Computational Modelling of the Behaviour of Potentiometric Membrane Biosensors

This paper presents a mathematical model of a potentiometric biosensor based on a potentiometric electrode covered with an enzyme membrane. The model is based on the reaction–diffusion equations containing a non-linear term related to theMichaelis–Menten kinetics of the enzymatic reaction. Using computer simulation the influence of the thickness of the enzyme membrane on the biosensor response was investigated. The digital simulation was performed using the finite difference technique. Results of the numerical simulation were compared with known analytical solutions.      

Atomic Force Microscopy Studies of Membranes: Effect of Surface Roughness on Double-Layer Interactions and Particle Adhesion

Atomic force microscopy in conjunction with the colloid (silica) probe technique has been used to quantify the variations in electrical double-layer interactions and adhesion at different locations on a rough reverse osmosis membrane (AFC99) surface in NaCl solutions. Prior scanning of the membrane surface with the colloid probe allowed precise location for force measurements. The membrane surface was composed entirely of peaks and valleys with a surface roughness substantially greater than that of most other types of polymeric membranes. The magnitude of the electrical double-layer repulsion between the colloid probe and the membrane at peaks on the membrane surface was greatly reduced compared to that in the valleys. Nevertheless, adhesion of the colloid probe was lower at the peaks on the membrane surface than in the valleys with the difference increasing with decreasing salt concentration, and reaching a factor of more than 20 in 10(-3) M solution. The study shows that minimization of membrane fouling by colloids could be achieved by choosing membranes with a roughness periodicity preventing penetration of foulants into valleys on the surface. Copyright 2000 Academic Press.     

Computational Modelling of Biosensors with Perforated and Selective Membranes

This paper presents a two-dimensional-in-space mathematical model of amperometric biosensors with perforated and selective membranes. The model is based on the diffusion equations containing a non-linear term of the Michaelis–Menten enzymatic reaction. Using numerical simulation of the biosensors action, the influence of the geometry of the perforated membrane on the biosensor response was investigated. The numerical simulation was carried out using finite-difference technique. The calculations demonstrated non-linear and non-monotonous change of the biosensor steady-state current at various degree of the surface of the perforated membrane covering. The non-monotonous behaviour of the biosensor response was also observed when changing the thickness of the perforated membrane.     

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca²⁺ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.     

Mass transfer in the membrane concentration polarization layer under turbulent cross flow : II. Application to the characterization of ultrafiltration membranes

Eleven commercial membranes with MW cut-off values between 5,000 and 25,000 daltons have been characterized using a 0.5% Dextran T10 solution as a test substance. In order to predict the concentration at the membrane wall, Sherwood correlations based on various turbulence models have been used. The effects of the roughness of the membrane and viscosity changes due to concentration polarization and suction through the membrane have also been considered. The results have been compared with an uncorrected film theory model in a Reynolds interval of 10000-20000. It is difficult to test the models unless a direct method of estimating the concentration at the membrane wall is developed. However, some conclusions can be drawn about the applicability of the suggested models, according to the type of membrane used. It was also noted that the differences between the models tended to diminish with increasing Reynolds numbers.     

Solvation forces between molecularly rough surfaces

Surface heterogeneity affects significantly wetting and adhesion properties. However, most of the theories and simulation methods of calculating solid-fluid interactions assume a standard thermodynamic model of the Gibbs' dividing solid-fluid interface, which is molecularly smooth. This assumption gives rise to a layering of the fluid phase near the surface that is displayed in oscillating density profiles in any theories and simulation models, which account for the hard core intermolecular repulsion. This layering brings about oscillations of the solvation (or disjoining) pressure as a function of the gap distance, which are rarely observed in experiments, except for ideal monocrystal surfaces. We present a detailed study of the effects of surface roughness on the solvation pressure of Lennard-Jones (LJ) fluids confined by LJ walls based on the quenched solid density functional theory (QSDFT). In QSDFT, the surface roughness is quantified by the roughness parameter, which represents the thickness of the surface "corona" - the region of varying solid density. We show that the surface roughness of the amplitude comparable with the fluid molecular diameter effectively damps the oscillations of solvation pressure that would be observed for molecularly smooth surfaces. The calculations were done for the LJ model of nitrogen sorption at 74.4 K in slit-shaped carbon nanopores to provide an opportunity of comparing with standard adsorption experiments. In addition to a better understanding of the fundamentals of fluid adsorption on heterogeneous surfaces and inter-particle interactions, an important practical outcome is envisioned in modeling of adsorption-induced deformation of compliant porous substrates.     

Evaluation of a mathematical model using experimental data and artificial neural network for prediction of gas separation

In recent times, membranes have found wide applications in gas separation processes. As most of the industrial membrane separation units use hollow fiber modules, having a proper model for simulating this type of membrane module is very useful in achieving guidelines for design and characterization of membrane separation units. In this study, a model based on Coker, Freeman, and Fleming＇s study was used for estimating the required membrane area. This model could simulate a multicomponent gas mixture separation by solving the governing differential mass balance equations with numerical methods. Results of the model were validated using some binary and multicomponent experimental data from the literature. Also, the artificial neural network （ANN） technique was applied to predict membrane gas separation behavior and the results of the ANN simulation were compared with the simulation results of the model and the experimental data. Good consistency between these results shows that ANN method can be successfully used for prediction of the separation behavior after suitable training of the network     

Study of temperature distribution of sliding block with asperity surface

In this study, a numerical thermal model is developed for sliding block contact under various loads, sliding velocities and surface roughness. The temperature distributions are shown for perfectly insulated thermal conditions along noncontact surfaces. For a particular five‐peaks contact model, the maximum temperature at the central peak is slightly lhigher than the others. The temperature profile decreases as the distance to the symmetry axis increases, and then decreases dramatically at the noncontact area. It is clear to see that the maximum temperature locates at the symmetry central peak of the asperity contact area instead of the leading head of the smooth surface. The maximum temperature rise parameter increases as the pressure, sliding velocity and asperity roughness increased or conductivity decreased. This phenomenon becomes obvious for cases at high pressure, velocity and roughness and low conductivity. Particularly, the influence of roughness is not significant for low velocity. Similar results are found for the maximum temperature rise parameter difference between peaks or peaks/valleys. The simulation results of this asperity surface sliding block contact model are able to provide essential information for the components of microelectro—mechanical systems (MEMS) and biochemical reaction mechanism. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of thickness of the PPO membranes on the surface morphology

The effect of membrane thickness on surface morphology has been studied for the case of polyphenylene oxide (PPO) membranes cast from solutions of PPO in trichloroethylene (TCE). Both roughness and nodule size decrease with increasing membrane thickness up to a certain point after which they begin to increase. Minima in roughness and nodule size are observed for 9–11 μm thick films. These minima depend on initial polymer concentration in the casting solution. At a membrane thickness greater than 11 μm, super nodular aggregates are formed. A mechanism based on Marangoni and Rayleigh number is presented.     

Pore surface fractal analysis of palladium-alumina ceramic membrane using Frenkel-Halsey-Hill (FHH) model

The alumina ceramic membrane has been modified by the addition of palladium in order to improve the H(2) permeability and selectivity. Palladium-alumina ceramic membrane was prepared via a sol-gel method and subjected to thermal treatment in the temperature range 500-1100 degrees C. Fractal analysis from nitrogen adsorption isotherm is used to study the pore surface roughness of palladium-alumina ceramic membrane with different chemical composition (nitric acid, PVA and palladium) and calcinations process in terms of surface fractal dimension, D. Frenkel-Halsey-Hill (FHH) model was used to determine the D value of palladium-alumina membrane. Following FHH model, the D value of palladium-alumina membrane increased as the calcinations temperature increased from 500 to 700 degrees C but decreased after calcined at 900 and 1100 degrees C. With increasing palladium concentration from 0.5 g Pd/100 ml H(2)O to 2 g Pd/100 ml H(2)O, D value of membrane decreased, indicating to the smoother surface. Addition of higher amount of PVA and palladium reduced the surface fractal of the membrane due to the heterogeneous distribution of pores. However, the D value increased when nitric acid concentration was increased from 1 to 15 M. The effect of calcinations temperature, PVA ratio, palladium and acid concentration on membrane surface area, pore size and pore distribution also studied.     

Simulation of pool boiling of nanofluids by using Eulerian multiphase model

In the present work, a new simulation of nanofluid/vapor two-phase flow inside the 2-D rectangular boiling chamber was numerically investigated. The Eulerian–Eulerian approach used to predict the boiling curve and the interaction between two phases. The surface modification during pool boiling of silica nanofluid represented by surface roughness and wettability is put into the account in this simulation. New closure correlations regarding the nucleation sites density and bubble departure diameter during boiling of silica nanofluid were inserted to extend the boiling model in this work. Besides, the bubble waiting time coefficient which involved in quenching heat flux under heat flux partitioning HFP model was corrected to improve the results of this study. The numerical results validated with experimental works in the literature, and they revealed good agreements for both pure water and nanofluids. The results found that when improving the heat flux partitioning model HFP by considering the surface modification of nucleate pool boiling parameters, it will give more mechanistic sights compared to the classical model, which is used for predicting of boiling heat transfer of pure liquid.

     

Mesoscopic simulation of an ionomeric membrane based on sulfonated aromatic poly(ether ether ketone)

Mesoscopic simulation in the framework of the mesoscopic dynamics method (a version of the dynamic density functional method) was performed for a proton conducting membrane based on sulfonated aromatic poly(ether ether ketone) in a wide range of water content in the system. For the selected parametric field, the model demonstrates microphase separation of hydrophilic and hydrophobic segments of the polymer. In the bulk of the membrane, a spatial network of water channels forms, whose walls consist of polar (sulfonated) units of the macromolecule. Independent molecular dynamics simulation for one set of parameters gives close values of the structural characteristics of the membrane, which confirms the correctness of the mesoscopic model.     

平板双分子层脂膜的制备及锌离子跨该膜的输运过程

制备了氧化胆固醇 卵磷脂(脑磷脂)平板双分子层脂膜,研究了膜配方对双分子层脂膜的稳定性和离子通透性的影响,得到了最佳制膜工艺,建立了锌离子跨卵 (脑 )磷脂膜的吸附 -扩散模型,其计算值与实验值基本吻合.     

Mussel‐inspired method to decorate commercial nanofiltration membrane for heavy metal ions removal

Nanofiltration (NF) membranes have been widely used for the treatment of electroplating, aerospace, textile, pharmaceutical, and other chemical industries. In this work, halloysite nanotubes (HNTs) were directly anchored on the surface of commercial nanofiltration (NF) membrane by dopamine modification following advantageous bio‐inspired methods. SEM and AFM images were used to characterize the HNTs decorated membrane surface in terms of surface morphology and roughness. Water contact angle (WCA) was employed in evidencing the incorporation of HNTs and dopamine in terms of hydrophilicity or hydrophobicity. Augmentation of HNTs was found to obviously enhance the hydrophilicity and surface roughness resulting in improved water permeability of membrane. More importantly, the rejection ratios of membrane also increased during the removal of heavy metal ions from wastewater. The permeability and Cu²⁺ rejection ratio of modified NF membrane were as high as 13.9 L·m^?2·h^?1·bar^?1 and 74.3%, respectively. Incorporation of HNTs was also found to enhance the anti‐fouling property and stability of membrane as evident from long‐term performance tests. The relative concentration of HNTs and dopamine on membrane surface was optimized by investigating the trade‐off between water permeability and rejection ratio.     

A Monte Carlo Simulation of the Development of Surface Roughness and Its Effect on the Dissolution Kinetics of SiO2 Aerogels

The development of surface roughness during dissolution of spherical particles is studied by the Monte Carlo method. The simulation results are used to analyze the dissolution kinetics of silicon dioxide aerogels in an aqueous solution of alkali (NaOH). The suggested model is shown to be suitable for describing the experimental dissolution curves obtained for aerogels with a small diameter of primary particles (3.5 and 2.9 nm). For aerogels with larger particles, a good agreement with the experiment can be achieved under the additional assumption that only part (p < 1) of the particle surface is originally active in dissolution; the best agreement is reached at p 0.5. In the kinetic regime of dissolution, the dissolution rate may be more than three times higher (owing to the formation of rough surfaces of primary particles with relatively large diameters, 40 atoms or more) than the rate calculated for the same parameters within the framework of the modified Delmon model, which does not make allowance for the development of roughness. Relatively small particles (with the diameter of less than 15 atoms) are dissolved before a significant roughness can be developed; therefore, the kinetic curves obtained for both models have virtually identical shapes in this case. The formation of roughness has an especially large effect on the dissolution of intermediate-size particles, whose dissolution time has the same order of magnitude as the time required for establishment of the steady-state roughness.     

The model of rough wetting for hydrophobic steel meshes that mimic Asparagus setaceus leaf

A comprehensive analytical model is proposed to provide a relationship between the macroscopic roughness and contact angle, which is used to develop macroscopic rough surface and to create biomimetic superhydrophobic surfaces. Using chemical surface modification of steel wires, an artificial hydrophobic surface was prepared. A steel mesh mimicking the Asparagus setaceus leaf was created by lowing the surface energy and enhancing macroscopic surface roughness. Water contact angles as high as 129.0° were achieved on the steel mesh with 200μm×200μm pore size. Bad agreement between the predictions based on the original Cassie-Baxter model and experiments was obtained. The version of the Cassie-Baxter model in current use could not be applied to this problem since the roughness magnitude changes from nano/microscopic to macroscopic. A new model, called macroscopic Cassie-Baxter (MCB) model, is constructed by the introduction of contact area density (δ) to original Cassie-Baxter model. It is shown that the measured data is in good agreement with the predicted data based on the MCB model. This model not only for solving macroscopic hydrophobic problems of meshes, but also can be used to solve that of other materials with macroscopic roughness.     

Electrical impedance spectroscopy characterisation of conducting membranes: II. Experimental

An electrical impedance spectroscopy (EIS) method and apparatus that eliminates the need for electrodes in the feed and permeate solutions was evaluated as a means of characterising physical and performance properties of polysulphone ultrafiltration membranes in situ. The membranes were sputter-coated on one side with platinum before assembly in the apparatus. Alternating electrical current used for impedance measurements was injected directly into the coat via dry electrical contacts with the edges of the membrane. As the frequency of the EIS measurement was increased the current increasingly dispersed into the solution via the interfacial region (double layer) and/or fouling layers that the coat formed with the solution. These spatial dispersions manifested as characteristic dispersions with frequency of the impedance of the system. Water flux measurements, field emission scanning electron microscopy and atomic force microscopy were also used to quantify the important membrane performance parameters of porosity and surface roughness. These estimates were in good agreement with the impedance model for the in situ membrane system that was fitted to the measured impedance dispersions. The study shows that EIS measurements potentially can quantify membrane performance parameters in situ better than those techniques that require disruption of the membrane separation process. The method also has the potential for monitoring the deposition of particulate that can lead to fouling.     

Preparation and characterisation of novel polysulfone membranes modified with Pluronic F-127 for reducing microalgal fouling

In this study, hydrophilic and fouling-resistant polysulfone (PS) membranes were fabricated using the phase inversion method to reduce membrane fouling caused by microalgal culture. The Pluronic F-127 polymer, which is used as a hydrophilic co-polymer, was added to the membranes to improve the membrane properties. Characteristic specifications of the fabricated membranes, such as morphology, surface roughness, chemical structures and hydrophobicity/hydrophilicity, were studied using scanning electron microscopy, atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDS), attenuated total reflection-fourier infrared (ATR-FTIR) spectroscopy and contact angle devices. According to the results obtained, it was observed that, with the increase of the Pluronic F-127 concentration in the membranes, the surface roughness of the membranes decreased and hydrophilicity and permeation fluxes increased notably. Furthermore, it was observed that the addition of the Pluronic F-127 polymer into the membranes reduced reversible/irreversible membrane fouling. Additionally, a characterisation of the fouled membranes was performed for the purpose of comprehensively understanding the membrane fouling mechanism caused by microalgal culture.     

Analysis of Contact Interactions between a Rough Deformable Colloid and a Smooth Substrate

A model was developed for the effect of van der Waals interactions between a rough, deformable, spherical colloid and a flat, smooth, hard surface in contact. The model demonstrates the significant effect of colloid roughness on removal force. Small changes in colloid roughness produce large changes in the predicted removal force. Several authors attribute discrepancies in the observed interaction force between particles and surfaces to colloid roughness, and our model supports their hypotheses. Experimental data documenting the force required to remove colloids of polystyrene latex from silica substrates in aqueous solution were collected during AFM studies of this system. When colloid roughness exists, as is the case in this work, our model bounds the observed removal force. The predicted range of removal forces is in better quantitative agreement with our removal force data than are forces predicted by classical DLVO theory. Copyright 2000 Academic Press.     

Evolution of a polysulfone nanofiltration membrane following ion beam irradiation

Ion beam irradiation was used to modify the surface of a sulfonated polysulfone water treatment membrane. A beam of 25 keV H (+) ions with four irradiation fluences (1 x 10 (13), 5 x 10 (13), 1 x 10 (14), and 5 x 10 (14) ions/cm (2)) was used to study the effects of ion beam irradiation on chemical structure, surface morphology, microstructure, and performance. XPS and ATR-FTIR analyses were performed on the virgin and irradiated membranes in order to determine the changes to chemical structure incurred by ion beam irradiation. The results showed that some sulfonic and C-H bonds were broken and new C-S bonds were formed after irradiation. AFM analysis showed that the roughness of the membranes decreased after irradiation, and the decrease in surface roughness was proportional to the increase in irradiation fluence. An increase in flux after ion beam irradiation was also observed along with a smaller flux decline during operation. Flux was not a function of irradiation fluence. Hydrophobicity, pore size distribution, and membrane rejection efficiencies were not affected by ion beam irradiation. Overall, irradiation led to an improvement in membrane performance.