AC Electroosmosis Research Articles

Optoelectrofluidic field separation based on light-intensity gradients.

Optoelectrofluidic field separation (OEFS) of particles under light -intensity gradient (LIG) is reported, where the LIG illumination on the photoconductive layer converts the short-ranged dielectrophoresis (DEP) force to the long-ranged one. The long-ranged DEP force can compete with the hydrodynamic force by alternating current electro-osmosis (ACEO) over the entire illumination area for realizing effective field separation of particles. In the OEFS system, the codirectional illumination and observation induce the levitation effect, compensating the attenuation of the DEP force under LIG illumination by slightly floating particles from the surface. Results of the field separation and concentration of diverse particle pairs (0.82-16 mum) are well demonstrated, and conditions determining the critical radius and effective particle manipulation are discussed. The OEFS with codirectional LIG strategy could be a promising particle manipulation method in many applications where a rapid manipulation of biological cells and particles over the entire working area are of interest.

Experimental and Theoretical Characterization of an AC Electroosmotic Micromixer

We have reported on a novel microfluidic mixer based on AC electroosmosis. To elucidate the mixer characteristics, we performed detailed measurements of mixing under various experimental conditions including applied voltage, frequency and solution viscosity. The results are discussed through comparison with results obtained from a theoretical model of AC electroosmosis. As predicted from the theoretical model, we found that a larger voltage (approximately 20 V(p-p)) led to more rapid mixing, while the dependence of the mixing on frequency (1-5 kHz) was insignificant under the present experimental conditions. Furthermore, the dependence of the mixing on viscosity was successfully explained by the theoretical model, and the applicability of the mixer in viscous solution (2.83 mPa s) was confirmed experimentally. By using these results, it is possible to estimate the mixing performance under given conditions. These estimations can provide guidelines for using the mixer in microfluidic chemical analysis.

Electrokinetic framework of dielectrophoretic deposition devices

Numerical modeling and experiments are performed investigating the properties of a dielectrophoresis-based deposition device, in order to establish the electrokinetic framework required to understand the effects of applied inhomogeneous electric fields while moving particles to desired locations. By capacitively coupling electrodes to a conductive substrate, the controlled large-scale parallel dielectrophoretic assembly of nanostructures in individually accessible devices at a high integration density is accomplished. Thermal gradients in the solution, which give rise to local permittivity and conductivity changes, and velocity fields are solved by coupling electric, thermal, and fluid-mechanical equations. The induced electrothermal flow (ETF) causes vortices above the electrode gap, attracting particles, such as single-walled carbon nanotubes (SWNTs), before they are trapped by the dielectrophoretic force and deposit across the electrodes. Long-range carbon nanotube transport is governed by hydrodynamic effects, while local trapping is dominated by dielectrophoretic forces in low concentration SWNT dispersions. Results show that by decreasing the ac frequency ac electroosmosis on the metallic electrodes occurs due to the emergence of an electric double layer, disturbing the initial flow pattern of the system. By superimposing a dc potential offset, a generated tangential electroosmotic fluid flow in the dielectric electrode gap also disrupts the ETF. Capacitive coupling is most efficient in the high frequency regime where it is the dominating impedance contribution. Understanding the occurrence and interaction of these different effects, including a self-limiting integration mechanism for individual nanostructures, allows an increased deposition yield at overall lower electric field strengths through a prudent choice of electric field parameters. The findings provide important avenues toward gentler particle handling, without direct current throughput, a relevant aspect for limiting process effects during device fabrication, all while increasing dielectrophoretic deposition efficiency in nanostructured networks.

Dynamic Manipulation by Light and Electric Fields: Micrometer Particles to Microliter Droplets

We demonstrate a new hybrid optoelectric technique that can manipulate objects across several length scales. The technique leverages a variety of different physical mechanisms to achieve the dynamic manipulation of droplets and also the in situ concentration of colloidal particles suspended in the droplets. Various physical mechanisms such as optoelectrowetting, electrothermal flows, and ac electroosmosis are leveraged through different modes of operation of the device. Each operational mode, which is activated through the proper combination of an applied ac bias and the illumination used, is characterized by the ability to manipulate objects on a certain length scale. We also demonstrate that the device lends itself to the active control of microstructure patterns that emerge from a droplet evaporation process.

The effect of electrostatic and electrohydrodynamic forces on the chaining of carbon nanofibres in liquid epoxy

The formation of chains of aligned carbon nanofibres (CNFs) in polymers is a subject of great interest in the field of multifunctional nanocomposites. The mechanism of CNF chain assembly and growth in a low viscosity epoxy is investigated by developing a finite element model of a chain attached to an electrode. The model examines the combined effects of electrostatic and electrohydrodynamic forces on chain morphology. The electrohydrodynamic forces are modelled using the theory of ac electro-osmosis. The predictions of the model are supported by experimental results. The experiments were conducted on a CNF/epoxy/amine mixture by applying an ac field at frequencies ranging from 100 to 100 000 Hz. The predictions of the model qualitatively capture the variations of chain morphology and growth rate as functions of ac frequency. Higher frequencies promote a more uniform and denser network of chains. The rate of growth of chains is highest at an intermediate frequency. A uniform network of chains was observed at frequencies of 1 kHz and greater in the experiments. The rate of growth of chains was maximized at a frequency of 1 kHz for a liquid viscosity of 0.03 Pa s.

Enhancement of heterogeneous immunoassays using AC electroosmosis

Enhancing the transport of analyte to a transducer surface is of critical importance to many biosensors, especially for those designed to capture and detect large biomolecules (nucleic acids, proteins, etc.) which diffuse slowly and therefore cause inordinately long detection times. For microfluidic-based sensors, which are characterized by low Reynolds number flow, the mass transport is purely diffusive in nature, which eliminates the possibility of turbulent mixing. In this article, enhancement of heterogeneous immunoassays using AC electroosmosis was studied through the use of finite element modeling and fluorescent immunoassays. Simulations of interdigitated planar electrodes indicate that optimum electrode geometry (electrode width and spacing) scales with the chamber height and that binding times can be reduced by up to a factor of 6. Optimum frequency of electrical excitation of the electrodes was between 100 and 200 Hz, which was confirmed by fluorescent immunoassays. Immunoassays also show that binding of secondary antibody on a functionalized surface can be significantly enhanced, especially at the ends of each electrode without noticeable nonspecific binding. Unexpectedly, the antibody binding was not uniformly distributed across the surface but patterned preferentially on the edges of the electrode, which indicates the presence of dielectrophoresis. Analysis of fluorescent intensity shows an enhancement of about 1.9 times at the center of the device and an enhancement of about 6.7 times at the edges of the best device type compared to a non-mixed counterpart. This work contributes to the growing field of electrokinetic enhancement of biosensors through the use of microelectrodes built directly onto, or near the binding region of a transducer.

Effect of the combined action of Faradaic currents and mobility differences in ac electro-osmosis

In this work, we extend previous analyses of ac electro-osmosis to account for the combined action of two experimentally relevant effects: (i) Faradaic currents from electrochemical reactions at the electrodes and (ii) differences in ion mobilities of the electrolyte. In previous works, the ac electro-osmotic motion has been analyzed theoretically under the assumption that only forces in the diffuse (Debye) layer are relevant. Here, we first show that if the ion mobilities of a 1-1 aqueous solution are different, the charged zone expands from the Debye layer to include the diffusion layer. We later include the Faradaic currents and, as an attempt to explore both factors simultaneously, we perform a thin-layer, low-frequency, linear analysis of the system. Finally, the model is applied to the case of an electrolyte actuated by a traveling-wave signal. A steady liquid motion in opposite direction to the applied signal is predicted for some ranges of the parameters. This could serve as a partial explanation for the observed flow reversal in some experiments.

J1103-1-2 交流電気浸透流を用いたマイクロアクチュエータの提案([1103-1]次世代アクチュエータシステム)

The paper presents a novel microactuator using AC electroosmosis. On surfaces of AC voltage applied plate elecrodes that contact with dielectric fluids, the gathered electric charges form electric double layers. Due to the AC electric field, the charged particles form convection flow, which is called AC electroosmosis. The proposed actuator transforms the covection flow to a unidirectional flow with channel walls and the unidirectional flow drives an fluid power actuator. The proposed actuator features simple and miniaturizable structure, low driving voltage, stable drive without electrolysis, and wide variety of the drivable fluids. In this paper, the microactuator is proposed and a basic pumping device is fabricated and tested. Based on the measured results, the no-load flow rate is estimated.

Size-selective immunofluorescence of Mycobacterium tuberculosis cells by capillary- and viscous forces

Rapid, low cost screening of tuberculosis requires an effective enrichment method of Mycobacterium tuberculosis (MTB) cells. Currently, microfiltration and centrifugation steps are frequently used for sample preparation, which are cumbersome and time-consuming. In this study, the size-selective capturing mechanism of a microtip-sensor is presented to directly enrich MTB cells from a sample mixture. When a microtip is withdrawn from a spherical suspension in the radial direction, the cells that are concentrated by AC electroosmosis are selectively enriched to the tip due to capillary- and viscous forces. The size-selectivity is characterized by using polystyrene microspheres, which is then applied to size-selective capture of MTB from a sample mixture. Our approach yields a detection limit of 800 cells mL(-1), one of the highest-sensitivity immunosensors to date.

In-depth imaging of dielectrophoresis by digital in-line holographic microscopy

Dielectrophoresis is usually viewed using traditional microscopy, basically in plane since the depth of focus is short. However, a basic dielectrophoretic platform induces particle mobility not only in electrode plane but also in the depth dimension as well. The electric field gradient also has depth dimension. Therefore it is essential to measure dielectrophoresis in that depth dimension as well. In this study we demonstrate negative dielectrophoresis; the phenomenon is imaged with digital in-line holographic microscopy. The imaging method requires a full platform transparency, which is realized with transparent indium-tin-oxide electrodes. The negative dielectrophoresis demonstrations utilized polystyrene carboxylate particles 4.08 μm in diameter in a potassium chloride dilution 5 mM in concentration. The size of the active region of the platform was 65 μm in diameter. By negative dielectrophoresis, particles approach the trap at the center of the electrode tips. Intense particle movement was also observed outside of the platform’s active region. This was a result of AC electro-osmosis or electro thermal flow.

A soft-lithographed chaotic electrokinetic micromixer for efficient chemical reactions in lab-on-chips

Mixing is one of the basic functions which automated lab-on-chips require for the effective management of liquid samples. In this paper we report on the working principle, design, fabrication and experimental characterization of a soft-lithographed micromixer for microfluidic applications. The device effectively mixes two liquids by means of chaotic advection obtained as an implementation of a Linked Twisted Map (LTM). In this sense it is chaotic. The liquids are electrokinetically displaced by generating rolls through AC electroosmosis on co-planar electrodes. The device performance has been tested on dyed DI-water for several voltages, frequencies and flow-rates, displaying good mixing properties in the range of $10 \div 100$kHz, at low peak-to-peak voltages ($\sim15 \div 20$ volts). Low voltage supply, small dimensions and possibility of fabrication via standard lithographic techniques make the device highly integrable in lab-on-a-chip platforms.

AN AFFINITY SENSOR IMPROVED BY EWOD ACTUATOR-BASED MICROFLUIDIC CHIP

We propose an improved immunoassay method with EWOD-based microfluidic biochip. Electro-wetting on dielectric (EWOD) is based on contact angle-changed phenomenon of droplet on the channel surface. The reactant-containing droplet can be moved from reservoir to a modified electrode area, which a stir function can be provided by controlling AC electro-osmosis flow (ACEOF). The immunochemistry reaction takes place on our modified area via electro-wetting actuation, and the signal from fluorescent or electrochemical impedance spectroscopy (EIS) was used to confirm the performance of designed element.

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.

Measurement of Molecular Diffusion Based on Optoelectrofluidic Fluorescence Microscopy

This technical note reports a method for measuring the diffusion coefficient of molecules based on an optoelectrofluidic platform. Optoelectrofluidic fluorescence microscopy, which is constructed with an optoelectrofluidic device and a conventional fluorescence microscope, is a useful tool for controlling and detecting local molecular concentration in a fluid with a single light source. When we project a light for fluorescence excitation and apply an ac signal of a few hundred Hertz frequency around 100 Hz to the optoelectrofluidic device, a sudden decay of molecular concentration occurs within the illuminated area due to several mechanisms, including optically induced ac electroosmosis, electrostatic interaction forces among the polarized molecules, and interactions among the molecules and between the molecules and the electric double layer. After the applied voltage was turned off, the dispersed molecules diffuse into the molecular depletion area and the fluorescence signal is recovered. On the basis of these phenomena, we successfully measured the diffusion coefficient of various dextran molecules. A statistical analysis to determine the significance of our experimental values compared to the previously reported values measured using fluorescence recovery after photobleaching technology was also performed. This new technique demonstrates the first analytical measurement of diffusion based on the optoelectrofluidic platform and can be a useful tool for measuring the mobility of molecules in a simple and easy way.

Mathematical modeling of AC electroosmosis in microfluidic and nanofluidic chips using equilibrium and non-equilibrium approaches

AC electroosmotic micropumps are suggested to be powerful tools for electrolyte dosing in various micro- and nanofluidic systems. In this paper, we compare two modeling approaches for studying the AC electroosmosis in the following micro and nanochannel systems: (i) a traveling-wave AC pump with a spatially continuous wave of electric potential applied on a planar boundary, (ii) a traveling-wave AC pump with a wave of electric potential applied on a set of discrete planar electrodes, and (iii) an AC pump with a set of non-planar electrodes. The equilibrium approach is based on the use of capacitor–resistor boundary conditions for electric potential and the slip boundary conditions for velocity at electrode surfaces. The non-equilibrium approach uses the mathematical model based on the Poisson equation and the non-slip boundary conditions. We have observed discrepancies between the predictions given by the both models and then we have identified their possible reasons. The comparison of the equilibrium and non-equilibrium results further showed three important actualities: (a) how the equilibrium model overestimates or underestimates the net velocity, (b) how the velocity maxima in the frequency characteristics can be shifted, if the equilibrium model assumptions are not satisfied, (c) the parametric region where the equilibrium model is applicable. Because the data are obtained in a dimensionless form, they can be exploited for AC electroosmotic studies. We discuss the limitations of the equilibrium and non-equilibrium models and compare selected predictions with available experimental data.

Nonlinear electrokinetics at large voltages

The classical theory of electrokinetic phenomena assumes a dilute solution of point-like ions in chemical equilibrium with a surface whose double-layer voltage is of order the thermal voltage, kBT/e=25 mV. In nonlinear ‘induced-charge’ electrokinetic phenomena, such as ac electro-osmosis, several volts ≈100kBT/e are applied to the double layer, and the theory breaks down and cannot explain many observed features. We argue that, under such a large voltage, counterions ‘condense’ near the surface, even for dilute bulk solutions. Based on simple models, we predict that the double-layer capacitance decreases and the electro-osmotic mobility saturates at large voltages, due to steric repulsion and increased viscosity of the condensed layer, respectively. The former suffices to explain observed high-frequency flow reversal in ac electro-osmosis; the latter leads to a salt concentration dependence of induced-charge flows comparable to experiments, although a complete theory is still lacking.

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.

Active Manipulation of Quantum Dots using AC Electrokinetics

The superior optical and electrical properties of quantum dot (QD) nanoparticles are being applied in a new generation of QD devices for microelectronic, nanophotonic, and biomedical systems. Research on integrated QD-based systems has been focused on passive manipulation of QDs including template-guided self-assembly, molecular scaffold-based assembly, and microbead-based assembly. However, little effort has been devoted to development of methods for active manipulation, such as positioning and concentration, of QDs. In this study, we show that 20 nm colloidal QDs can be effectively positioned and concentrated using a combination of dielectrophoresis (DEP) and AC electro-osmosis (ACEO). The long-range fluid motion generated by ACEO entrains QDs to the area near the electrode surface, which facilitates the trapping of QDs by DEP. A systematic investigation was performed to examine electrokinetic processes at different applied frequencies and voltages using a concentric electrode configuration. We demonstrate that QDs can be dynamically positioned with an electric field strength as small as 10 kV/m and define the operating parameters for increasing the concentration of colloidal QDs by 2 orders of magnitude within one minute.

Flow Reversal in Traveling-Wave Electrokinetics: An Analysis of Forces Due to Ionic Concentration Gradients

Pumping of electrolytes using ac electric fields from arrays of microelectrodes is a subject of current research. The behavior of fluids at low signal amplitudes (<2-3 V(pp)) is in qualitative agreement with the prediction of the ac electroosmosis theory. At higher voltages, this theory cannot account for the experimental observations. In some cases, net pumping is generated in the direction opposite to that predicted by the theory (flow reversal). In this work, we use fluorescent dyes to study the effect of ionic concentration gradients generated by Faradaic currents. We also evaluate the influence of factors such as the channel height and microelectrode array shape in the pumping of electrolytes with traveling-wave potentials. Induced charge beyond the Debye length is postulated to be responsible for the forces generating the observed flows at higher voltages. Numerical calculations are performed in order to illustrate the mechanisms that might be responsible for generating the flow.

Numerical study on AC electroosmosis in microfluidic channels

This paper deals with parametrical studies focused on determination of selected characteristics of AC electroosmotic micropumps in non-linear (high-amplitude) regimes in the interdigitated arrangement. The net velocity and current–voltage phase shift are computed. Non-equilibrium Navier–Stokes–Poisson–Nernst–Planck model that describes the transport processes in the entire time and spatial domains including electric double layers (EDLs) was derived and analyzed. The obtained results were qualitatively interpreted by means of a simple relation derived for RC circuit in series. Locations of the high net velocity regions in the parametric space were found. The model thus can contribute to development and optimization of microfluidic devices driven by AC electric fields. Flow reversals were observed in frequency and geometric scale characteristics. We suggest that the flow reversals do not necessarily rely on the electric charge injection or the condensed layer formation. The direction of the net flow probably depends only on an interplay of the electric, viscous and pressure forces.