Electroosmotic Effects Research Articles

Numerical Analysis of the Zeta Potential Regarding the Characteristics of a Ring-Type Electro-Osmotic Mixer

The micromixer that is applied for the electro-osmotic effect in this study, which is a passive type, takes two fluids from different inlets and combines them into a single channel. The fluids then enter the mixing chamber with different inner and outer radii. Four microelectrodes are positioned on the outer wall of the mixing chamber. The electric potentials on the four microelectrodes are sinusoidal for certain values of the time, voltage, zeta potential, and frequency. In order to check the validity of the two-dimensional numerical analysis that already has been performed, we performed a three-dimensional numerical analysis using a commercial code, COMSOL. The incompressible Navier-Stokes equation is solved in this model, with a slip boundary condition on the inner and outer walls of the mixing chamber. The results of two- and three-dimensional models are graphically depicted and compared from the viewpoint of streamlines and concentration.

Maximum efficiency of the electro-osmotic pump

Electro-osmotic effect in a porous medium arises from the electrically charged double layer at the fluid-solid interface, whereby an externally applied electric field can give rise to fluid flow. The electro-osmotic pump (EOP) is potentially useful for a variety of engineering and biorelated applications, but its generally low efficiency is a negative factor in this regard. A study to determine the optimal efficiency of the EOP and the condition(s) under which it can be realized is therefore of scientific interest and practical importance. We present the results of a theoretical and experimental study on the maximum efficiency optimization of the electrokinetic effect in artificially fabricated porous media with controlled pore diameters. It is shown that whereas the EOP efficiency increases with decreasing channel diameter, from 4.5 to 2.5 μm for samples fabricated on oxidized silicon wafers as expected for the interfacial nature of the electro-osmotic effect, the opposite trend was observed for samples with much smaller channel diameters fabricated on anodized aluminum oxide films, with the pore surface coated with silica. These results are in agreement with the theoretical prediction, based on the competition between interfacial area and the no-slip flow boundary condition, that an optimal efficiency of ~1% is attained at a microchannel diameter that is five times the Debye length, with a zeta potential of ~100 mV.

Electroosmotic-capillary penetration in circular microchannels

Electroosmotic forces can be used to promote penetration of electrolytes in microcapillaries. Analytical solutions for the dynamics of penetration are provided. The investigation includes both the electroosmotic and Poiseuille flows. For horizontal microcapillaries, the expression extends the Lucas–Washburn equation to account for the presence of electroosmotic effects. An analytical solution is also obtained for the case where gravity effects come into play.

Streaming potential generated by a pressure-driven flow over superhydrophobic stripes

The streaming potential generated by a pressure-driven flow over a weakly charged slip-stick surface [the zeta potential of the surface is smaller than the thermal potential (25 mV)] with an arbitrary double layer thickness is theoretically studied by solving the Debye–Huckel equation and Stokes equation. A series solution of the streaming potential is derived. Approximate expressions for the streaming potential in the limits of thin double layers and thick double layers are also given in excellent agreement with the full solution. To understand the impact of the slip, the streaming potential is compared against that over a homogeneously charged smooth surface. Our results indicate that the streaming potential over a superhydrophobic surface can only be enhanced under certain conditions. Moreover, as the double layer thickness increases, the advantage of the superhydrophobic surface diminishes. In addition, the Onsager relation which directly relates the magnitude of electro-osmotic effect to that of the streaming current effect has been explicitly proved to be valid for thin and thick double layers and homogeneously charged superhydrophobic surfaces. Comparisons between the streaming current and electro-osmotic mobility for an arbitrary electric double layer thickness under various conditions indicate that the Onsager relation seems applicable for arbitrary weakly charged superhydrophobic surfaces although there is no general proof. Knowledge of the streaming potential over a slip-stick surface can provide guidance for designing novel and efficient microfluidic energy-conversion devices using superhydrophobic surfaces.

Study of single particle charge and Brownian motions with surface plasmon resonance

We demonstrated a method to accurately measure the zeta potentials and surface charges of individual particles by surface plasmon resonance microscopy (SPRM). The principle is based on the sensitive dependence of surface plasmons in a metal surface on the distance between a particle and the surface. By applying a periodic (ac) electric field to the metal surface, the charged particle oscillates, which is measured with SPRM, from which the zeta potential and the surface charge of the particle can be determined. The ac method reduces the electro-osmotic effect and noises induced by Brownian motions and allows for the rapid determination of the zeta potentials of individual particles.

Study of sulfur dioxide crossover in proton exchange membrane fuel cells

As one of the most deleterious impurities to proton exchange membrane fuel cells (PEMFCs), sulfur dioxide (SO2) in air can pass through the membrane from the cathode to the anode and poison the catalyst of the two electrodes. The phenomenon of SO2 crossover is investigated electrochemically in this paper. The influences of SO2 concentration, relative humidity, gas pressure and current density on SO2 crossover are discussed. Experimental results reveal that the anode tends to be poisoned heavily with the increasing concentration of SO2 in the cathode. The coverage of the anode catalyst by SO2 permeating from the cathode enlarges with the decreasing relative humidity in the anode. The rate of SO2 crossover from the anode to the cathode is promoted at high current density when SO2 is directly introduced into the anode side instead of the cathode side, which can be ascribed to the electro-osmotic drag effect. Gas pressures show no obvious effects on SO2 crossover. A co-permeation mechanism of SO2 with water is deduced based on the overall analysis.

Microfluidic on-chip fluorescence-activated interface control system.

A microfluidic dynamic fluorescence-activated interface control system was developed for lab-on-a-chip applications. The system consists of a straight rectangular microchannel, a fluorescence excitation source, a detection sensor, a signal conversion circuit, and a high-voltage feedback system. Aqueous NaCl as conducting fluid and aqueous glycerol as nonconducting fluid were introduced to flow side by side into the straight rectangular microchannel. Fluorescent dye was added to the aqueous NaCl to work as a signal representing the interface position. Automatic control of the liquid interface was achieved by controlling the electroosmotic effect that exists only in the conducting fluid using a high-voltage feedback system. A LABVIEW program was developed to control the output of high-voltage power supply according the actual interface position, and then the interface position is modified as the output of high-voltage power supply. At last, the interface can be moved to the desired position automatically using this feedback system. The results show that the system presented in this paper can control an arbitrary interface location in real time. The effects of viscosity ratio, flow rates, and polarity of electric field were discussed. This technique can be extended to switch the sample flow and droplets automatically.

Microfluidic switch based on combined effect of hydrodynamics and electroosmosis

This paper presents theoretical and experimental investigations on valveless microfluidic switch using the coupled effect of hydrodynamics and electroosmosis. Switching of a non-conducting fluid stream is demonstrated. The first part of the investigation focused on flow switching of a non-conducting fluid, while the second part focused on switching of aqueous liquid droplets in a continuous oil stream. Two sheath streams (aqueous NaCl and glycerol) and a sample stream (silicon oil) are introduced by syringe pumps to flow side by side in a straight rectangular microchannel. External electric fields are applied on the two sheath streams. The switching process using electroosmotic effect for different flow rate and viscosity of sample stream is investigated. The results indicate that the switching response time is affected by the electric fields, flow rate, and viscosity of the sample. At constant inlet volumetric flow rates, the sample streams or droplets can be delivered to the desired outlet ports using applied voltages.

Electro-microfiltration treatment of water containing natural organic matter and inorganic particles

Natural organic matter (NOM) and inorganic particles are ubiquitous in many natural surface waters that serve as sources of drinking water. In addition to reacting with chlorine to form chlorinated disinfection by-products (DBPs), NOM and NOM sorbed on inorganic particles cause serious membrane fouling problems. We evaluated the feasibility of using a novel laboratory-scale electro-microfiltration (EMF) system to remove NOM and inorganic particles from water. Synthetic water samples containing 5 mg/L humic acid (HA) only (Kao0), 5 mg/L HA and 5 mg/L kaolin (Kao1), and 5 mg/L HA and 50 mg/L kaolin (Kao2) were used in the EMF study. Kao2 demonstrated the largest flux decline in the absence of an electrical field. When an electrical field was applied, the filtration flux, NOM removal, and decrease in specific ultraviolet absorbance (SUVA) were enhanced because of electrophoretic and electroosmotic effects. When the applied electrical field strength was greater than a critical electrical field strength ( E critical), system operations and efficiency did not increase further. These results suggest that the applied electrical field should be close to E critical for maximum efficiency and cost-effectiveness. This study demonstrates that an EMF system is a highly effective way to treat water containing NOM and inorganic particles.

Analysis of Electrokinetic Mixing Techniques Using Comparative Mixing Index

The performance of micro-mixers is evaluated in terms of deviations from perfectly mixed state and mixing length (i.e., device length required to achieve perfect mixing). Different variations of T-mixer are reported for improved mixing performance, including geometric constrictions/obstacles embedded in the channel wall, heterogeneously charged walls, grooves on channel base, etc. Most of the reported designs provide improved mixing at the expense of reduced flow rate; there exists therefore a tradeoff between mixing and transport. The reduced flow rate, which affects species residence time, is unfortunately not taken into account in most micro-mixing performance analyses. This issue is addressed by the comparative mixing index (CMI), which evaluates mixing performance more appropriately by normalizing the effect of residence time among different designs. In this study, the performance of several mixing strategies are evaluated based on the CMI; these are mixer designs that incorporate (a) physical constrictions, (b) induced charge electro-osmotic (ICEO) effects, and (c) heterogeneously charged walls. The present analysis clearly identifies conditions under which a given mixer design is superior to a T-mixer.

Experimental investigation on possibility of electro-osmotic regeneration for solid desiccant

This paper aims to investigate the possibility and performance of a novel electro-osmotic regeneration method for the solid desiccant system, which could adsorb water from the moist air and be regenerated by the electro-osmotic force at the same time, by detailed experimental validation. The zeolite powder with both electro-osmotic and adsorptive characteristics was selected to make the solid desiccant. The quantity of the removed water was measured in the condition with and without an applied electric field to validate the possibility of the electro-osmotic regeneration. The test data show that there was an obvious electro-osmotic regeneration effect in the experiments. The whole electro-osmosis process was divided into three phases because of the Joule heating effect and electrode corrosion, which limited the performance of the experimental system. In particular, a filter cloth under the cathode made a superior electro-osmotic regeneration effect than without the cloth. The experimental results illustrate that the system has the potential of energy-saving and low regeneration temperature. So the electro-osmotic regeneration method is a potential regeneration method for the solid desiccant system.

Improved estimates of power consumption during dewatering of mine tailings using electrokinetic geosynthetics (EKGs)

Electrokinetic geosynthetics (EKGs) were developed about a decade ago, producing a material that did not experience the same corrosion problems as metal electrodes when used in electro-osmotic (EO) dewatering applications. The conductive polymeric material from which EKGs are made has the additional advantage of being able to be formed into a variety of shapes, including drainage tubes such as those used in this study. One of the primary concerns regarding EO dewatering applications is the rate of energy consumption. This paper reports experiments at three different scales, on different mine tailings materials, that illustrates the potentially misleading results obtained from small scale laboratory tests. Using large, outdoor tests, power consumption rates for treating a kaolinitic mineral sands tailings and a smectitic diamond tailings were found to be up to 30 times lower than the power consumption rates obtained from laboratory tests using small samples in a purpose built EO cell. The primary reason for the improved performance is thought to be the lower voltage gradient used in the field tests, plus the geometric advantages offered by a field layout where a greater volume of material is exposed to the EO effect than only the material directly between the electrodes, which is the case in the EO cell tests. Finally, in the field tests the EO treatment caused the early development of vertical cracks in the tailings, resulting in increased exposure of the tailings to the effects of solar drying. This effect cannot obviously be reproduced in the EO cell laboratory tests, and can be a major advantage for the in situ treatment of mine tailings lagoons, where in some materials the onset of cracking is significantly delayed by the development of an impermeable surface crust.

Single colloid electrophoresis

Optical tweezers enable one to trap a single particle without any mechanical contact and to measure its position and the forces acting on it with high resolution (±4 nm, ±160 fN). Taking advantage of a specially designed microfluidic cell the electrophoretic response of the colloid under study and the electroosmotic effect on the surrounding medium are determined using the identical colloid. The former is found to be by more than one order of magnitude larger than the electroosmotic effect. It is shifted in phase with respect to the external field, hence giving rise to a complex electrophoretic mobility which can be theoretically described by a strongly damped driven harmonic oscillator model. By exchanging the medium surrounding the colloid it is possible to deduce the (KCl) concentration dependence of the single colloid electrophoretic response. The results are compared with conventional Zetasizer measurements.

Molecular dynamics simulations on the ionic current through charged nanopores

Molecular dynamics (MD) simulation was performed to investigate the ionic current through charged nanopores, and the results were compared with the calculation of Poisson–Nernst–Planck (PNP) equations based on the continuum theory. Results show that the current obtained by MD simulation is lower than the current calculated by PNP equations, and the discrepancy depends on the surface charge density of the nanopores. Also, MD simulation shows that the contribution of the electro-osmotic flow effect on ionic current could be 10% higher than the results obtained by solving PNP equations. Since the PNP equations do not take the effect of the pore wall into consideration, we suggest that adjusting the diffusion coefficient in the PNP equations can obtain more accurate results when calculating the ionic current through charged nanopores.

Enhanced biodegradation of pentachlorophenol in unsaturated soil using reversed field electrokinetics

This study investigated the use of electrokinetics in unsaturated soil to promote biodegradation of pentachlorophenol through increased contact between bacteria and contaminant. Soil microcosms, contaminated with approximately 100 mg kg −1 pentachlorophenol (containing [ 14C]-PCP as a tracer), and inoculated with a specific pentachlorophenol-degrading bacterium ( Sphingobium sp. UG30–1 × 10 8 cfu g −1) were subjected to constant and regularly reversed electric currents (10 mA). The former caused large pH and moisture content changes due to water electrolysis and electroosmotic effects, with subsequent negative impacts on biodegradation parameters including enzyme activity and contaminant mineralisation (as measured by 14CO 2 evolution rate). The reversed field caused little change in pH and moisture content and led to more rapid contaminant mineralisation, lower soil contaminant concentration in the majority of the microcosms and increased soil enzyme activity (with the exception of soil immediately adjacent to the anode). The presence of an electric field, if suitably applied, may therefore enhance contaminant biodegradation in unsaturated soil.

Electroosmotic control of width and position of liquid streams in hydrodynamic focusing

This paper presents theoretical and experimental investigations on electroosmotic control of stream width in hydrodynamic focusing. In the experiments, three liquids (aqueous NaCl, aqueous glycerol and aqueous NaCl) are introduced by syringe pumps to flow side by side in a straight rectangular microchannel. External electric fields are applied on the two aqueous NaCl streams. Under the same inlet volumetric flow rates, the applied electric fields are varied to control the interface positions and consequently the width of the focused aqueous glycerol stream. The electroosmotic effect on the width of the aqueous glycerol is measured using fluorescence imaging technique. The electroosmotic effect under different flow rates, different viscosity, and aspect ratio are investigated. The results indicate that the electroosmotic effect on the pressure-driven flow becomes weaker with the increase in flow rates, viscosity ratio or aspect ratio of the channel. The measured results of the focused width of the non-conducting fluid agree well with the analytical model.

Interfacial Phenomena and Dynamic Contact Angle Modulation in Microcapillary Flows Subjected to Electroosmotic Actuation

The dynamic evolution of an incompressible liquid meniscus inside a microcapillary is investigated, under the combined influences of viscous, capillary, intermolecular, pondermotive, and electroosmotic effects. In the limit of small capillary numbers, an advancing meniscus shape is shown to merge smoothly with the precursor film, using matched asymptotic analysis. A scaling relationship is also established for the dynamic contact angle as a nondimensional function of the capillary number and the applied electrical voltage. The analysis is further generalized by invoking a kinetic slip model for overcoming the constraints of meniscus tip singularity. The kinetic slip model is subsequently utilized to analyze the interfacial dynamics from the perspective of the results obtained from the matched asymptotic analysis. A generalization is achieved in this regard, which may provide a sound basis for controlling the topographical features of a dynamically evolving meniscus in a microcapillary subjected to electrokinetic effects. These results are also in excellent agreement with the experimental findings over a wide range of capillary number values.

Electrophoretic Motion of a Spherical Particle with a Symmetric Nonuniform Surface Charge Distribution in a Nanotube

The electrophoretic motion of a spherical nanoparticle, subject to an axial electric field in a nanotube filled with an electrolyte solution, has been investigated using a continuum theory, which consists of the Nernst-Planck equations for the ionic concentrations, the Poisson equation for the electric potential in the solution, and the Stokes equation for the hydrodynamic field. In particular, the effects of nonuniform surface charge distributions around the nanoparticle on its axial electrophoretic motion are examined with changes in the bulk electrolyte concentration and the surface charge of the tube's wall. A particle with a nonuniform charge distribution is shown to induce a corresponding complex ionic concentration field, which in turn influences the electric field and the fluid motion surrounding the particle and thus its electrophoretic velocity. As a result, contrary to the relatively simple dynamics of a particle with a uniform surface charge, dominated by the irradiating electrostatic force, that with a nonuniform surface charge distribution shows various intriguing behaviors due to the additional interplay of the nonuniform electro-osmotic effects.

Field-induced phases of an orientable charged particle in a dilute background of point charges

We report numerical simulations of a two-dimensional dynamical model comprised of a rodlike particle surrounded by a cloud of smaller particles of the same charge, in the presence of an alternating electric field inside a box. We show that this system displays a remarkable dynamical effect; at low forcing frequencies the rod tends to align perpendicularly to the external field, whereas for higher field frequencies the standard orientation (parallel to the field) prevails. Interestingly, the transition between orientations is abrupt enough to resemble a phase transition. The fact that the "anomalous" orientation (perpendicular to the field) takes place is also interesting in the light of some recent laboratory experiments on colloidal solutions, where anomalous orientation at low frequencies was observed. Our toy model suggests that future physically realistic simulations of these systems should explore whether the anomalous orientation may be due to the collective dynamics of the colloidal particles, without necessarily involving more sophisticated electro-osmotic effects.

Study of the Influence of Fibrous Pericellular Matrix in the Cortical Interstitial Fluid Movement With Hydroelectrochemical Effects

Fluid flow within cortical bone tissue is modeled through an upscaling approach of a local description of the fluid movement. At the pore scale, the coupled phenomena (Poiseuille effect, osmosis, and electro-osmosis) governing the interstitial fluid movement are considered. Thus, actions of electro-osmotic and osmotic motions, in addition to the classical Poiseuille flow, are studied at the canaliculus scale by deriving a coupled Darcy law. The addition of a Brinkman-like term in this macroscopic result helps us to take into account the influence of the pericellular matrix on the coupled transport phenomena. At the canaliculus scale, the general trends that can be drawn from this study are as follows: (i) The presence of the fibrous matrix tends to reduce the fluid flow considerably; (ii) the role of osmotic and electro-osmotic effects is no longer negligible for dense fibrous media.