Electroosmotic Effects Research Articles

Cytoplasmic Electric Fields and Electroosmosis: Possible Solution for the Paradoxes of the Intracellular Transport of Biomolecules

The objective of the paper is to show that electroosmotic flow might play an important role in the intracellular transport of biomolecules. The paper presents two mathematical models describing the role of electroosmosis in the transport of the negatively charged messenger proteins to the negatively charged nucleus and in the recovery of the fluorescence after photobleaching. The parameters of the models were derived from the extensive review of the literature data. Computer simulations were performed within the COMSOL 4.2a software environment. The first model demonstrated that the presence of electroosmosis might intensify the flux of messenger proteins to the nucleus and allow the efficient transport of the negatively charged phosphorylated messenger proteins against the electrostatic repulsion of the negatively charged nucleus. The second model revealed that the presence of the electroosmotic flow made the time of fluorescence recovery dependent on the position of the bleaching spot relative to cellular membrane. The magnitude of the electroosmotic flow effect was shown to be quite substantial, i.e. increasing the flux of the messengers onto the nucleus up to 4-fold relative to pure diffusion and resulting in the up to 3-fold change in the values of fluorescence recovery time, and therefore the apparent diffusion coefficient determined from the fluorescence recovery after photobleaching experiments. Based on the results of the modeling and on the universal nature of the electroosmotic flow, the potential wider implications of electroosmotic flow in the intracellular and extracellular biological processes are discussed. Both models are available for download at ModelDB.

An electro-hydrodynamics-based model for the ionic conductivity of solid-state nanopores during DNA translocation

A solid-state nanopore can be used to sense DNA (or other macromolecules) by monitoring ion-current changes that result from translocation of the molecule through the pore. When transiting a nanopore, the highly negatively charged DNA interacts with a nanopore both electrically and hydrodynamically, causing a current blockage or a current enhancement at different ion concentrations. This effect was previously characterized using a phenomenological model that can be considered as the limit of the electro-hydrodynamics model presented here. We show theoretically that the effect of surface charge of a nanopore (or electro-osmotic effect) can be equivalently treated as modifications of electrophoretic mobilities of ions in the pore, providing an improved physical understanding of the current blockage (or enhancement).

Influence of the ion–solvent interactions on ionic transport through ion-exchange-membranes

Ionic transport through ionic exchange membrane cannot be interpreted by the Nernst–Planck equation if the ion density is high, particularly in the membrane. In order to extend the density range, excess terms must be added to the chemical potential. These terms are computed by considering charged hard spheres embedded in a dielectric continuum. In this aim and owing to heterogeneity of the electrolytic solution the density function theory (DFT) is used. A previous work has been carried out with a homogeneous and amorphous solvent. Here an extension including the finite size of the solvent molecule and the local dielectric properties via the dielectric coefficient is presented. The electro-osmosis is also examined. The selectivity at equilibrium, the density profiles and the voltammograms are analysed. The numerical results obtained with NaCl and CaCl2 show that the physical properties of the solvent decrease the selectivity and increases the conductivity of the membrane systems. In the same time, the dielectric properties increase the electro-osmotic effects on the ionic transport. The approach described in this work can be used to study the solvent confinement effect inside the membrane on the ionic transport.

Electroosmotic Augmentation in Flexural Plate Wave Micropumps

We investigate the effectiveness and applicability of electroosmotic augmentation in flexural plate wave (FPW) micropumps for enhanced capabilities. Flow rates generated in FPW micro-scale flow systems are restricted particularly when the channel height is greater than the acoustic wave length. The proposed concept can be exploited to integrate micropumps into complex microfluidic chips improving the portability of micro-total-analysis systems along with the capabilities of actively controlling acoustics and electrokinetics for micro-mixer applications. A computational study of electroosmotic augmentation in FPW micropumps is presented where FPWs are considered by a moving wall model. A transient analysis of compressible flows of water is performed for microchannels. An isothermal equation of state for water is employed. The nonlinear Poisson-Boltzmann and Laplace equations are used to model the induced electric double layer (EDL) potential and the applied electric potential. Coupled electroosmotic and acoustics cases are investigated for two channel heights while the electric field intensity of the electrokinetic body forces and actuation frequency of acoustic excitations are varied. For deeper microchannels, increasing the actuation frequency of the wall motion does not improve the generated flow rate significantly. Inclusion of electroosmotic effects is more efficient than increasing the intensity of acoustic perturbations whenever high flow rates are required in micro-mixer applications.

A Modified Device for Pressurized Planar Electrochromatography and Preliminary Results with On-Line Sample Application

Pressurized planar electrochromatography (PPEC) is a separating technique in which an electric field is applied to force the mobile phase movement through a porous media (electroosmotic effect). High separation efficiency, fast separations and changes in separation selectivity in comparison to liquid chromatography, especially thin layer chromatography (planar chromatography, TLC), are features of this technique. Constructional methodological challenges to PPEC are obstacles to its development and application in laboratory practice. In this article, an attempt to overcome the challenges related to device construction and sample application/injection is described. The introduced device enables both prewetting of the adsorbent layer and electrochromatogram development with a single PPEC device. It also enables simultaneous application/injection of six samples on a chromatographic plate in a stream of the mobile phase (on-line application/injection). In addition, the PPEC chamber was equipped with a thermostat. The device is characterized by an impressive throughput in comparison to the other planar technique, TLC/HPTLC. Although the developed device still needs improvement, it is, in our opinion, a considerable step toward possible automation of this planar separation technique.

Electrokinetic transport of nanoparticles to opening of nanopores on cell membrane during electroporation

Nanoparticle transport to the opening of the single nanopore created on the cell membrane during the electroporation is studied. First, the permeabilization of a single cell located in a microchannel is investigated. When the nanopores are created, the transport of the nanoparticles from the surrounding liquid to the opening of one of the created nanopores is examined. It was found that the negatively charged nanoparticles preferably move into the nanopores from the side of the cell membrane that faces the negative electrode. Opposite to the electro-osmotic flow effect, the electrophoretic force tends to draw the negatively charged nanoparticles into the opening of the nanopores. The effect of the Brownian force is negligible in comparison with the electro-osmosis and the electrophoresis. Smaller nanoparticles with stronger surface charge transport more easily to the opening of the nanopores. Positively charged nanoparticles preferably enter the nanopores from the side of the cell membrane that faces the positive electrode. On this side, both the electrophoretic and the electro-osmotic forces are in the same directions and contribute to bring the positively charged particles into the nanopores.

Numerical Modelling of Electroosmotic Driven Flow in Nanoporous Media by Lattice Boltzmann Method

The lattice Boltzmann method was employed to simulate electroosmotic driven flow and Debye layer screening in conducting electrolyte around a porous structure with average size of 40 nm. The charge screening around the nanopores was investigated by solving the vector-superpositioned potential equilibrium distribution function and adding electro-kinetic force term to the evolution equation. In this intermediate case of moderate Debye length, the electrophoresis problem becomes complicated. The motion of the particles distorts the screening cloud, which becomes asymmetric, resulting in very complex interactions between the electrolyte, the screening cloud and the particle; but the Electroosmotic Flow (EOF) behaviour was still considered based on the Helmoholtz-Smoluchowski model with adaptation to fit nanoporous flow in the porous structure. In the present approach, the flow in the nanopores is directly modelled; the detailed flow information can be obtained by simplifying the repeated macrostructure. Due to the symmetry of the domain, the size of computational domain can be largely reduced by less repeated spherical nanoparticles. Each pore of the medium contains several lattice nodes on the simplified curved edges and potential gradients are produced by adjusting the zeta potential value. The velocity results for pressure-driven and EOF flows agree well with the analytical solutions and recent experimental results. In particular, the interface between solid particles and fluids, the influences of porosity, solid particle diameter, yield stress and electric parameters in EOF were investigated. The anti-adhesion effect of electroosmotic pumping effect was evaluated, and the pulsed DC was applied in order to enhance the performance of the electroosmotic pumping. The results demonstrate that the present lattice Boltzmann model is capable of modelling flow through nanoporous media at certain restrictions while some results deviate from the predictions based on the macroscopic theories.

Experimental investigation of the electrokinetic flow in microchannels with internal electrodes

AbstractWe investigate the electrokinetic flow in microchannels with internal electrodes. Experiments and numerical simulations are performed. The micro–particle–image velocimetry method is used to measure two–dimesional, two–component velocity fields over the complete height of the microchannel. Based on this measurements, the third velocity component, which cannot be measured directly, is calculated by an integration of the continuity equation. Due to the fact that microparticles, used for the μPIV are electrically non‐neutral leads to the problem that these particles experience electrophoretic forces. That means that the particle movement appears to be a superposition of electroosmotic and electrophorectic effects. To verify the influence of electrophoretic effects on the microparticles, additional numerical calculations are made. (© 2012 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evaluation of electro-osmotic pumping effect on microporous media flow

In this paper, the electro-osmotic flow (EOF) in micro-porous media is investigated regarding to the flow rate caused by the combined effect of electro-osmosis force and the gravitational force. It is noticeable that the actual electro-osmosis pumping force can be achieved at flow rate of 10 μL/min in our designed system. With a constant external DC electric field supplied, in which the external electric field exerts the same effect of voltage density on the whole particle interface, the flow resistance inside the pores deceases when water entering into the electro-osmosis pumping section. Moreover, the mass flow rate by the effect of electro-osmosis pumping can be achieved at 1.49 × 10−4 g/s under 5 V DC supply. To simplify the practical flow pattern in the micro-porous media, a specific micro-channel is assumed with a compact spherical stacking-up model. To simulate this geometrical model, a modified lattice Boltzmann method is set up in order to form the electro-osmotic flow of a practical problem from our experiment. The numerical results are obtained and analysed for fitting the present model with the micro-porous media. Both the experimental and numerical results have provided some useful instructions of electro-osmosis performance for designing micro-porous channels in such as filtering the compounds of drug delivery, particle purification and liquid separation applications etc.

The performance degradation of a passive direct formic acid fuel cell and its improvement by a hydrophobic filter

A direct formic acid fuel cell, DFAFC, can generate a higher power density compared with other direct liquid fuel cells, however, its performance decreases with time. Especially in a passive operation, the performance degradation was caused not only by catalyst poisoning at the anode but also serious flooding at the cathode due to high current operation. In this study, the effect of the flooding on the power generation characteristics of the DFAFC operated with three different air supplying modes, i.e., air breathing, air flowing and oxygen flowing, was investigated by comparing the cathode over-potential before and after 2 h operation at a constant voltage. Moreover, in order to inhibit the flooding, a hydrophobic filter, HF, was introduced at the surface of the cathode, and the effect of the HF on the flooding was investigated. It was found that the water crossover from the anode to the cathode significantly increased by the electroosmotic effect at a high current density, and it caused serious flooding when DFAFC was operated without HF. On the other hand, by inserting the HF, the performance degradation could be inhibited by reducing the flooding at the cathode surface, and the cathode over potentials could be improved. The HF is necessary for higher power generation and a stable operation of the passive DFAFC.

In situ particle zeta potential evaluation in electroosmotic flows from time‐resolved microPIV measurements

A time-resolved microPIV method is presented to measure in an EOF the particles zeta potential in situ during the transient start-up of a microdevice. The method resolves the electrophoretic velocity of fluoro-spheres used as tracer particles in microPIV. This approach exploits the short transient regime of the EOF generated after a potential drop is imposed across a microchannel and before reaching quasisteady state. During the starting of the transient regime, the electrophoretic effect is dominant in the center of the channel and the EOF is negligible. By measuring the velocity of the tracer particles with a microPIV system during that starting period, their electrophoretic velocity is obtained. The technique also resolves the temporal evolution of the EOF with three regions identified. The first region occurs before the electroosmotic effect reaches the center of the channel, the second region extends until the EOF reaches steady state, and thereafter is the third region. The two time constants separating these regions are also obtained and compared to the theory. The zeta potential of 860 nm diameter polystyrene particles is calculated for different solutions including borate buffer, sodium chloride, and deionized water. Results show that the magnitudes of the electrophoretic and electroosmotic velocities are in the range of |300| to |700| μm/s for these measurements. The zeta potential values are compared to the well-established closed cell technique showing improved accuracy. The method also resolves the characteristic response time of the EOF, showing small but important deviations from current analytical predictions. Additionally, the measurements can be performed in situ in microfluidic devices under actual working EOF conditions and without the need for calibrations.

Soil reinforcement experiment inside large-scale bucket foundation in muddy soil

The large-scale bucket foundation with 30 m in diameter and 6 m in height was used as the foundation of wind turbine. The wide-shallow foundation is different from the traditional bucket foundation with high ratio of height to diameter. The cover-load-bearing mode of the new type foundation can resist more external loadings. To achieve the bearing mode, the muddy soil inside the bucket should be reinforced, which will improve the soil strength and make the soil and foundation into a whole part to resist the external loadings. The vacuum and electro-osmotic soil reinforcing methods were used in the experiments. The results showed that the bearing behavior of the muddy soil were effectively improved by the negative pressure and electro-osmotic effect, and the improved muddy soil with better strength could work together with the bucket foundation, meaning that the top-cover bearing mode of the new bucket foundation was achieved. During the soil reinforcing process, the foundation moved downward, i.e., the settlement of foundation was almost finished during the pre-loading process caused by the vacuum and electro-osmotic method.

The Electro-Osmotic Device for Extensive Soil Dehydration

In order to improve the electro-osmotic effect in high water ratio soil dehydration consolidation, this paper introduced an electro-osmotic installation system for extensive soil draining which were composed by special power source, electrode, vacuum pump and flow measurement device etc. Among those above, the development of special power source was the core of the system, which was mainly introduced in this paper with power electrical technology and single chip SCM technology and constitutive digital high frequency switch mode power source. The system supported a plenty of technological operations. It was available in worksite.

Electrically assisted liquid hydrocarbon transport in porous media

Electrically assisted mass transport of hydrocarbon compounds in natural porous media can be used to recover residual oil from clay rich rock formations, for which other means of recovery is infeasible. Momentum transfer from electrosmotic flow of water achieved on the water wet pore surfaces, and increased permeability due to enlargement of capillaries by colloidal dislodging are thought to be some of the direct mechanisms responsible for oil transport under an applied electric field. In this paper, the efficacy of electrically assisted hydrocarbon recovery in laboratory and field cores of hydrocarbon bearing formations is investigated. In all the laboratory experiments, the mass transport was induced under a constant applied potential gradient, without supplemental water flooding under pressure. It was found that oil recovery was maximized for surrogate cores containing 10–15% clay by mass. At higher clay contents the recovery was limited by low charge efficiency, while at lower clay contents the recovery appeared to be influenced by colloidal transport. The electroosmotic pumping effect was investigated and found to potentially contribute to the oil transport to the cathode reservoir. Low permeability sandstone cores showed increased permeability under applied electric field. Larger oil recovery was observed in the cathode reservoir than in the anode reservoir.

A Variational Model for Two-Phase Immiscible Electroosmotic Flow at Solid Surfaces

AbstractWe develop a continuum hydrodynamic model for two-phase immiscible flows that involve electroosmotic effect in an electrolyte and moving contact line at solid surfaces. The model is derived through a variational approach based on the On-sager principle of minimum energy dissipation. This approach was first presented in the derivation of a continuum hydrodynamic model for moving contact line in neutral two-phase immiscible flows (Qian, Wang, and Sheng, J. Fluid Mech. 564, 333-360 (2006)). Physically, the electroosmotic effect can be formulated by the Onsager principle as well in the linear response regime. Therefore, the same variational approach is applied here to the derivation of the continuum hydrodynamic model for charged two-phase immiscible flows where one fluid component is an electrolyte exhibiting electroosmotic effect on a charged surface. A phase field is employed to model the diffuse interface between two immiscible fluid components, one being the electrolyte and the other a nonconductive fluid, both allowed to slip at solid surfaces. Our model consists of the incompressible Navier-Stokes equation for momentum transport, the Nernst-Planck equation for ion transport, the Cahn-Hilliard phase-field equation for interface motion, and the Poisson equation for electric potential, along with all the necessary boundary conditions. In particular, all the dynamic boundary conditions at solid surfaces, including the generalized Navier boundary condition for slip, are derived together with the equations of motion in the bulk region. Numerical examples in two-dimensional space, which involve overlapped electric double layer fields, have been presented to demonstrate the validity and applicability of the model, and a few salient features of the two-phase immiscible electroosmotic flows at solid surface. The wall slip in the vicinity ofmoving contact line and the Smoluchowski slip in the electric double layer are both investigated.

Feedback/feedforward modeling and control of electroosmotic flow in a T-shape microchannel

Electroosmotic effect is usually utilized to generate the flow field in microfluidic systems. In many of these microdevices, an accurate control over the output flow rate of the microfluidic part is necessary for the successful operation of the whole system. In this study, a combined feedback/feedforward strategy is proposed to control the output flow rate in a micro-T-junction. First, finite element model of the electroosmotic flow in the T-junction is generated; second, using the adaptive neural fuzzy inference system, the finite element model forms a basis for generating training data for building an inverse model of the flow in the micro-T-junction. This inverse model serves as a controller in the feedforward part of the system. Then, in order to make the controller robust against disturbances and uncertainties such as dimensional tolerances, a Mamdani-type fuzzy logic controller is incorporated in the feedback part of the controller. Finally, simulation results are presented in order to proof the performance of the designed controller.

Numerical modeling of tunable optofluidics lens based on combined effect of hydrodynamics and electroosmosis

A numerical model of liquid-core liquid-cladding optofluidics lens under the combined effect of hydrodynamics and electroosmosis are presented in this paper. In the numerical simulation, a combined formulation using only one set of conservation equations to treat both fluids are employed. The coupled electric potential equation and Navier–Stokes equation are solved using the finite volume method. The level-set method is used to capture the interface between the fluids. To overcome a weakness in the level-set method, the localized mass correction scheme is applied to ensure mass conservation. The validity of the numerical scheme is evaluated by comparing with the experimental results; numerical results highlight the electroosmotic effect; the combined effect of pressure driven and electroosmosis can form optically smooth interfaces with arc-shape between the cladding fluids and the core fluid. Under fixed cladding flow rates, the same electric field forms symmetric biconvex lens only. Different electric fields can form biconvex lens, plane-convex lens, and meniscus lens. The results also present the velocity profiles and flow fields of micro lens. There is a good agreement between numerical and experimental results.

Fully-developed thermal transport in combined electroosmotic and pressure driven flow of power-law fluids in microchannels

Thermally fully-developed heat transfer has been analyzed for combined electroosmotic and pressure driven flow of power-law fluid in a microchannel. Analytical expressions for transport parameters are presented in terms of the flow behavior index, the length scale ratio (ratio of Debye length to half channel height), dimensionless pressure gradient, and Joule heating parameter (ratio of Joule heating to surface heat flux). Closed form solutions are obtained for some specific values of the flow behavior index, while numerical solutions are presented for general cases. The results show that the temperature variation across the channel increases with increasing the pressure gradient. To reduce the length scale ratio is found to decrease the temperature variation, particularly for shear-thinning fluids. To increase the Joule heating parameter is to enlarge the temperature variation in the channel, especially for shear-thickening fluids. The Nusselt number can be increased by decreasing the length scale ratio due to the electroosmotic effect. Also, the Nusselt number increases with decreasing the values of flow behavior index and dimensionless pressure gradient.

Characterization and performance of the electroosmotic pumping effect for different porous media

High flow rate electroosmotic, EO, pumps are of great interest due to their simple design and non-moving parts. EO pumps were fabricated from two types of membranes: a never before tested, microcapillary array and the popular, anodic aluminum oxide (AAO). Flow rates and power consumption were measured directly for both membranes while efficiency was measured indirectly for the AAOs. A normalized flow rate of 1.90 mL/min/V/cm2 was recorded which is the highest normalized flow rate published. Large inefficiencies can occur due to electrolysis and electrode spacing and thus methods to decreasing these problems are discussed. Electroosmotic pumps were driven by constant voltage and asymmetric voltage pulses. The asymmetric voltage pulses negated electrolysis while producing a net flow. Voltage losses due to electrode spacing were minimized by platinum coatings.

Transport phenomena in proton exchange membrane fuel cells and over-potential distribution of membrane electrode assembly

To study the coupled phenomena occurring in proton exchange membrane fuel cells, a two-phase, one-dimensional, non-isothermal model is developed. The model includes water phase change, proton transport in the membrane and electro-osmotic effect. The thinnest, but most complex layer in the membrane electrode assembly, catalyst layer, is considered an interfacial boundary between the gas diffusion layer and the membrane. Mass and heat transfer and electro-chemical reaction through the catalyst layer are formulated into equations, which are applied to boundary conditions for the gas diffusion layer and the membrane. Detail accounts of the boundary equations and the numerical solving procedure used in this work are given. The polarization curve is calculated at different oxygen pressures and compared with the experimental results. When the operating condition is changed along the polarization curve, the change of physicochemical variables in the membrane electrode assembly is studied. In particular, the over-potential diagram presents the usage of the electro-chemical energy at each layer of the membrane electrode assembly. As the fuel cell reaction becomes more limited by mass transfer, it is found that higher over-potential is uselessly concentrated on the cathode catalyst layer. The over-potential for the anode reaction was usually ignored in other studies, but the ratio of the anode over-potential to the cathode over-potential increases at a higher current condition. Water content is distributed more unevenly in the membrane, as the cell current is increased. That causes the proton conductivity of membrane to decrease and the water content to increase in the cathode side, which hampers O 2 transfer. The effect of electro-osmotic property, one of important property of membrane, on cell performance is also studied.