Electroosmotic Effects Research Articles

Imaging alpha-hemolysin with molecular dynamics: ionic conductance, osmotic permeability, and the electrostatic potential map.

alpha-Hemolysin of Staphylococcus aureus is a self-assembling toxin that forms a water-filled transmembrane channel upon oligomerization in a lipid membrane. Apart from being one of the best-studied toxins of bacterial origin, alpha-hemolysin is the principal component in several biotechnological applications, including systems for controlled delivery of small solutes across lipid membranes, stochastic sensors for small solutes, and an alternative to conventional technology for DNA sequencing. Through large-scale molecular dynamics simulations, we studied the permeability of the alpha-hemolysin/lipid bilayer complex for water and ions. The studied system, composed of approximately 300,000 atoms, included one copy of the protein, a patch of a DPPC lipid bilayer, and a 1 M water solution of KCl. Monitoring the fluctuations of the pore structure revealed an asymmetric, on average, cross section of the alpha-hemolysin stem. Applying external electrostatic fields produced a transmembrane ionic current; repeating simulations at several voltage biases yielded a current/voltage curve of alpha-hemolysin and a set of electrostatic potential maps. The selectivity of alpha-hemolysin to Cl(-) was found to depend on the direction and the magnitude of the applied voltage bias. The results of our simulations are in excellent quantitative agreement with available experimental data. Analyzing trajectories of all water molecule, we computed the alpha-hemolysin's osmotic permeability for water as well as its electroosmotic effect, and characterized the permeability of its seven side channels. The side channels were found to connect seven His-144 residues surrounding the stem of the protein to the bulk solution; the protonation of these residues was observed to affect the ion conductance, suggesting the seven His-144 to comprise the pH sensor that gates conductance of the alpha-hemolysin channel.

Electrically Controllable Biomimetic Actuators Made with Multiwalled Carbon Nanotube(MWNT) Loaded Ionomeric Nanocomposites

Biomimetic actuators that can produce soft-actuation but large force generation capability are of interest. Nafion, an effective ionomeric material from DuPont, has been shown to produce large deformation under low electric fields (<10V/mm). This response is caused by a direct electro-osmotic effect due to the existence and mobility of cations and subsequent swelling and deswelling of the material. In this effort, multiwalled carbon nanotube (MWNT)/Nafion nanocomposites were prepared by a casting in order to investigate the effect of MWNT loading in the range of 0 to 7 wt% on electromechanical properties of the MWNT/Nafion nanocomposites. The measured elastic modulus and actuation force of the MWNT/Nafion nanocomposites are drastically different, showing larger elastic modulus and improved electromechanical coupling, from the one without MWNT, implying that the effective MWNT loading is crucial to develop high-performance biomimetic actuators.

Development and characterization of single wall carbon nanotube–Nafion composite actuators

The development and characterization of thin film polymeric actuators has been performed for single wall carbon nanotube (SWNT)–Nafion composite systems. Previous work in our laboratory illustrated that incorporation of highly conductive SWNTs into an ionomeric matrix promoted an actuation response by enhancing the electro-osmotic effect at relatively low doping levels, ∼0.1% (w/w). Further investigation has shown the effects of frequency and applied voltage on the composite bimorph actuator systems. The results indicate a displacement response that is linearly dependent on voltage and inverse to frequency.

Effect of Electric Field on the Mobility of Carboxyl-Terminated Dendrimers

The effect of an applied electric field on the mobilities of carboxyl-terminated dendrimers has been measured by capillary electrophoresis at applied voltages V ranging from 5 to 30 kV. The mobility increased with the field strength at moderate ionic strength, but varied inversely with V at low ionic strength. These results did not arise from Joule heating or from electroosmotic flow effects. It is suggested that the remarkably high velocities of these dendrimers enables them to outstrip their ion atmospheres at high V, in a manner related to the increase in conductance at high field strength first described by Wien in 1927. At low ionic strength, the increase in the size of the ion atmosphere leads to increased frictional drag with increasing field strength.

Single-phase convective heat transfer in microchannels: a review of experimental results

A bibliographical review on the convective heat transfer through microchannels is presented. The available experimental works quoted in the open literature are critically analysed in order to highlight the main results obtained on the friction factor, on the laminar-to-turbulent transition and on the Nusselt number in channels having a hydraulic diameter less than 1 mm. A comparison of the experimental results quoted in the open literature is made. In many cases the experimental data of the friction factor and of the Nusselt number in microchannels disagree with the conventional theory but they also appear to be inconsistent with one another. Various reasons have been proposed to account for these differences. Rarefaction and compressibility effects, viscous dissipation effects, electro-osmotic effects (EDL), property variation effects, channel surface conditions (relative roughness) and experimental uncertainties have been invoked to explain the anomalous behaviour of the transport mechanisms through microchannels. By comparing the available experimental data on single-phase convective heat transfer through microchannels, it is evident that further systematic studies are required to generate a sufficient body of knowledge of the transport mechanism responsible for the variation of the flow structure and heat transfer in microchannels.

ANALYSIS OF THE ELECTROFILTRATION MECHANISM BASED ON MULTIPHASE FILTRATION THEORY

A mathematical model based on multiphase filtration theory is developed to describe the electrofiltration process. The model takes both electrophoretic and electroosmotic effects into account. The electrophoretic migration velocity of solid particles is predicted from the model using the filtrate volume time data obtained from electrofiltration experiments.

Electroosmotic enhancement of the binding of a neutral molecule to a transmembrane pore.

The flux of solvent water coupled to the transit of ions through protein pores is considerable. The effect of this electroosmotic solvent flow on the binding of a neutral molecule [beta-cyclodextrin (betaCD)] to sites within the staphylococcal alpha-hemolysin pore was investigated. Mutant alpha-hemolysin pores were used to which betaCD can bind from either entrance and through which the direction of water flow can be controlled by choosing the charge selectivity of the pore and the polarity of the applied potential. The Kd values for betaCD for individual mutant pores varied by >100-fold with the applied potential over a range of -120 to +120 mV. In all cases, the signs of the changes in binding free energy and the influence of potential on the association and dissociation rate constants for betaCD were consistent with an electroosmotic effect.

Biochemical analysis with microfluidic systems.

Microfluidic systems are capillary networks of varying complexity fabricated originally in silicon, but nowadays in glass and polymeric substrates. Flow of liquid is mainly controlled by use of electroosmotic effects, i.e. application of electric fields, in addition to pressurized flow, i.e. application of pressure or vacuum. Because electroosmotic flow rates depend on the charge densities on the walls of capillaries, they are influenced by substrate material, fabrication processes, surface pretreatment procedures, and buffer additives. Microfluidic systems combine the properties of capillary electrophoretic systems and flow-through analytical systems, and thus biochemical analytical assays have been developed utilizing and integrating both aspects. Proteins, peptides, and nucleic acids can be separated because of their different electrophoretic mobility; detection is achieved with fluorescence detectors. For protein analysis, in particular, interfaces between microfluidic chips and mass spectrometers were developed. Further levels of integration of required sample-treatment steps were achieved by integration of protein digestion by immobilized trypsin and amplification of nucleic acids by the polymerase chain reaction. Kinetic constants of enzyme reactions were determined by adjusting different degrees of dilution of enzyme substrates or inhibitors within a single chip utilizing mainly the properties of controlled dosing and mixing liquids within a chip. For analysis of kinase reactions, however, a combination of a reaction step (enzyme with substrate and inhibitor) and a separation step (enzyme substrate and reaction product) was required. Microfluidic chips also enable separation of analytes from sample matrix constituents, which can interfere with quantitative determination, if they have different electrophoretic mobilities. In addition to analysis of nucleic acids and enzymes, immunoassays are the third group of analytical assays performed in microfluidic chips. They utilize either affinity capillary electrophoresis as a homogeneous assay format, or immobilized antigens or antibodies in heterogeneous assays with serial supply of reagents and washing solutions.

Transport of Th(IV) and U(VI) through barium silico-phosphate composite membrane using electric field

Summary The present paper describes the preparation of a novel barium silico-phosphate filter paper supported membrane. It is based on precipitation reaction of barium silico-phosphate on the outer surface and in the interstices of a filter paper by means of electrodialysis. The main physical and electrical properties of the membrane are given and its electrodialysis behaviour is assessed for Th(IV) and U(VI). The transport of Th(IV) in presence of U(VI) was studied. The cationic fluxes of Th(IV) and U(VI) were found to be 1.2 × 10−8 and 6.5 × 10−9 g eq cm−2 s−1, respectively. Transport of Th(IV) and U(VI) in presence of EDTA was investigated. The cationic flux of U(VI) is found to be 9.8 × 10−9 g eq cm−2 s−1 at a current density of 25 mA/cm2. A comparative study on the electro osmotic effect was carried out using the developed membrane and commercially available Nafion membranes. In this context, different parameters like current density, electrolyte concentration, etc. were investigated. The electro-osmotic permeability coefficient, D e, of Th(IV) through barium silico-phosphate and Nafion membranes were 6.9 × 10−2 and 1.0 × 10−2 cm3/A s, respectively. It can be concluded that inorganic membranes have very marked electro-osmotic properties unlike their organic counterparts.

Electric field and Charged Molecules Mediated Self-Assembly for Electronic Devices

ABSTRACTIn this paper we present techniques, utilizing dielectrophoresis and electrohydrodynamics, which can possibly be used for assembling devices suspended in a solution onto a binding site on a substrate. We explored the concepts using micro-scale negatively charged polystyrene beads and rectangular silicon blocks. Dielectrophoretic forces on devices in buffer solutions were examined as a function of frequency of the applied AC signal. The observed results can be explained by taking in account electro-thermal and AC electroosmotic effects. The study described in the paper can be used for placing and assembling micro and nano-electronic devices and objects at specific sites on various substrates, in combination with bio-inspired biological binding techniques such as DNA hybridization, antigen-antibody interactions, and ligand-receptor (avidin-biotin) interactions.

Electrodialysis of uranium(VI) through cation exchange membranes and modeling of electrodialysis processes

The transport of ionic species through ion exchange membranes found several applications for water effluents purification and metal ion separation. To enhance the transport performance, the effect of electric fields was suggested in this work. The transport of U(VI) species in nitric acid solutions across cation exchange membranes was investigated. Different parameters affecting the transport of U(VI) were studied. These parameters include: nitric acid concentration in the feed solution, stripping solution concentration and applied electric field. From the results obtained, the cationic flux of U(VI) was 6.5.10–8 geq.cm–2.s–1 at the optimal conditions of 10–3M HNO3 in the feed solution, 0.5M Na2CO3 in the stripping solution and 30 V. The modeling of the electrodialysis process was also made. The model correlates the mass transfer as a function of current density and voltage as variables and takes into account the electro-osmotic effect. The model is applied to the experimental data.

Electric field and Charged Molecules Mediated Self-Assembly for Electronic Devices

AbstractIn this paper we present techniques, utilizing dielectrophoresis and electrohydrodynamics, which can possibly be used for assembling devices suspended in a solution onto a binding site on a substrate. We explored the concepts using micro-scale negatively charged polystyrene beads and rectangular silicon blocks. Dielectrophoretic forces on devices in buffer solutions were examined as a function of frequency of the applied AC signal. The observed results can be explained by taking in account electro-thermal and AC electroosmotic effects. The study described in the paper can be used for placing and assembling micro and nano-electronic devices and objects at specific sites on various substrates, in combination with bio-inspired biological binding techniques such as DNA hybridization, antigen-antibody interactions, and ligand-receptor (avidin-biotin) interactions.

Electroosmotic effects in the determination of ion mobilities of carrier-free radionuclides in free aqueous electrolyte solutions

A device for on-line electromigration studies of radio-elements in free electrolytes has been tested by measuring the absolute individual ion mobilities of no-carrier-added single charged anions [99mTc]TcO4 - and [18F]F- and differently charged cations [137Cs]Cs+, [57Co]Co2+ and [241Am]Am3+ in inert aqueous electrolytes at 298.1 K. The observed ion mobilities were found to be influenced by electroosmosis in the migration tube. A relation to correct this effect is proposed. In addition, it was shown that the effect of electroosmosis may be suppressed by chemical treatment of the surface of the migration tube.

Two-dimensional numerical modelling of a direct methanol fuel cell

The results of a numerical simulation of a direct methanol fuel cell (DMFC) with liquid methanol feed are presented. A two-dimensional numerical model of a DMFC is developed based on mass and current conservation equations. The velocity of the liquid is governed by gradients of membrane phase potential (electroosmotic effect) and pressure. The results show that, near the fuel channel, transport of methanol is determined mainly by the pressure gradient, whereas in the active layers, and in the membrane, diffusion transport dominates. ‘Shaded’ zones, where there is a lack of methanol, are formed in front of the current collectors. The results reveal a strong influence of the hydraulic permeability of the backing layer K p BL on methanol crossover through the membrane. If the value of K p BL is comparable to that of the membrane and active layers, electroosmotic effects lead to the formation of an inverse pressure gradient. The flux of liquid driven by this pressure gradient is directed towards the anode and reduces methanol crossover.

Electrokinetic geophysics — a review

Electrokinetic phenomena occur in porous rocks and include streaming potential, electro‐osmotic and electrophoretic effects. Geophysical techniques can generate acoustic body waves that stimulate electrokinetic coupling at subsurface boundaries. The electromagnetic effects generated are observed using surface electric field sensors. Improved methodologies, aided by recent modelling of the coupled acoustic and electromagnetic wave equations, are re‐exploring electrokinetic behaviour in the near surface. The methodologies are aimed at providing geophysical tools that may characterize environmentally sensitive parameters such as the fluid content, the fluid geochemistry and the microstructural fabric of the subsurface.

Recycling electrophoretic separations: modeling of isotachophoresis and isoelectric focusing

The conventional, one-dimensional model that is the foundation for the dynamic simulation of electrophoretic separations is modified to describe the essential features of recycling electrophoresis instruments. Simulations are performed for the isoelectric focusing (IEF) and the isotachophoresis (ITP) modes. Results are compared with experimental data from prototype recycling free-flow IEF and ITP devices. Agreement between the experiments and the simulations is favorable, though electrode and electroosmotic effects, which are omitted from the model, are noticeable in the IEF instrument. Three parameters relevant to the design of recycling electrophoresis devices, viz. the number of ports for the recycle manifold, the residence time in the separation chamber per cycle, and the current density, are investigated with the model.

Correlations of filtration flux enhanced by electric fields in crossflow microfiltration.

The steady state filtration flux in electrically-enhanced crossflow microfiltration is estimated using a correlation equation proposed for several kinds of suspensions. Baker's yeast and Rhodotorula glutinis were used as model samples of microbial cells, and PMMA particles were used as samples of non-living solids. Application of the electric field in crossflow microfiltration is a useful method for improving the filtration flux of these samples. High flux levels for the cells were achieved when an electric field above 3000 V/m was applied. The effect of the electric field in increasing the filtration flux of the steady state was analyzed theoretically using a force balance model where the viscous drag force, FJ, the electrophoretic force, FE, and the re-entraining force, FB, were considered to act on a particle on the membrane surface under a steady state of filtration, respectively. From force balance analysis, it is found that on application of an electric field, the electro-osmotic effect can be neglected in the present study, so that the filtration flux of the steady state, JES, can be presented by, JES = UEPE + J0S where UEP is the electrophoretic mobility of particles and E is the electric field applied. J0S is the filtration flux in the absence of an electric field, which is correlated with the operating parameters for suspensions tested.

Comment on "Modeling of metal electrodeposits: Analytical solutions"

When the equations of ionic motion for electrodeposition in a one-dimensional cell filled with a dilute binary electrolyte are solved under fixed-boundary conditions, a diffusion-limited current, independent of applied potential, is obtained. This result is well expected in the framework of the quasineutrality approximation. In this framework, the assumption by Huang and Hibbert [Phys. Rev. E. 52, 5065 (1995)] of an electrical-migration term in the evolution equation for the concentration is incorrect. However, a term of the same form, though smaller, may appear either from the concentration dependence of the mobilities or from an electro-osmotic effect if the electrolyte is embedded in a gel or a porous medium. \textcopyright{} 1996 The American Physical Society.

Electrokinetic effects by the electric work method

The electric work method is used to derive new expressions for electrokinetic effects produced by flowing fluids. The expressions are functions of channel geometry. The results are obtained from an equation of motion by superposition of reversible and irreversible phenomena. The equation of motion is an extended Navier-Stokes equation. The momentum transference factors, which describe the reversible conversion between mechanical and electrical energy or vice versa, are central new parameters. It follows from the equations that the streaming potential is opposite and equal to the electroosmotic flux, and that the streaming current is equal to the electroosmotic pressure. We also show how the streaming potential and the electroosmotic pressure effects are related; they differ mainly in their capability of storing electric energy. The new theory predicts a different dependence on electrolyte conductivity as does the classical theory. The streaming potential has not been explained, ie by surface conductivities, but by momentum transference properties between the wall and the solution.

Analysis of phenoxyalkyl acid herbicides and chlorophenols by capillary zone electrophoresis

A capillary zone electrophoresis system (CZE) with on-line UV detection was used for the determination of two phenoxyalkyl acid herbicides (2,4-D and 2,4,5-TP) and two chlorophenols (2,4-dichlorophenol and 2,4,5-trichlorophenol). Under an applied voltage of 23 kV, all compounds migrated towards the cathode within 10 min. Phosphate buffer was used and the separation of different species was achieved by combining electrophoretic and electro-osmotic effects. The coefficient of electro-osmotic flow and the electrophoretic mobilities of individual compounds were calculated. A linear relationship between peak area and sample amount was obtained in the range of 4–56 pg for 2,4-D, 2,4,5-TP and 2,4,-dichlorophenol and of 10–46 pg for 2,4,5-trichlorophenol. The detection limit was lower than 6 pg. The coefficients of variation of peak area and retention time were 3.4–6.1% and 0.2%, respectively.