AC Electroosmotic Flow Research Articles

Application of astigmatism μ-PTV to analyze the vortex structure of AC electroosmotic flows

The three-dimensional (3D) flow due to AC electroosmotic (ACEO) forcing on an array of interdigitated symmetric electrodes in micro-channels is experimentally analyzed using astigmatism micro-particle tracking velocimetry (astigmatism μ-PTV). Upon application of the AC electric field with a frequency of 1,000 Hz and a voltage of 2 Volts peak–peak, the obtained 3D particle trajectories exhibit a vortical structure of ACEO flow above the electrodes. Two alternating time delays (0.03 and 0.37 s) were used to measure the flow field with a wide range of velocities, including error analysis. Presence and properties of the vortical flow were quantified. The steady nature and the quasi-2D character of the vortices can combine the results from a series of measurements into one dense data set. This facilitates accurate evaluation of the velocity field by data-processing methods. The primary circulation of the vortices due to ACEO forcing is given in terms of the spanwise component of vorticity. The outline of the vortex boundary is determined via the eigenvalues of the strain-rate tensor. Overall, astigmatism μ-PTV is proven to be a reliable tool for quantitative analysis of ACEO flow.

Ion Fluxes and Electro-osmotic Fluid Flow in Electrolytes around a Metallic Nanowire Tip under Large Applied ac Voltage

Motivated by the analysis of electrochemical growth of metallic nanowires from solution, we studied ion fluxes near nanoelectrodes in a binary symmetric electrolyte on the basis of the modified Poisson-Nernst-Planck equations in the strongly nonlinear region at large applied ac voltage. For an approximate calculation of the electric field near the nanowire tip, concentric spherical blocking electrodes were considered with radius of the inner electrode being of typically a few ten nanometers. The spatiotemporal evolution of the ion concentrations within this spherical model was calculated numerically by using the finite element method. The potential drop at the electric double layer, the electric field enhancement at the electrode surface, and the field screening in the bulk solution were determined for different bulk concentrations, ac voltages, and frequencies. The appearance of ac electro-osmotic fluid flow at the tip of a growing metallic nanowire is discussed, based on an estimation of the body force in the liquid near the nanowire tip, which was modeled by a cylinder with hemispherical cap. Electric field components tangential to the electrode surface exist near the contact between cylinder and hemisphere. Our analysis suggests that ac electro-osmotic flow causes an additional convective transport of metal complexes to the tip of the growing metal nanowire and thus affects the nanowire growth velocity.

AC electro-osmotic micromixer using a face-to-face, asymmetric pair of planar electrodes

This paper is the first to report the mixing performance of an AC electro-osmotic (AC-EO) micromixer based on the two sets of vortices generated by a face-to-face, asymmetric pair of planar electrodes. The larger top electrode of the asymmetric electrode pair was rectangular (30×60mm2) and the much smaller bottom electrode was also rectangular (500×700μm2) and featured two protrusions. Due to the highly three-dimensional flow pattern generated by the unique shape of the face-to-face electrode pair, the mixing enhancement factor (mixing time by diffusion/mixing time by the micromixer) exhibited by the reported micromixer ranged from 290 to 360 and the mixing performance values were uniform across the entire height of the fluid cavity. The usefulness of the mixer in hybridizing single-stranded deoxyribonucleic acids and mixing Escherichia coli with RNA stains was also demonstrated experimentally. A numerical investigation was conducted to study the three-dimensional AC-EO flow patterns induced by the face-to-face, asymmetric electrode pair. This analysis made it possible to identify a detailed flow pattern and the formation mechanism of the vortices. The confluence of inward AC-EO flows from the edges of the bottom-electrode protrusion locally induced a large inward-pumping force and was responsible for the formation of the four vortices at the four corners of the bottom electrode. The numerical and experimental results of this research provide information that will be useful in future applications and research in the area of microfluidic manipulation.

Mapping alternating current electroosmotic flow at the dielectrophoresis crossover frequency of a colloidal probe

AC electroosmotic (ACEO) flow above the gap between coplanar electrodes is mapped by the measurement of Stokes forces on an optically trapped polystyrene colloidal particle. E²-dependent forces on the probe particle are selected by amplitude modulation (AM) of the ACEO electric field (E) and lock-in detection at twice the AM frequency. E²-dependent DEP of the probe is eliminated by driving the ACEO at the probe's DEP crossover frequency. The location-independent DEP crossover frequency is determined, in a separate experiment, as the limiting frequency of zero horizontal force as the probe is moved toward the midpoint between the electrodes. The ACEO velocity field, uncoupled from probe DEP effects, was mapped in the region 1-9 μm above a 28 μm gap between the electrodes. By use of variously sized probes, each at its DEP crossover frequency, the frequency dependence of the ACEO flow was determined at a point 3 μm above the electrode gap and 4 μm from an electrode tip. At this location the ACEO flow was maximal at ∼117 kHz for a low salt solution. This optical trapping method, by eliminating DEP forces on the probe, provides unambiguous mapping of the ACEO velocity field.

Controlling SWCNT assembling density by electrokinetics

Single-walled carbon nanotubes (SWCNTs) as the colloid in a colloidal solution can be polarized in a non-uniform electric field and experience a net force that is the so-called dielectrophoresis (DEP) force, due to the interaction between the induced dipoles and the electric field. The positive DEP force can be used to position and assemble arrays of SWCNTs. Inversely, the negative DEP force can be utilized to separate SWCNTs in terms of their electronic properties. Moreover, Joule heating generated by the electric field can lead to other electrokinetics forces in the colloidal solution, which give rise to fluidic motion of the solution. Additionally, at low frequencies, the electrical double layer also induces a steady fluidic motion, a phenomenon known as AC electroosmotic flow. These fluidic motion in turn exerts a drag force on the nanotubes. Hence, to controllably assemble SWCNTs using DEP force is a non-trivial task. In this article, the mechanisms of electrokinetics and electrohydrodynamics are systematically analyzed through numerical simulations for a set of parameters that are typically used for assembling SWCNTs between metal electrodes. Finally, experimental results from the frequency-dependent assembly of SWCNTs using this set of parameters are described and discussed. These results show that the density of SWCNTs assembled between electrodes can be varied by controlling the electrokinetics parameters.

Measurement of periodic micro flows using micro-particle image velocimetry with phase sampling

A phase-sampling method has been developed to measure periodic flows at a high temporal resolution using conventional micro-particle image velocimetry (PIV). In this technique, the sampling is set such that each velocimetry dataset represents a unique point in phase of a periodic flow. The flow characteristics over a single cycle are reconstructed from measurements over a number of cycles, thus allowing measurement at a higher temporal resolution than the PIV system. The flow measurements were performed for AC electroosmotic flows and verified with results from the phase-locking technique. The temporal resolution is limited by the shortest camera exposure time and the time separation between laser pulses. The theoretical sampling resolution can be as low as 20 μs for 100 Hz periodic flows. A resolution of 200 μs was obtained in the experiment using 40 velocimetry datasets.

Improved particle concentration by cascade AC electroosmotic flow

The importance of electrokinetics in microfluidic technology has been growing owing to its versatility and simplicity in fabrication, implementation, and handling. Alternating-current electroosmosis (ACEO), which is the motion of fluid due to the ion movement by an interaction between AC electric field and an electrical double layer on the electrode surface, has a potential for a particle concentration method to detecti rare samples flowing in a microchannel. This study investigates an improved ACEO-based particle concentration by cascade electrokinetic approach. Flow field induced by ACEO and accumulation behavior of particles were parametrically measured to discuss the concentrating mechanism. The accumulation of particles by ACEO can be explained by a balance between the attenuating electroosmotic flow to transport particles and the inherent diffusive motion of the particles, which is hindered due to the near-wall location. Although a parallel double-gap electrode geometry enables particles to be collected at the center of electrode very sharply, it has scattering zones with accumulated particles at sidewalls of the channel. This drawback can be overcome by applying sheath flow or introducing cascade electrode pattern upstream of the focusing zone. As a result, total concentration efficiency was 98.4 % for all the particles flowing in the cascade device. The resultant concentrated particles exist on the electrode surface within 5 μm, and three-dimensional concentration of particle with the concentration factor as large as 700 is possible using a monolithic channel, co-planar electrode, and sheathless solution feeding. This cascade electrokinetic method provides a new and effective preconcentrator for ultra-sensitive detection of rare samples.

Heat-transfer enhancement in AC electro-osmotic micro-flows

Heat transfer in micro-flows is essential to emerging technologies as advanced microelectronics cooling systems and chemical processes in lab-on-a-chip applications. The present study explores the potential of AC electro-osmotic (ACEO) flow forcing, a promising technique for the actuation and manipulation of micro-flows, for heat-transfer enhancement. Subjects of investigation include the 3D flow structure due to ACEO forcing via an array of electrodes in a micro-channel by way of 3D velocity measurements. Presence and properties of vortical structures of the 3D flow are quantified in laboratory experiments. Typical outcomes of the experimental study result from a number of 3D particle trajectories obtained by using 3D micro-Particle-Tracking Velocimetry (3D μ-PTV). The steady nature of the flow enables combination of results from a series of measurements into one dense data set. This facilitates accurate evaluation of quantities relevant for heat transfer by data-processing methods. The primary circulation is given above one half of an electrode in terms of the spanwise component of vorticity. The outline of the vortex boundary is determined via the eigenvalues of the strain-rate tensor. To estimate convective heat transfer, wall shear rate above one half of an electrode is quantitatively analyzed as function of voltage amplitude and frequency. These results yield first insights into the characteristics of 3D ACEO flows and ways to exploit and manipulate them for heat-transfer enhancement.

KT-Scale Colloidal Interactions in High-Frequency Inhomogeneous AC Electric Fields. II. Concentrated Ensembles

We report nonintrusive optical microscopy measurements of ensembles of polystyrene colloids in inhomogeneous AC electric fields as a function of field frequency and particle size. By using an inverse Monte Carlo (MC) simulation analysis of time-averaged particle microstructures, we sensitively measure induced dipole-dipole interactions on the kT energy scale. Measurements are reported for frequencies when the particle polarizability is greater and less than the medium, as well as the crossover between these conditions when dipole-dipole interactions vanish. By using measured single dipole-field interactions and associated parameters from Part I as input in the inverse analysis, the dipole-dipole interactions in this work are accurately modeled with no adjustable parameters for conditions away from the crossover frequency (i.e., |f(CM)| > 0). As dipolar interactions vanish at the crossover, a single frequency-dependent parameter is introduced to account for microstructures that appear to result from weak AC electro-osmotic flow induced interactions. By connecting quantitative measures of equilibrium microstructures and kT-scale dipole-field and dipole-dipole interactions, our findings provide a basis to understand colloidal assembly in inhomogeneous AC electric fields.

Transport of particles and microorganisms in microfluidic channels using rectified ac electro-osmotic flow

A new method is demonstrated to transport particles, cells, and other microorganisms using rectified ac electro-osmotic flows in open microchannels. The rectified flow is obtained by synchronous zeta potential modulation with the driving potential in the microchannel. Experiments were conducted to transport both neutral, charged particles, and microorganisms of various sizes. A maximum speed of 50 μm∕s was obtained for 8 μm polystyrene beads, without any electrolysis, using a symmetrical square waveform driving electric field of 5 V∕mm at 10 Hz and a 360 V gate potential with its polarity synchronized with the driving potential (phase lag=0°).

A three-phased circular electrode array for electro-osmotic microfluidic pumping

A circular micro-electrode array with three phases is designed and prototyped using PolyMUMPs process for AC electro-osmotic flow pumping. Finite element model of the micro electrode array has been developed using COMSOL Multiphysics™. Performance of the electrode array is simulated and a double peak velocity phenomenon is found for this design, which is confirmed by experimental testing. Using ethanol as testing medium, the two time-averaged peak flow velocities are approximate 320 μm/s at 7 Hz and 850 μm/s at 100 Hz. It is found that the simulated and experimental results agree well.

Effects of Faradaic currents on AC electroosmotic flows with coplanar symmetric electrodes

Microelectrodes subjected to AC potentials are usually employed to induce electrolyte flow, commonly known as AC electroosmotic (ACEO) flows. In this work, we apply novel theoretical results for analysing the influence of Faradaic reactions on these flows. Specifically, we choose to analyse the electrokinetic flow induced by a pair of coplanar symmetric microelectrodes, as the simplest microelectrode structure used in experiments. The liquid is assumed to be a 1:1 electrolyte with ions of different diffusivities. For simplicity, in the model only the cations are responsible for the Faradaic reactions while the anions do not react at the electrodes. We show how to use the novel model to numerically obtain the electrokinetic flow induced by the pair of electrodes. A remarkable new prediction of the model is that, for certain values of the parameters, fluid flow can occur in opposite direction to that obtained in the absence of Faradaic reactions.

Three-dimensional non-linear AC electro-osmotic flow induced by a face-to-face, asymmetric pair of planar electrodes

This paper demonstrates a three-dimensional, non-linear, AC electro-osmotic extensional flow induced by the capacitive charging of a face-to-face pair of planar electrodes. These two planar electrodes facing each other had different sizes and shapes. The extensional flow was attributed to the rectification of the stagnation-point flow in the center region by the three-dimensional vortices at the corners of the smaller electrode. The measured flow velocity near the edge of the electrode was on the order of millimeters per second. The shape of the electrode affected the distribution of the electric field and thereby modified the electro-osmotic flow. A change in the shape of the smaller electrode would significantly affect the flow fields and could cause a variation of a factor of 10 in the measured flow velocity. An equivalent electronic circuit of the experimental apparatus was developed. Based on the equivalent electronic circuit, the calculated critical frequency was in good agreement with the measured optimal velocity frequency. The quadratic dependence of the velocity on the voltage drop across the liquid, a characteristic of the capacitive charging process, was also reported in this paper.

Applications of electrohydrodynamics and Joule heating effects in microfluidic chips: A review

This review article presents an overview on the application of electrohydrodynamics and Joule heating effects in microfluidic chips. A brief introduction of microfluidic chips and a classification of electrohydrodynamics as well as the applications in microfluidic devices are first given. Then basic theories and governing equations of classical electromagnetics are summarized and electroviscous effects in pressure driven flows in a microchannel are presented. Principles and applications of DC electrokinetics, including DC electroosmotic flow, DC electrophoresis, as well as principles of AC electrokinetics, including AC electroosmotic flow and dielectrophoresis are also reviewed. Finally, Joule heating effects in both DC and AC electrokinetics, especially the newly discovered electrothermal flow, are summaried.

Individual evaluation of DEP, EP and AC-EOF effects on λDNA molecules in a DNA concentrator

Bio-concentrators have been proposed to concentrate and detect tiny amount of cells, DNAs, or proteins in a microfluidic system. Concentrating processes have been experimentally studied and discussed as a result of concentration caused by multiple electrokinetic phenomena. Here, we have investigated individual electrokinetic phenomenon in a DNA concentrator to propose a design guide for a micro-bio-concentrator. Three dominant electrokinetic phenomena: dielectrophoresis (DEP), electrophoresis (EP), and AC electroosmotic flow (AC-EOF) are experimentally and theoretically evaluated. DNA mobility due to DEP was separately studied by utilizing quadrupole electrodes generating a uniform field gradient, which enables us to derive a DEP force on the order of 10 −13 N acting on a single DNA molecule. Using the theoretical model the DEP force on the order of 10 −12 N, close to the experimental result, was calculated. The EP force estimated from the average charge per base pair of DNA molecule was on the order of 10 −9 N. Compared with these two forces, AC-EOF generating a flow velocity of 100–200 μm/s, globally dominates the manipulation of DNA molecules throughout the concentrator. However, the three-dimensional observation of molecule distribution reveals that the EP force has a significant role in holding the DNA molecules at the electrode centre. The results not only support the proposal that the biomolecular concentrator should be designed to utilize AC-EOF and EP [1,2], but also provide approximate estimates for the DEP and EP forces, and the AC-EOF velocity which will allow us to design a concentrator with a given experimental condition.

Electrokinetic DNA concentration in microsystems

Active DNA concentration on a specific site in microsystems was demonstrated by the use of AC electroosmosis and electrophoresis. Two different sizes of DNA molecules, double-stranded λ-phage DNA (48.5 kbp) and single-stranded DNA in 20 nucleotides, were utilized in this study. In the DNA concentrator, AC electroosmosis induced by AC electric field could generate bulk fluid flow to transport DNA molecules from a large effective region to the electrode surface. Numerical simulation was done to show the motion of the bulk fluid flow. Electrophoresis induced by DC bias was applied simultaneously to hold the DNA molecules on the surface of the DNA concentrator. The applied electric field required only a low electric potential of a few volts. The motion of the DNA molecules under electrokinetic forces was observed under a fluorescence microscope. Experimental results showed the λ-phage DNA molecules could be concentrated by AC electroosmosis alone, and the single-stranded DNA molecules required the combined electrokinetic forces. The magnitude of AC electroosmotic flow and electrophoretic force controlled by the applied electric potential and frequency was also demonstrated through these concentration experiments.

Configurable AC electroosmotic generated in-plane microvortices and pumping flow in microchannels

This paper proposes the patterned AC electroosmotic flows (AC-EOF) by simply grouping discrete electrodes together to form various electrode configurations for generation of in-plane microvortices with clockwise/counter-clockwise rotation, and pumping flow in a microchannel. The rotational direction of in-plane microvortices and pumping flow direction can be controlled using the same electrode pattern by simple switching of the voltages on the electrodes. Microparticle image velocimetry (μPIV) was used to characterize the flow fields of the generated in-plane microvortices and pumping flow. The rotational strength of microvortices and flow rates of pumping flows were found to increase with the increase of the applied voltages, and an optimal value was achieved at an appropriately applied frequency. Moreover, the dependency of the applied voltages, frequencies, and the heights of the measured planes on the rotational strength of in-plane microvortices between the interdigitated and discrete electrode configurations were examined. The discrepancy in electrode geometry results in a small performance reduction, whereas it can be compensated for the ability of switching the rotational direction of in-plane microvortices using the same device. The configurable in-plane microvortices and pumping flow in microchannels provide the potential for micromixing applications and for the integration into a lab-on-a-chip system.

Numerical prediction of ac electro-osmotic flows around polarized electrodes

In this paper we present an interactive numerical method that can predict ac electro-osmotic flows around completely polarized electrodes. In this method the slip velocity on the electrode surface is calculated by numerically solving the Laplace equation for the potential in the bulk coupled with the dynamical equation for the surface charge density on the electrodes; here the dynamical equation has been derived from the asymptotic solutions of the Poisson-Nernst-Planck equation for the potential drop across the electrical thin layer near the electrode. A unique feature of this study is that the effect of nonspecific ion adsorption is considered. The numerical code was applied to the two-dimensional ac electro-osmotic flow above a pair of coplanar electrodes, and the solutions compared well with the experimental data reported in the literature. We investigated the effect of various parameters on the slip velocity distribution, such as the ac frequency, the electrode length, the effective Stern-layer thickness, and the adsorption coefficients.

Single-stranded DNA concentration by electrokinetic forces

Electrokinetics provide an effective molecular manipulation technique in micro/nano fluidic environments, which match the length scale of various biological objects. In this work, two circular gold electrodes are fabricated on a glass substrate as DNA concentrators. A combination of alternating current (ac) electro-osmosis and electrophoresis is used to concentrate single-stranded DNA (ssDNA) molecules as small as 20 nucleotides in length. To understand the phenomenon of ac electro-osmotic flow, numerical simulation of the flow field is done and shows that the effective fluid flow is up to 100 µm above the electrode surface. Hence, ssDNA molecules labeled with fluorescent dye are utilized to demonstrate the concentration effect on the DNA concentrator. ac electro-osmotic flow induced by the ac electric field can stir the bulk fluid, and ssDNA molecules can be transported from a large effective region to the surface of DNA concentrator. In addition, electrophoretic force induced by direct current (dc) bias is applied simultaneously to attract and hold ssDNA molecules. The motion of ssDNA molecules under electrokinetic forces is observed under a fluorescence microscope. The experimental results show that the ssDNA molecules can be concentrated on the electrode surface instantly. ssDNA concentration under different conditions is also compared and the results generated are discussed.

A study on the slip velocity on a pair of asymmetric electrodes for AC-electroosmosis in a microchannel

In numerical studies on microscale electroosmotic flows, the electric double layer (EDL) effect is usually predicted by solving the traditional Navier-Stokes equation subjected to the slip velocity induced by the electric-charged wall as a boundary condition. Recently, Suh and Kang (Physical Review E 77, 2008) presented the asymptotic solutions of the ion transport equations near a polarized electrode under the action of an AC field, and then proposed a new theoretical model of the slip velocity on the electrode considering the ion adsorption effect. In the present paper, we apply the model to a two-dimensional AC-electroosmotic flow in a microchannel to calculate the slip velocity on a pair of coplanar asymmetric electrodes embedded on the bottom wall, and then experimentally measure the slip velocity using the micro-PIV technique to validate the theoretical model. Comparison shows an excellent overall match between the theoretical and experimental results, except for on the narrow electrode at low frequencies. Next, we numerically perform parametric studies regarding the AC frequency, effective Stern-layer thickness and ion adsorption effect to further understand the characteristics of the AC electroosmotic flow. Results show that, as the frequency increases, the slip velocity also increases. In addition, the velocity decreases with increasing either Stern-layer thickness or ion adsorption effect.