、毛细压力分布、气体扩散率和液相相对渗透率随饱和度变化,并从孔隙尺度角度探究了沟槽的作用机制。研究结果表明：沟槽改变了GDL的毛细压力分布,提供了液态水直接传输路径并优化了GDL内氧气和液态水的分布,从而提高了氧气有效扩散率。沟槽位置对氧气传输有明显影响,对液相传输的影响取决于是否形成贯穿GDL的传输路径;沟槽加深,氧气和液态水传输性能增强,沟槽穿透GDL时传输性能达到最佳;沟槽变宽,液相传输性能增强,氧气传输性能在低饱和度范围内先增强后减弱。综合各因素,给出了氧气和液态水传输性能最优时的沟槽参数。     

Numerical simulation of the electrodeionization (EDI) process for producing ultrapure water

A steady-state model was established to simulate EDI process for producing ultrapure water (MixEDI), the dilute compartment of which is filled with mixed cation and anion-exchange resins. By calculating the mathematical model which includes water dissociation mechanism, ionic status of ion-exchange resin, concentration polarization status and the concentration distribution of water dissociation products are obtained. The influence of water dissociation on the current efficiency, removal rate and pH value of EDI effluent is investigated. The existence of water dissociation catalyst at anion-exchange membrane (AM) makes the water dissociation current of the AM much larger than that of CM. The result is that the amount of electro-regenerated cation-exchange resins is much larger than that of anion resins. This is the reason why the removal rate of salt cation much larger than that of salt anion in EDI for producing ultrapure water. Thus, at the target percentage removal, water dissociation at AM surface is excessive and the one at CM surface is insufficient. We assume that there is also some water dissociation catalyst at CM surface. It is found that the improved water dissociation at CM could increase the percentage removal of salt anions and the current efficiency.     

Modelling Approach for Planar Self‐Breathing PEMFC and Comparison with Experimental Results

This paper presents a model‐based analysis of a proton exchange membrane fuel cell (PEMFC) with a planar design as the power supply for portable applications. The cell is operated with hydrogen and consists of an open cathode side allowing for passive, self‐breathing, operation. This planar fuel cell is fabricated using printed circuit board (PCB) technology. Long‐term stability of this type of fuel cell has been demonstrated. A stationary, two‐dimensional, isothermal, mathematical model of the planar fuel cell is developed. Fickian diffusion of the gaseous components (O₂, H₂, H₂O) in the gas diffusion layers and the catalyst layers is accounted for. The transport of water is considered in the gaseous phase only. The electrochemical reactions are described by the Tafel equation. The potential and current balance equations are solved separately for protons and electrons. The resulting system of partial differential equations is solved by a finite element method using FEMLAB (COMSOL Inc.) software. Three different cathode opening ratios are realized and the corresponding polarization curves are measured. The measurements are compared to numerical simulation results. The model reproduces the shape of the measured polarization curves and comparable limiting current density values, due to mass transport limitation, are obtained. The simulated distribution of gaseous water shows that an increase of the water concentration under the rib occurs. It is concluded that liquid water may condense under the rib leading to a reduction of the open pore space accessible for gas transport. Thus, a broad rib not only hinders the oxygen supply itself, but may also cause additional mass transport problems due to the condensation of water.     

Heat and mass transfer on the wavy film absorption process

Recently the absorption heat pumps and chillers have received considerable attention due to their low electricity consumption rate. Therefore, it is important to understand the transport mechanism of an absorption process. In this paper, a numerical study of the heat and mass transfer taking place on a wavy falling liquid film of an absorption process is presented. With previously solved periodic wavy film flow solutions, the finite difference method is employed to solve the heat and mass transport equations. The numerical solution indicates that the waves significantly increase the transport rates. A comparison of the transfer rates of the wavy film to that of the smooth film is presented to show that the mass transfer rate can be doubled.