An electroacoustic and high-frequency dielectric response study of stagnant layer conduction in emulsion systems

Stagnant layer conduction (or anomalous surface conduction) in perfluoromethyldecalin (PFMD) and n-hexadecane emulsions has been measured by electroacoustics and verified by high-frequency dielectric response experiments. The electroacoustic technique can detect the presence of stagnant layer conduction from the salt dependence of the dynamic mobility. As the indifferent electrolyte concentration is increased from low values (<5 mM), the zeta-potential and droplet size, estimated from the dynamic mobility by the normal procedures, gradually increase in magnitude until the size plateaus and the zeta-potential begins to decrease with added salt in the usual fashion. When stagnant layer conduction is taken into account, the dynamic mobility can be fitted to a constant size distribution and more realistic zeta-potential values with varying electrolyte concentration. High-frequency dielectric response has been used to measure the total conduction in a PFMD emulsion system. Very good agreement between these two independent techniques verifies the existence of conduction behind the shear plane and demonstrates that electroacoustics alone can detect and quantify its extent. This is possible because of the unique character of the AcoustoSizer procedure, which estimates both particle size and zeta-potential from the same signal.     

On surface conduction and its role in electrokinetics

A review is given of the role of surface conduction in electrokinetics. Experimental evidence is considered to indicate when such conduction has to be considered and to what extent this conduction is carried by charges within the stagnant layer. For non-penetrable surfaces, lateral mobilities of monovalent ions in the stern layer are not much lower than those in bulk and under certain conditions conduction behind the slip plane may be of the same order of magnitude as that beyond it. New experiments on the conductivity of polystyrene latex plugs illustrate these phenomena.     

Determination of stagnant layer conductivity in polystyrene suspensions: temperature effects

In the classical theory of electrokinetic phenomena, it is admitted that the whole electrokinetic behavior of any colloidal system is fully determined by the zeta potential, zeta, of the interface. However, both experimental data and theoretical models have shown that this is an incomplete picture, as ions in the stagnant layer (the region between the solid surface and the slip plane--the plane where the equilibrium potential equals zeta) may respond to the field. In this paper, we aim at the evaluation of this contribution by the estimation of both K(SL)(sigma) (the surface conductivity of the stagnant layer) and K(d)(sigma) (the conductivity associated with the diffuse layer). This will be done by measuring the high-frequency dielectric dispersion (HFDD) in polystyrene suspensions; here "high-frequency" means the frequency interval where Maxwell-Wagner-O'Konski relaxation takes place (typically at MHz frequencies). Prior to any conclusions, a treatment of electrode polarization effects in the measurements was needed: we used two methods, and both led to similar results. Simulating the existence of surface conductivity by bulk conductivity, we reached the conclusion that no consistent explanation can be given for our HFDD and additional electrophoresis data based on the existence of diffuse-layer conductivity alone. We thus show how K(SL)(sigma) can be estimated and demonstrate that it can be explained by an ionic mobility very close to that characteristic of ions in the bulk solution. Such mobility, and hence also the values of K(SL)(sigma), increases with temperature as expected on simple physical grounds.     

Faradaic double layer depolarization in electrokinetics: Onsager relations and substrate limitations

More often than not, the measurement of interfacial potentials by means of electrokinetic techniques is affected by interfering processes that may relax or even annihilate their primary response function. Among these processes are faradaic ones, provided that the substrate is sufficiently conducting and a redox function is available, and non-faradaic ones, if geometrical constraints are in effect. Ample experimental evidence is available, e.g., in the collapse of streaming potentials generated by metal/electrolyte solution interfaces, the bipolar microelectrodic redox processes in fluidized beds of metallic particles, and the "superfast" electrophoresis of dispersed ion exchanger particles and electron-conducting particles. Common feature of these apparently disparate phenomena is that the lateral electric field is affected by coupling with transversal depolarization fields, or by conductance gradients due to Donnan effects. Recent work has rigorously analyzed the deformation of the lateral electric field in a (streaming potential) slit cell by electron transfer reactions at the interface, taking into account both convective diffusion of the electroactive species and kinetics of the interfacial electron transfer reaction. Here a common, generic basis for faradaic and non-faradaic double layer depolarization is formulated along the lines set by Onsager, and methodologies for retrieving the underlying electrokinetic parameters from experimental data are evaluated. Particular attention is paid to the limitations of double layer polarization, as posed by the substrate.     

Modeling electrokinetics in ionic liquids

Using direct numerical simulations, we provide a thorough study regarding the electrokinetics of ionic liquids. In particular, modified Poisson–Nernst–Planck equations are solved to capture the crowding and overscreening effects characteristic of an ionic liquid. For modeling electrokinetic flows in an ionic liquid, the modified Poisson‐Nernst‐Planck equations are coupled with Navier–Stokes equations to study the coupling of ion transport, hydrodynamics, and electrostatic forces. Specifically, we consider the ion transport between two parallel charged surfaces, charging dynamics in a nanopore, capacitance of electric double‐layer capacitors, electroosmotic flow in a nanochannel, electroconvective instability on a plane ion‐selective surface, and electroconvective flow on a curved ion‐selective surface. We also discuss how crowding and overscreening and their interplay affect the electrokinetic behaviors of ionic liquids in these application problems.     

The water-amorphous silica interface: analysis of the Stern layer and surface conduction

To explain why dynamical properties of an aqueous electrolyte near a charged surface seem to be governed by a surface charge less than the actual one, the canonical Stern model supposes an interfacial layer of ions and immobile fluid. However, large ion mobilities within the Stern layer are needed to reconcile the Stern model with surface conduction measurements. Modeling the aqueous electrolyte-amorphous silica interface at typical charge densities, a prototypical double layer system, the flow velocity does not vanish until right at the surface. The Stern model is a good effective model away from the surface, but cannot be taken literally near the surface. Indeed, simulations show no ion mobility where water is immobile, nor is such mobility necessary since the surface conductivity in the simulations is comparable to experimental values.     

Calculation of the dynamic impedance of the double layer on a planar electrode by the theory of electrokinetics

Applications of microelectromechanical systems in the biotechnological arena (bioMEMS) are a subject of great current interest. Accurate calculation of electric field distribution in these devices is essential to the understanding and design of processes such as dielectrophoresis and AC electroosmosis that drive MEMS-based devices. In this paper, we present the calculation of the electrical double-layer impedance (Z(el)) of an ideally polarizable plane electrode using the standard model of colloidal electrokinetics. The frequency variation of the electrical potential drop across the double layer above a planar electrode in a general electrolyte solution is discussed as a function of the electrode zeta potential zeta, the Debye length kappa(-1), the electrolyte composition and the bulk region thickness L.     

Effect of chemical composition on electrokinetics of diaspore

The effect of chemical composition of diaspores on their electrokinetics was studied. Increasing SiO(2) content in a diaspore sample was found to decrease its isoelectric point. The X-ray diffraction and SEM microanalysis showed the absence of distinct SiO(2) phases. A linear correlation was found to exist between the measured isoelectric point and alumina to silica mass ratio in diaspore samples. The linear regression analysis of the experimental data suggests a more significant impact of silicon content than aluminum content, indicating a preferential adsorption of dissolved silicon on diaspore samples and/or preferential dissolution of aluminum from diaspore samples.     

Aging effects in the electrokinetics of colloidal iron oxides

We analyze in this contribution the effect of aging on the electrokinetic properties of magnetite (Fe(3)O(4)) and hematite (alpha-Fe(2)O(3)). In both cases, high-purity commercial samples and monodisperse synthetic particles were studied. Commercial magnetite showed a rather erratic dependence of its electrophoretic mobility u(e) with the concentration of NaCl. Furthermore, sufficient concentrations of the latter were able to change the sign of the mobility. When KNO(3) solutions were used, although no such change was observed, no clear effect of [KNO(3)] on the mobility was found, and, in addition, an intense aging effect was detected, as the mobility became increasingly positive in suspensions that were stored over 1 day. The picture was radically different with synthetic magnetite spheres, as the expected overall decrease of u(e) with either NaCl or KNO(3) concentration was measured. However, also in this case the aging effect was clearly observed: u(e) tended in this case to more negative values upon suspension storage, and a steady value of the mobility was reached only after 5 days in NaCl (and even longer in KNO(3) solutions). Because of the crystal structure similarities between magnetite and maghemite (gamma-Fe(2)O(3)), it has been shown that the final step of magnetite oxidation is maghemite. This is confirmed in the present study, as the mobility-pH trends of magnetite progressively approach those of maghemite after about 7 days of storage. Since hematite is chemically more stable than magnetite, our study focused in this case on the comparison between commercial and synthetic particles. The former showed a negative mobility at pH 5.5 under all conditions, suggesting an isoelectric point well below the value accepted for hematite (>/=7). The effect of aging on commercial samples was again very significant, as u(e) decreased in absolute value, apparently without limit as the time since preparation was longer. In contrast, synthetic hematite showed a more predictable dependence on ionic strength, and more limited aging effects, as u(e) reached equilibrium values after around 5 days in NaCl; longer times were required in KNO(3) solutions.     

AC electrokinetics of conducting microparticles: A review

This paper reviews both theory and experimental observation of the AC electrokinetic properties of conducting microparticles suspended in an aqueous electrolyte. Applied AC electric fields interact with the induced charge in the electrical double layer at the metal particle–electrolyte interface. In general, particle motion is governed by both the electric field interacting with the induced dipole on the particle and also the induced-charge electro-osmotic (ICEO) flow around the particle. The importance of the RC time for charging the double layer is highlighted. Experimental measurements of the AC electrokinetic behaviour of conducting particles (dielectrophoresis, electro-rotation and electro-orientation) are compared with theory, providing a comprehensive review of the relative importance of particle motion due to forces on the induced dipole compared with motion arising from induced-charge electro-osmotic flow. In addition, the electric-field driven assembly of conducting particles is reviewed in relation to their AC electrokinetic properties and behaviour.     

Hybrid continuum-atomistic approach to model electrokinetics in nanofluidics

In this study, for the first time, a hybrid continuum-atomistic based model is proposed for electrokinetics, electroosmosis and electrophoresis, through nanochannels. Although continuum based methods are accurate enough to model fluid flow and electric potential in nanofluidics (in dimensions larger than 4 nm), ionic concentration is too low in nanochannels for the continuum assumption to be valid. On the other hand, the non-continuum based approaches are too time-consuming and therefore is limited to simple geometries, in practice. Here, to propose an efficient hybrid continuum-atomistic method of modelling the electrokinetics in nanochannels; the fluid flow and electric potential are computed based on continuum hypothesis coupled with an atomistic Lagrangian approach for the ionic transport. The results of the model are compared to and validated by the results of the molecular dynamics technique for a couple of case studies. Then, the influences of bulk ionic concentration, external electric field, size of nanochannel, and surface electric charge on the electrokinetic flow and ionic mass transfer are investigated, carefully. The hybrid continuum-atomistic method is a promising approach to model more complicated geometries and investigate more details of the electrokinetics in nanofluidics.     

High ionic strength electrokinetics of melamine-formaldehyde latex

The electrokinetic potential of melamine-formaldehyde latex at high ionic strengths was measured by means of two different instruments. The present study confirms that the zeta potentials in 1 M 1-1 electrolyte solutions can be as high as +/-20 mV. The IEP of latex at low ionic strengths was at pH 11. The increase in the electrolyte concentration induced a shift in the IEP to low pH for all studied salts, and this indicates specific adsorption of the anions. The magnitude of the shift depends chiefly on the nature of the anion and increases in the series Cl < NO(3) = Br < I, and the nature of the cation (Li, Na, K, Cs) plays a rather insignificant role.     

Concentration polarization-based nonlinear electrokinetics in porous media: induced-charge electroosmosis

We have investigated induced-charge electroosmotic flow in a fixed bed of ion-permselective glass beads by quantitative confocal laser scanning microscopy. Externally applied electrical fields induce concentration polarization (CP) in the porous medium due to coupled mass and charge transport normal to the charge-selective interfaces. These data reveal the generation of a nonequilibrium electrical double layer in the depleted CP zones and the adjoining anodic hemispheres of the (cation-selective) glass beads above a critical field strength. This initiates CP-based induced-charge electroosmosis along curved interfaces of the quasi-electroneutral macropore space between glass beads. Caused by mutual interference of resulting nonlinear flow with (flow-inducing) space charge regions, an electrohydrodynamic instability can appear locally and realize turbulent flow behavior at low Reynolds numbers. It is characterized by a local destruction of the CP zones and concomitant removal of diffusion-limited mass transfer. More efficient pore-scale lateral mixing also improves macroscopic transport, which is reflected in the significantly reduced axial dispersion of a passive tracer.     

Faradaic depolarization in the electrokinetics of the metal-electrolyte solution interface

Streaming potentials (E(str)) have been measured in a flat thin-layer cell with gold and aluminum surfaces. The conventional relation between E(str) and the zeta-potential is shown to be applicable only as long as charge transfer reactions at the metal-electrolyte solution interface are insignificant in terms of the ensuing contribution to the overall cell conductivity. Owing to the irreversibility of the reduction/oxidation of water at most metal surfaces, streaming potentials can be obtained over a very broad range of pressure gradients for metallic substrates in electrolytes such as KNO3. The situation changes drastically in the presence of a reversible redox couple like Fe(CN)(6)3-/Fe(CN)(6)4-. Even small streaming potentials are then greatly diminished due to the extensive conduction that results from the bipolar electrolysis at the metal surface. For gold and aluminum in the presence of various electroinactive and electroactive electrolytes, the measured values for E(str) are shown to be consistent with their conventional voltammetric characteristics.     

Formation of a colloidal band via pH-dependent electrokinetics

Electroosmosis on nonuniformly charged surfaces often gives rise to intriguing flow behaviors, which can be utilized in applications such as mixing processes and designing micromotors. Here, we demonstrate nonuniform electroosmosis induced by electrochemical reactions. Water electrolysis creates pH gradients near the electrodes that cause a spatiotemporal change in the wall zeta potential, leading to nonuniform electroosmosis. Such nonuniform EOFs induce multiple vortices, which promote the continuous accumulation of particles that subsequently form a colloidal band. The band develops vertically into a “wall” of particles that spans from the bottom to the top surface of the chamber. Such a flow-driven colloidal band can be potentially used in colloidal self-assembly and separation processes irrespective of the particle surface properties. For instance, we demonstrate these vortices can promote rapid segregation of soft colloids such as oil droplets and fat globules.     

Hard versus soft particle electrokinetics of silica colloids

To verify the existence of a gel layer at the surface of silica, dependences of the electrophoretic mobility of fresh and aged colloidal silica particles on the KCl concentration are measured. These dependences, corrected for the relaxation/polarization effect, are fitted by analytical expressions based on the model of hard, soft, and brush surfaces. A bad fit is obtained for both silicas when its surface is considered ideal (hard). Much better fits are achieved with the invariable soft layer model for the fresh silica but especially for the aged silica whose surface is less charged probably as a result of an extension and/or loosening of the layer. A perfect fit is found for aged silica when applying a trivial model of the soft polyelectrolyte layer combined with the scaling model of polyelectrolyte brushes.     

Nonlinear effects on electrokinetics of a highly charged porous sphere

The motivation of the present study is to provide a correct estimate of the electrophoretic mobility of a charged porous particle for wide-range electrokinetic parameters, such as particle charge density, permeability, and Debye length. Based on the Nernst–Planck equation, which takes into account the external electric field and fluid convection on ion transport, we have estimated the mobility of the particle by establishing a force balance. We have validated our results with the linear model due to Hermans and Fujita (K Nederl Akad Wet Proc Ser B 58:182–187, 1955) and the computed solution based on perturbation of the Poisson–Boltzmann model as obtained by Hsu and Lee (J Colloid Interface Sci 390:85–95, 2013). For the case of thin double layer, our computed results agree with the linear model even for large values of charge density of the particle. The linear model overpredicts our computed solution for mobility when the thick Debye layer is considered. However, a large discrepancy of the present model from the results based on the perturbation of the Boltzmann model is observed for all the cases considered. We have analyzed the double-layer polarization and counterion condensation through the distribution of counterions, net charge density, and the effective charge density of the particle.     

Recent applications of AC electrokinetics in biomolecular analysis on microfluidic devices

AC electrokinetics is a generic term that refers to an induced motion of particles and fluids under nonuniform AC electric fields. The AC electric fields are formed by application of AC voltages to microelectrodes, which can be easily integrated into microfluidic devices by standard microfabrication techniques. Moreover, the magnitude of the motion is large enough to control the mass transfer on the devices. These advantages are attractive for biomolecular analysis on the microfluidic devices, in which the characteristics of small space and microfluidics have been mainly employed. In this review, I describe recent applications of AC electrokinetics in biomolecular analysis on microfluidic devices. The applications include fluid pumping and mixing by AC electrokinetic flow, and manipulation of biomolecules such as DNA and proteins by various AC electrokinetic techniques. Future prospects for highly functional biomolecular analysis on microfluidic devices with the aid of AC electrokinetics are also discussed.