Electroosmotic Research Articles

The influence of pulp and interfacial chemistry and mode of electrical power input on electroosmotic dewatering of Na-exchanged smectite dispersions

Abstract In this study, pulp and interfacial chemistry, gravity settling/drainage and electroosmotic (EO) dewatering behaviour of two types of fine, Na-exchanged smectite clay (high and low Na content montmorillonites) slurries were investigated as a function of background electrolyte ionic strength and time at pH 7.5 with constant voltage (0.5 vs. 1 V/cm initial field strength) or current density (12.5 vs. 25 A/m2). The two smectite variants displayed significantly different mineral chemistry and particle zeta potential, the latter dependent on Ca(II) background. Suppressed swelling and marked reduction in zeta potential magnitude occurred at 0.03–0.3 M Ca(II) ionic strength, the extent of which was greater for low than high Na-smectite pulps. Ca(II) ionic strength of ~0.1 M improved pre-EO gravity dewatering from

Surface Modification of Electroosmotic Silicon Microchannel Using Thermal Dry Oxidation.

A simple fabrication method for the surface modification of an electroosmotic silicon microchannel using thermal dry oxidation is presented. The surface modification is done by coating the silicon surface with a silicon dioxide (SiO2) layer using a thermal oxidation process. The process aims not only to improve the surface quality of the channel to be suitable for electroosmotic fluid transport but also to reduce the channel width using a simple technique. Initially, the parallel microchannel array with dimensions of 0.5 mm length and a width ranging from 1.8 µm to 2 µm are created using plasma etching on the 2 cm × 2 cm silicon substrate <100>. The oxidation of the silicon channel in a thermal chamber is then conducted to create the SiO2 layer. The layer properties and the quality of the surface are analyzed using scanning electron microscopy (SEM) and a surface profiler, respectively. The results show that the maximum oxidation growth rate occurs in the first 4 h of oxidation time and the rate decreases over time as the oxide layer becomes thicker. It is also found that the surface roughness is reduced with the increase of the process temperature and the oxide thickness. The scallop effect on the vertical wall due to the plasma etching process also improved with the presence of the oxide layer. After oxidation, the channel width is reduced by ~40%. The demonstrated method is suggested for the fabrication of a uniform channel cross section with high aspect ratio in sub-micro and nanometer scale that will be useful for the electroosmotic (EO) ion manipulation of the biomedical fluid sample.

Optimizing the longitudinal and transverse electroosmotic pumping in a rectangular channel with horizontal baffle plates

This paper presents a continued study to our previous work on electroosmotic (EO) flow in a channel with vertical baffle plates by further investigating EO flow through an array of baffle plates arranged in parallel to the channel walls. The flow may be driven either in the direction along or in the direction transverse to the plates, thus distinguishing the longitudinal EO pumping (LEOP) and the transverse EO pumping (TEOP). In both types of EO pumping, it is more interesting to examine the cases when the baffle plates develop a higher zeta potential (denoted by α) than that on the channel walls (β). This semi-analytical study enables us to compare between LEOP and TEOP in the pumping efficiency under similar conditions. The TEOP case is more difficult to solve due to the higher order governing partial differential equations caused by the induced non-uniform pressure gradient distribution. In particular, we examine how the EO pumping rates deviate from those predicted by the Helmholtz-Smoluchowski velocity and illustrate the general trend of optimizing the EO pumping rates with respect to the physical and geometric parameters involved.

Separating large microscale particles by exploiting charge differences with dielectrophoresis

Dielectrophoresis (DEP), the migration of particles due to polarization effects under the influence of a nonuniform electric field, was employed for characterizing the behavior and achieving the separation of larger (diameter >5 μm) microparticles by exploiting differences in electrical charge. Usually, electrophoresis (EP) is the method of choice for separating particles based on differences in electrical charge; however, larger particles, which have low electrophoretic mobilities, cannot be easily separated with EP-based techniques. This study presents an alternative for the characterization, assessment, and separation of larger microparticles, where charge differences are exploited with DEP instead of EP. Polystyrene microparticles with sizes varying from 5 to 10 μm were characterized employing microdevices for insulator-based dielectrophoresis (iDEP). Particles within an iDEP microchannel were exposed simultaneously to DEP, EP, and electroosmotic (EO) forces. The electrokinetic behavior of four distinct types of microparticles was carefully characterized by means of velocimetry and dielectrophoretic capture assessments. As a final step, a dielectropherogram separation of two distinct types of 10 μm particles was devised by first characterizing the particles and then performing the separation. The two types of 10 μm particles were eluted from the iDEP device as two separate peaks of enriched particles in less than 80 s. It was demonstrated that particles with the same size, shape, surface functionalization, and made from the same bulk material can be separated with iDEP by exploiting slight differences in the magnitude of particle charge. The results from this study open the possibility for iDEP to be used as a technique for the assessment and separation of biological cells that have very similar characteristics (shape, size, similar make-up), but slight variance in surface electrical charge.

Enhanced cell trapping throughput using DC-biased AC electric field in a dielectrophoresis-based fluidic device with densely packed silica beads.

This paper presents the use of DC-biased AC electric field for enhancing cell trapping throughput in an insulator-based dielectrophoretic (iDEP) fluidic device with densely packed silica beads. Cell suspension is carried through the iDEP device by a pressure-driven flow. Under an applied DC-biased AC electric field, DEP trapping force is produced as a result of non-uniform electric field induced by the gap of electrically insulating silica beads packed between two mesh electrodes that allow both fluid and cells to pass through. While the AC component is mainly to control the magnitude of DEP trapping force, the DC component generates local electroosmotic (EO) flow in the cavity between the beads and the EO flow can be set to move along or against the main pressure-driven flow. Our experimental and simulation results show that desirable trapping is achieved when the EO flow direction is along (not against) the main flow direction. Using our proposed DC-biased AC field, the device can enhance the trapping throughput (in terms of the flowrate of cell suspension) up to five times while yielding almost the same cell capture rates as compared to the pure AC field case. Additionally, the device was demonstrated to selectively trap dead yeast cells from a mixture of flowing live and dead yeast cells.

Facilitated polymer capture by charge inverted electroosmotic flow in voltage-driven polymer translocation.

The optimal functioning of nanopore-based biosensing tools necessitates rapid polymer capture from the ion reservoir. We identify an ionic correlation-induced transport mechanism that provides this condition without the chemical modification of the polymer or the pore surface. In the typical experimental configuration where a negatively charged silicon-based pore confines a 1 : 1 electrolyte solution, anionic polymer capture is limited by electrostatic polymer-membrane repulsion and the electroosmotic (EO) flow. Added multivalent cations suppress the electrostatic barrier and reverse the pore charge, inverting the direction of the EO flow that drags the polymer to the trans side. This inverted EO flow can be used to speed up polymer capture from the reservoir and to transport weakly or non-uniformly charged polymers that cannot be controlled by electrophoresis.

Highly enhanced liquid flows via thermoosmotic effects in soft and charged nanochannels.

Enhancing nanoscale liquid flows remains an existing challenge in nanofluidics. Here we propose the generation of highly augmented thermoosmotic (TOS) liquid flows in soft nanochannels (or nanochannels functionalized by grafting with end-charged polyelectrolyte or PE brushes) by employing an axial temperature gradient. The TOS transport is a combination of the induced-electric-field electroosmotic (EOS) transport and a thermo-chemioosmotic (TCOS) transport with the latter resulting from an induced pressure gradient on account of the changes associated with the imposition of the axial temperature gradient. The end-charged brushes develop an electric double layer (EDL) localized at the charged, non-grafted brush end. Depending on the system parameters, this EDL localization massively augments the influence of the EOS body force and the induced pressure-gradient resulting in a TOS transport in soft nanochannels that can be more than one order of magnitude larger than that in brush-free nanochannels. Given the existing notion that the presence of the brushes invariably reduces the flow strength, this result of massive flow augmentation is extremely significant and non-trivial serving as a paradigm shift in the study of liquid transport in brush-grafted nanochannels.

Analysis of electro-osmotic flow over a slightly bumpy plate

The present study is aimed to investigate the effects of wavy roughness on electro-osmotic (EO) flow over a wavy plate. The waviness of the plate is modeled by the product of two cosinoidal functions, and the roughness (ε) is defined to be the ratio of the wavy amplitude to the Debye length. The effects are examined with respect to the roughness ε and different wave numbers (α and β) of the plate waviness. The analysis of the EO flow over the wavy plate is carried out for the applied electric potential, the potential for the electric double layer, as well as the EO flow velocity and pressure field under the Debye-Hückel approximation by using a boundary perturbation method. It is found that the velocity component along the direction of the applied electric field is modified by a second-order term of the roughness, though the same velocity component near the wavy wall exhibits periodic behaviors in phase with the plate waviness. The mean flow rate deficit (ε2μ2) due to the surface roughness presents a sophisticated dependence on the longitudinal wave number (α) and transverse wave number (β) of the plate waviness, yet the flow deficit is linear in α for small α at β = 0, and shows a long wavelength limit singularity at β = 0 for α ≠ 0.

Electroosmotic flow of a two-layer fluid in a slit channel with gradually varying wall shape and zeta potential

This study aims to investigate electroosmotic (EO) flow of a two-layer fluid through a slit microchannel where the wall shape as well as zeta potential may vary slowly and periodically with axial position. The two-layer EO flow is a model for the flow of two immiscible fluids: a non-conducting working fluid being dragged into motion by a conducting sheath fluid. Electric double layers may develop in the conducting fluid near the interface between the two fluids, and near a wall that is assumed to be non-uniform in both shape and zeta potential distribution. Because of these geometrical and electrical non-uniformities, pressure is internally induced. The two-fluid flow is therefore driven by electrokinetic and hydrodynamic forcings, while subjected to the combined effects of the axial variations of the wall shape and potential distribution. The present problem is formulated by invoking the lubrication approximation, for a nearly parallel flow of low Reynolds number, and the governing equations are solved as analytically as possible. The induced pressure gradient and the deformed shape of the interface, which are functions of axial position, as well as the flow rates of the two fluids, are determined by an iterative trial-and-error numerical scheme. Results are generated to show the effects due to various factors, including the applied pressure difference, interfacial potential, viscosity ratio, wall undulation, and phase shift between the wall shape and potential distribution. Some of the effects on flow in a non-uniform channel can be qualitatively different from that in a uniform channel.

Hydrodynamic dispersion in a combined magnetohydrodynamic- electroosmotic-driven flow through a microchannel with slowly varying wall zeta potentials

The effective dispersion coefficient of a neutral solute in the combined electroosmotic (EO) and magnetohydrodynamic (MHD)-driven flow of a Newtonian fluid through a parallel flat plate microchannel is studied. The walls of the microchannel are assumed to have modulated and low zeta potentials that vary slowly in the axial direction in a sinusoidal manner. The flow field required to obtain the dispersion coefficient is solved using the lubrication approximation theory. The solution of the electrical potential is based on the Debye-Hückel approximation for a symmetric (Z:Z) electrolyte solution. The EO and MHD effects, together with the variations in the zeta potentials of the walls, are observed to notably modify the axial distribution of the effective dispersion coefficient. The problem is formulated for two cases of the zeta potential function. Note that the dispersion coefficient primarily depends on the Hartmann number, on the ratio of the half height of the microchannel to the Debye length, and on the assumed variation in the zeta potentials of the walls.

Optimizing electroosmotic pumping rates in a rectangular channel with vertical gratings

The Helmholtz-Smoluchowski (H-S) velocity is known to be an accurate and useful formula for estimating the electro-osmotic (EO) flow rates in a simple micro-channel with a thin electric-double layer. However, in case the channel cross section is not so simple, the usefulness of H-S velocity could be sharply limited. A case of fundamental interest representing this situation is a rectangular channel (comprising parallel plates) with built-in vertical gratings, in which the surfaces inside the channel may develop different normalized zeta potentials α (on the gratings) and β (on the side walls). In this study, analytical solutions are pursued under the Debye-Hückel approximation to obtain EO pumping rates in a rectangular channel with vertical gratings. In particular, we identify the conditions under which the H-S formula can be properly applied and investigate how the EO flow rates may deviate from those predicted by the H-S velocity with varying physical parameters. Moreover, a diagram of the optimal EO pumping rates on the α-β plane is introduced that accounts for the general features of the analysis, which is consistent with a mathematical model and may serve as a convenient guide for engineering design and applications.

Electrothermal Transport in Biological Systems: An Analytical Approach for Electrokinetically Modulated Peristaltic Flow

A mathematical model is presented to study the combined viscous electro-osmotic (EO) flow and heat transfer in a finite length microchannel with peristaltic wavy walls in the presence of Joule heating. The unsteady two-dimensional conservation equations for mass, momentum, and energy conservation with viscous dissipation, heat absorption, and electrokinetic body force, are formulated in a Cartesian co-ordinate system. Both single and train wave propagations are considered. The electrical field terms are rendered into electrical potential terms via the Poisson–Boltzmann equation, Debye length approximation, and ionic Nernst Planck equation. A parametric study is conducted to evaluate the impact of isothermal Joule heating and electro-osmotic velocity on axial velocity, temperature distribution, pressure difference, volumetric flow rate, skin friction, Nusselt number, and streamline distributions.

Massively Enhanced Electroosmotic Transport in Nanochannels Grafted with End-Charged Polyelectrolyte Brushes.

We establish that nanochannels grafted with pH-responsive, end-charged polyelectrolyte (PE) brushes demonstrate a massive augmentation in the strength of the electroosmotic (EOS) transport in the presence of an external electric field. This contradicts the existing understanding that the EOS transport is severely retarded in channels grafted with the PE brushes due to the brush-induced enhanced drag force. Our mathematical model, developed on the basis of the fact that the ion concentration polarization (ICP) effect can be neglected, explains this enhancement of the EOS transport by noting that the end-charged PE brushes demonstrate a unique ability of localizing the electric double layer (or the EDL) or equivalently localizing the maximum charge density of the electrolyte ions at the location of its end, i.e., away from the grafting surface. Accordingly, the maximum EOS driving force on the liquid, which is proportional to this charge density, can be maximum at a location far away from the wall. As a consequence, the resulting local EOS velocity suffers very little retardation due to the wall shear stress enabling such massive augmentation of the EOS transport. We anticipate that the present paper will unravel a completely new paradigm in the employment of functionalized interfaces in regulating the nanofluidic transport for a myriad of applications.

An on-chip electroosmotic micropump with a light- addressable potentiometric sensor

An on-chip electroosmotic (EO) micropump (EOP) was integrated in a microfluidic channel combined with a light-addressable potentiometric sensor (LAPS). The movement of EO flow towards right and left directions can be clearly observed in the microfluidic channel. The characteristics of photocurrent-time and photocurrent-bias voltage are obtained when buffer solution passes through the sensing region. The results demonstrate that the combination of an on-chip EOP with an LAPS is feasible.

A bubble-assisted electroosmotic micropump for a delivery of a droplet in a microfluidic channel combined with a light-addressable potentiometric sensor

A bubble-assisted electroosmotic (EO) micropump is proposed as a miniaturized actuating element to be integrated in a microfluidic chemical sensing system. In this device, a bubble is generated in a narrow channel to form a thin water sheet between the bubble and the wall. Due to the high surface-to-volume ratio of the water sheet, an efficient EO flow is realized at relatively low pumping voltages of 20–60V without using a porous material. The direction and the flow rate of the EO flow depended on the polarity and the magnitude of the pumping voltage, and the highest flow rate of about ±500pL/s was obtained at ±60V. A bubble-assisted EO micropump was combined with a LAPS for measuring the pH value of a droplet with a volume of 1nL.

Rotating electroosmotic flow of an Eyring fluid

A perturbation analysis is presented in this paper for the electroosmotic (EO) flow of an Eyring fluid through a wide rectangular microchannel that rotates about an axis perpendicular to its own. Mildly shear-thinning rheology is assumed such that at the leading order the problem reduces to that of Newtonian EO flow in a rotating channel, while the shear thinning effect shows up in a higher-order problem. Using the relaxation time as the small ordering parameter, analytical solutions are deduced for the leading- as well as first-order problems in terms of the dimensionless Debye and rotation parameters. The velocity profiles of the Ekman–electric double layer (EDL) layer, which is the boundary layer that arises when the Ekman layer and the EDL are comparably thin, are also deduced for an Eyring fluid. It is shown that the present perturbation model can yield results that are close to the exact solutions even when the ordering parameter is as large as order unity. By this order of the relaxation time parameter, the enhancing effect on the rotating EO flow due to shear-thinning Eyring rheology can be significant.

Maximizing fluid delivered by bubble-free electroosmotic pump with optimum pulse voltage waveform.

In generating high electroosmotic (EO) flows for use in microfluidic pumps, a limiting factor is faradaic reactions that are more pronounced at high electric fields. These reactions lead to bubble generation at the electrodes and pump efficiency reduction. The onset of gas generation for high current density EO pumping depends on many parameters including applied voltage, working fluid, and pulse duration. The onset of gas generation can be delayed and optimized for maximum volume pumped in the minimum time possible. This has been achieved through the use of a novel numerical model that predicts the onset of gas generation during EO pumping using an optimized pulse voltage waveform. This method allows applying current densities higher than previously reported. Optimal pulse voltage waveforms are calculated based on the previous theories for different current densities and electrolyte molarity. The electroosmotic pump performance is investigated by experimentally measuring the fluid volume displaced and flow rate.

Electro-kinetically driven peristaltic transport of viscoelastic physiological fluids through a finite length capillary: Mathematical modeling

Analytical solutions are developed for the electro-kinetic flow of a viscoelastic biological liquid in a finite length cylindrical capillary geometry under peristaltic waves. The Jefferys’ non-Newtonian constitutive model is employed to characterize rheological properties of the fluid. The unsteady conservation equations for mass and momentum with electro-kinetic and Darcian porous medium drag force terms are reduced to a system of steady linearized conservation equations in an axisymmetric coordinate system. The long wavelength, creeping (low Reynolds number) and Debye–Hückel linearization approximations are utilized. The resulting boundary value problem is shown to be controlled by a number of parameters including the electro-osmotic parameter, Helmholtz–Smoluchowski velocity (maximum electro-osmotic velocity), and Jefferys’ first parameter (ratio of relaxation and retardation time), wave amplitude. The influence of these parameters and also time on axial velocity, pressure difference, maximum volumetric flow rate and streamline distributions (for elucidating trapping phenomena) is visualized graphically and interpreted in detail. Pressure difference magnitudes are enhanced consistently with both increasing electro-osmotic parameter and Helmholtz–Smoluchowski velocity, whereas they are only elevated with increasing Jefferys’ first parameter for positive volumetric flow rates. Maximum time averaged flow rate is enhanced with increasing electro-osmotic parameter, Helmholtz–Smoluchowski velocity and Jefferys’ first parameter. Axial flow is accelerated in the core (plug) region of the conduit with greater values of electro-osmotic parameter and Helmholtz–Smoluchowski velocity whereas it is significantly decelerated with increasing Jefferys’ first parameter. The simulations find applications in electro-osmotic (EO) transport processes in capillary physiology and also bio-inspired EO pump devices in chemical and aerospace engineering.

Optimal electro-osmotic pumping of a micro-duct with finned structures

This article is aimed to present an analytic study of electro-osmotic (EO) pumping of a micro-duct with inserted fin vanes. Finned structures are known to be an efficient and very important tool in conducting heat generated in fluid transport system, such as Joule’s heating in micro-fluidic devices. The present semi-analytical analysis is performed under the Debye–Hückel approximation (DHA), enabling us to explicitly investigate the combined effects of various parameters for optimizing the EO pumping rate while retaining substantial fin vanes for heat removal. A mathematical model based on the solutions of two fundamental EO flows is introduced to explain how the choice of the fin vane width may optimize the EO pumping rate in the general case. Moreover, we present the optimized EO flow rates in diagrams plotted on the plane of zeta potentials, which may serve as an easily used reference for engineering design and applications.

Effect of solvent polarization on electroosmotic transport in a nanofluidic channel

In this paper, we develop a theory based on the Langevin–Bikerman approach to study the electroosmotic (EOS) transport in a nanofluidic channel in the presence of finite solvent polarization effect (SPE). At the outset, we conduct an analysis based on practically achievable parameters to highlight the consequence of SPE in the variation in the electric double-layer (EDL) electrostatics. We witness that SPE invariably increases the effective EDL thickness; our numerical results are justified through a scaling analysis. More importantly, we unravel that the EOS transport, most remarkably, shows negligible influence on the qualitative variation in the EDL electrostatic potential; rather, it is dictated by the ratio of the effective to the actual EDL thicknesses. This finding, supported by scaling analysis, ensures that for the chosen set of parameters, SPE invariably enhances the EOS transport. Apart from shedding light on this extremely non-intuitive nanoscopic electroosmotic flow phenomenon, we anticipate that the present study will embolden us to better control the nanofluidic transport for a plethora of biological and industrial applications.