AC Electroosmotic Flow Research Articles

A Bubble-Free AC Electrokinetic Micropump Using the Asymmetric Capacitance-Modulated Microelectrode Array for Microfluidic Flow Control

A novel ac electrokinetic micropumping device based on ac electro-osmotic flow induced by asymmetrically capacitance/chemistry-modulated microelectrode arrays has been successfully developed and demonstrated. Asymmetric capacitance modulation is made of comb electrode arrays and parts of individual electrode surfaces are modulated/deposited with a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> dielectric layer. This proposed design can be utilized to shift the optimal operation frequency of maximum velocity from tens of kilohertz to megahertz to minimize electrolytic bubble generation and enhance micropumping performance. The pumping velocity, described in this paper, is measured via the tracing of microbeads and is a function of applied potential, signal frequency, buffer concentration, and dielectric layer thickness. A maximum pumping velocity up to 290 ¿m ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in 5-mM buffer solution with the applied potential of 10 Vpp is observed in our prototype device, and the estimated maximum flow rate is up to 26.1 ¿l ·h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . This is the first successful demonstration regarding bubble-free ac electrokinetic micropumping via such an asymmetrically capacitance-modulated electrode arrays. Design, simulation, microfabrication, experimental result, and theoretical model are described in this paper to characterize and exhibit the performance of proposed novel bubble-free ac electrokinetic micropump.

Assessment of three AC electroosmotic flow protocols for mixing in microfluidic channel

This study performs an experimental investigation into the micromixer capabilities of three different protocols of AC electroosmotic flow (AC EOF), namely capacitive charging (CC), Faradaic charging (FC) and asymmetric polarization (AP). The results reveal that the vortices generated by the FC protocol (the frequency is around 50-350 Hz) are stronger than those induced by the CC protocol (the frequency is higher than 350 Hz), and therefore provide an improved mixing effect. However, in the FC protocol, the frequency of the external AC voltage must be carefully controlled to avoid damaging electrodes as a result of Faradaic reactions. The experimental results indicate that the AP polarization effect (the applied voltage and frequency are V(1) = 1 V(pp) and V(2) = 20 V(pp)/5 kHz) induces more powerful vortices than either the CC protocol or the FC protocol, and therefore yields a better mixing performance. Two AP-based micromixers are fabricated with symmetric and asymmetric electrode configurations, respectively. The mixing indices achieved by the two devices after an elapsed time of 60 seconds are found to be 56.49 % and 71.77 %, respectively. This result shows that of the two devices, an asymmetric electrode configuration represents a more suitable choice for micromixer in microfluidic devices.

DC-biased AC-electroosmotic and AC-electrothermal flow mixing in microchannels

This paper presents a novel approach of mixing two laminar flowing streams in microchannels. The mixer consists of a pair of electrodes disposed along a fluidic channel. By energizing the electrodes with a DC-biased (2.5 V) AC voltage (20 Vpp), an electrokinetic flow is induced with a flow profile perpendicular to that of the incoming laminar streams of liquids to be mixed. As a result, the flow lines of the incoming streams and the induced flow are forced to crossover and very efficient stirring and mixing at short mixing length can be achieved. The mixer can be operated from the AC-electroosmotic (ACEO) (sigma=1 mS/m, f=100 kHz) to the AC-electrothermal (ACET) (sigma=500 mS/m, f=500 kHz) flow regimes. The mixing efficiency in the ACEO regime was 92%, with a mixing length of 600 microm (Q=2 microL/min), an estimated mixing time of 69 ms and an induced ACEO flow velocity of approximately 725 microm/s. The mixing efficiency in the ACET regime was 65% for a mixing length of approximately 1200 microm. The mixer is efficient and suitable for mixing reagents in a fluid media from low to high conductivity as required in diverse microfluidic applications.

Light-Actuated AC Electroosmosis for Nanoparticle Manipulation

We present a novel light-actuated ac electroosmosis (LACE) mechanism that allows the concentration and transportation of micro- and nanoscopic particles using light-patterned dynamically reconfigured microfluidic vortices on a photoconductive surface. LACE is realized by sandwiching an aqueous liquid medium between a featureless photoconductive surface and a transparent indium tin oxide electrode. By applying an ac electrical bias with a frequency that is close to the electric double-layer relaxation frequency, a light-patterned virtual electrode can induce ac electroosmotic flow to concentrate and transport nanoscopic particles on the photoconductive surface. By integrating with a spatial light modulator such as a digital micromirror device microdisplay, we can create 31000 microfluidic vortices on a 1.3 times 1-mm2 area for massively parallel trapping of 2- and 1-mum polystyrene beads. We have also demonstrated LACE concentration and transportation of nanoscopic particles including 200- and 50-nm polystyrene beads, lambda-phage DNA molecules, and quantum dots.

An experimental study on the AC electroosmotic flow around a pair of electrodes in a microchannel

This paper presents an experimental study on the AC electroosmotic flow in a microchannel having a pair of rectangular electrodes on the bottom wall with narrow gap. The microchannel was made of PDMS (Polydimethylsiloxane) and the electrodes of ITO (Indium Tin Oxide). The electrodes were arranged such that the electric field is mainly perpendicular to the channel’s longitudinal direction, thus creating a transversal secondary flow. The primary flow was driven by a pressure force through the fluid-level difference on both reservoirs of the channel. To measure the velocity distributions around the electrodes, we used a micro-PTV (particle tracking velocimetry) technique. We find that on the surface of the electrodes the flow velocity caused by the AC electroosmosis is directed from the electrode edge toward the side wall of the channel, and the maximum crosswise velocity occurs at the frequency 120Hz. A smooth profile of the crosswise velocity component along a vertical line was successfully obtained from the present experimental technique, and it shows a flow reversal due to the mass conservation principle.

Frequency Dependence of Gold Nanoparticle Superassembly by Dielectrophoresis

Dielectrophoresis is an effective method for capturing nanoparticles and assembling them into nanostructures. The frequency of the dielectrophoretic alternating current (ac) electric field greatly influences the morphology of resultant nanoparticle assemblies. In this study, frequency regimes associated with specific gold nanoparticle assembly morphologies were identified. Gold nanoparticles suspended in water were captured by microelectrodes at different electric field frequencies onto thin silicon nitride membranes. The resultant assemblies were examined by transmission electron microscopy. For this system, the major frequency-dependent influence on morphology appears to arise not from the Clausius-Mossotti factor of the dielectrophoretic force itself, but instead from ac electroosmotic fluid flow and the influence of the electrical double layer at the electrode-solution interface. Frequency regimes of technological interest include those forming one-dimensional nanoparticle chains, microwires, combinations of microwires and nanoparticle chains suitable for nanogap electrode formation, and dense three-dimensional assemblies with very high surface area.

Using nonlinear ac electrokinetics vortex flow to enhance catalytic activities of sol-gel encapsulated trypsin in microfluidic devices.

A novel microstirring strategy is applied to accelerate the digestion rate of the substrate N(alpha)-benzoyl-L-arginine-4-nitroanilide (L-BAPA) catalyzed by sol-gel encapsulated trypsin. We use an ac nonlinear electrokinetic vortex flow to stir the solution in a microfluidic reaction chamber to reduce the diffusion length between the immobilized enzyme and substrate in the solution. High-intensity nonlinear electroosmotic microvortices, with angular speeds in excess of 1 cms, are generated around a small ( approximately 1.2 mm) conductive ion exchange granule when ac electric fields (133 Vcm) are applied across a miniature chamber smaller than 10 mul. Coupling between these microvortices and the on-and-off electrophoretic motion of the granule in low frequency (0.1 Hz) ac fields produces chaotic stream lines to stir substrate molecules sufficiently. We demonstrate that, within a 5-min digestion period, the catalytic reaction rate of immobilized trypsin increases almost 30-fold with adequate reproducibility (15%) due to sufficient stirring action through the introduction of the nonlinear electrokinetic vortices. In contrast, low-frequency ac electroosmotic flow without the granule, provides limited stirring action and increases the reaction rate approximately ninefold with barely acceptable reproducibility (30%). Dye molecules are used to characterize the increases in solute diffusivity in the reaction reservoir in which sol-gel particles are placed, with and without the presence of granule, and compared with the static case. The solute diffusivity enhancement data show respective increases of approximately 30 and approximately 8 times, with and without the presence of granule. These numbers are consistent with the ratios of the enhanced reaction rate.

Modeling the mechanisms driving ac electro-osmotic flow on planar microelectrodes

The authors introduce an electrical model of the electrode-electrolyte interface and use it to discuss the mechanisms responsible for the generation of ac electro-osmotic flows at the surface of planar electrodes. They propose that such flows are generated by two distinct mechanisms: nonuniform diffusion of ionic species from the bulk to the electrode surface and inhomogeneous electrochemical transfer across the electrode surface. They then proceed to explain experimental observations in two situations where one mechanism dominates over the other, in each case validating the inclusion of specific components in their model.

Double layer overlap in ac electroosmosis

The frequency-dependent flow of electrolytes between pairs of parallel plate micro-electrodes is analyzed in this paper, for the cases in which electric double layers formed in vicinity of the solid boundaries may strongly interact with each other. Closed form expressions for the potential distributions are first developed under certain simplifying assumptions, depicting the interactions between the oscillating electric field and charge density distribution within the double layer. It is revealed that the impact of double layer overlap on ac electroosmotic flows turns out to be more predominant at frequencies of the order of relaxation frequency of the electrode–electrolyte system. At higher frequencies, potential drop across the double layer tends to zero, due to polarization of the electrode-solution interface, and virtually no electroosmotic flows can be obtained in such cases.

AC electroosmotic generated in-plane microvortices for stationary or continuous fluid mixing

This paper proposes a novel design of the patterned AC electroosmotic flows (AC-EOF) by various electrode configurations to locally and simultaneously generate in-plane microvortices of clockwise, counter-clockwise, or pair rotation in a microchannel for micromixing applications. The device was operated under the application of an AC signal with a voltage and frequency below 20 V p–p and 10 kHz, respectively, for all cases. Microparticle image velocimetry (μPIV) was used to characterize the flow fields of the generated in-plane microvortices. The sizes of the active regions and the magnitudes of the rotational strength of microvortices were found to increase with the increase of the applied voltages and an optimal value was achieved at an appropriately applied frequency. By employing the microvortices, AC-EOF micromixers were successfully carried out for either rapid local mixing of two stationary fluids with high viscosity ratio or continuous mixing of two flowing fluids introduced into a Y-shaped microchannel of 400 μm wide. The mixing times for the stationary fluids with viscosity ratio of 450 and the continuously flowing fluids were 2 min and 0.86 s, respectively, which are 15-fold and 30-fold faster than that of mixing directly by diffusion. The AC-EOF micromixer provides a great mixing enhancement with a shorter mixing length for the application of flows in low Peclet numbers ( Pe < 3 × 10 3). The AC-EOF micromixer can be employed locally and simultaneously for either stationary or continuous flows in microchannels, and integrated into a lab-on-a-chip system.

Analysis of Electrically Induced Swirling Flow of Isotonic Saline in a Mixing Microchannel

We have designed a prototype microfluidic device to mix suspended particles with isotonic saline by use of electrically induced swirling flow in the microchannel. However, the principles underlying microfluidic rotation induced by AC electrodes are not well understood, and the characteristics of the rotation velocity are unpredictable. Furthermore, these properties have not been studied using a highly conductive liquid like isotonic saline, which is an important fluid in the medical and biological fields. The lack of such studies causes uncertainty in the design required for high-performance microfluidic devices. We have examined the electrical rotational properties of the microfluid at an isotonic concentration of saline using computer simulation, and here we show that buoyant flow, which has previously been largely ignored, has a significant effect in channels of 100-μm depth or deeper, and that AC electroosmotic flow is not induced at isotonic saline concentrations.

Electrokinetic particle aggregation patterns in microvortices due to particle-field interaction

A complex and dynamic particle banding phenomenon in an electric field is reported. A single cylindrical vortex flow is generated by a dc-biased ac electro-osmotic flow on parallel electrodes. Charged particles are attracted to the vortex by positive dielectrophoresis (DEP) to form rotating cylindrical structures. As the particle concentration increases, the cylinder undergoes longitudinal symmetry breaking, producing concentrated rotating bands caused by field screening effects. The focusing of the particles into bands is shown to obey negative diffusion dynamics of a long-wave instability. Funnels and butterfly-like patterns also form because of secondary longitudinal DEP forces from nonuniform screening effects.

AC electroosmotic flow in a DNA concentrator

Experimental velocity measurements are conducted in an AC electrokinetic DNA concentrator. The DNA concentrator is based upon Wong et al. (Transducers 2003, Boston, pp 20–23, 2003a; Anal Chem 76(23):6908–6914, 2004)and consists of two concentric electrodes that generate AC electroosmotic flow to stir the fluid, and dielectrophoretic and electrophoretic force fields that trap DNA near the centre of the inside electrode. A two-colour micro-PIV technique is used to measure the fluid velocity without a priori knowledge of the electric field in the device or the electrical properties of the particles. The device is also simulated computationally. The results indicate that the numerical simulations agree with experimental data in predicting the velocity field structure, except that the velocity scale is an order of magnitude higher for the simulations. Simulation of the dielectrophoretic forces allows the motion of the DNA within the device to be studied. It is suggested that the simulations can be used to study the phenomena occurring in the device, but that experimental data is required to determine the practical conditions under which these phenomena occur.

AC electroosmotic micromixer for chemical processing in a microchannel

A rapid micromixer of fluids in a microchannel is presented. The mixer uses AC electroosmotic flow, which is induced by applying an AC voltage to a pair of coplanar meandering electrodes configured in parallel to the channel. To demonstrate performance of the mixer, dilution experiments were conducted using a dye solution in a channel of 120 microm width. Rapid mixing was observed for flow velocity up to 12 mm s(-1). The mixing time was 0.18 s, which was 20-fold faster than that of diffusional mixing without an additional mixing mechanism. Compared with the performance of reported micromixers, the present mixer worked with a shorter mixing length, particularly at low Peclet numbers (Pe < 2 x 10(3)).

Dynamic aspects of electroosmotic flow

This article presents an analysis of the frequency and time dependent electroosmotic flow in open-end and closed-end microchannels of arbitrary cross-section shape. In the numerical model, the modified Navier–Stokes equation governing the AC electroosmotic flow is solved using the control volume method. The iterative approach is used to determine the induced backpressure gradient. The potential distribution of the EDL in the channel is obtained by solving the non-linear 2D Poisson–Blotzmann equation. The comparison between the control volume formulation and the Green’s function method for the case of a rectangular microchannel shows a good agreement. The time evolution of the electroosmotic flow and the effect of a frequency-dependent AC electric field on the oscillating electroosmotic flow are also examined. The effect of the induced backpressure gradient with the frequency of the applied electric field is also shown.

Electro-osmotic pumping on application of phase-shifted signals to interdigitated electrodes

Alternating-current (ac) electro-osmotic flow can be implemented in microfluidic pumping systems with the advantage over conventional electro-osmosis that it requires low applied potentials that can be supplied by normal batteries so that it is highly suitable for use in portable lab-on-a-chip systems. We report how the combination of applying phase shifted signals to the interdigitated electrodes and the use of a ground layer causes ac electro-osmotic flow of aqueous electrolytes. The direction of pumping may be changed by varying the phase difference. Velocities up to 100 μ m/s were observed for applied potentials of less than 1 V rms . The maximum velocity was observed at phase differences of around ± 90 ° . How surface conductivity affects ac electro-osmosis can be inferred from the effect of varying electrolyte concentration on the frequency of peak velocity.

Lateral separation of colloids or cells by dielectrophoresis augmented by AC electroosmosis

Colloidal particles and biological cells are patterned and separated laterally adjacent to a micropatterned electrode array by applying AC electric fields that are principally oriented normally to the electrode array. This is demonstrated for yeast cells, red blood cells, and colloidal polystyrene particles of different sizes and ζ-potentials. The separation mechanism is observed experimentally to depend on the applied field frequency and voltage. At high frequencies, particles position themselves in a manner that is consistent with dielectrophoresis, while at low frequencies, the positioning is explained in terms of a strong coupling between gravity, the vertical component of the dielectrophoretic force, and the Stokes drag on particles induced by AC electroosmotic flow. Compared to high frequency dielectrophoretic separations, the low frequency separations are faster and require lower applied voltages. Furthermore, the AC electroosmosis coupling with dielectrophoresis may enable cell separations that are not feasible based on dielectrophoresis alone.

Frequency-dependent velocity and vorticity fields of electro-osmotic flow in a closed-end cylindrical microchannel

The frequency-dependent electro-osmotic flow in closed-end cylindrical microchannels is analyzed in this study. A dynamic ac electro-osmotic flow field is obtained analytically by solving the Navier–Stokes equation using the Green function formulation in combination with a complex variable approach. Onsager's principle of reciprocity is demonstrated to be valid for transient and ac electro-osmotic flow. The effect of a frequency-dependent ac electric field on the oscillating electro-osmotic flow is studied. The induced pressure gradient is analyzed under the effects of the channel dimension and the frequency of electric field. Based on the Stokes second problem, the solution of the slip velocity approximation is presented for comparison with the results obtained from the analytical solution developed in this study. In addition, the expression for the electro-osmotic vorticity field is derived, and the characteristic of the vorticity field in ac electro-osmotic flow is discussed.

Electro-osmotic streaming on application of traveling-wave electric fields

We describe ac electro-osmotic flow of an aqueous electrolyte on application of a traveling-wave electric field. Depending on the frequency of the applied traveling wave, the interaction of the electric double layer charge and the tangential electric field leads to fluid flow in the direction of the traveling wave. We have derived two theoretical models that describe this flow as a function of the amplitude of the applied electric potential, the signal frequency, and the material properties of the system. The first is based on a capacitative model and is limited to frequencies much lower than the double layer relaxation frequency. The second is an analytical solution of the electrokinetic equations and is also valid at higher frequencies. We provide experimental evidence that streaming takes place on application of a traveling wave of potential by tracing the movements of fluorescent latex beads over a spiral electrode structure. Streaming takes place at applied potentials low enough for the method to be easily integrated into lab-on-a-chip devices.

AC electroosmosis in microchannels packed with a porous medium

This paper presents a theoretical study on ac-driven electroosmotic flow in both open-end and closed-end microchannels packed with uniform charged spherical microparticles. The time-periodic oscillating electroosmotic flow in an open-end capillary in response to the application of an alternating (ac) electric field is obtained using the Green function approach. The analysis is based on the Carman–Kozeny theory. The backpressure associated with the counter-flow in a closed-end capillary is obtained by analytically solving the modified Brinkman momentum equation. It is demonstrated that in a microchannel with its two ends connected to reservoirs and subject to ambient pressure, the oscillating Darcy velocity profile depends on both the pore size and the excitation frequency; such effects are coupled through an important aspect ratio of the tubule radius to the Stokes penetration depth. For a fixed pore size, the magnitude of the ac electroosmotic flow decreases with increasing frequency. With increasing pore size, however, the magnitude of the maximum velocity shows two different trends with respect to the excitation frequency: it gets higher in the low frequency domain, and gets lower in the high frequency domain. In a microchannel with closed ends, for a fixed excitation frequency, use of smaller packing particles can generate higher backpressure. For a fixed pore size, the backpressure magnitude shows two different trends changing with the excitation frequency. When the excitation frequency is lower than the system characteristic frequency, the backpressure decreases with increasing excitation frequency. When the excitation frequency is higher than the system characteristic frequency, the backpressure increases with increasing excitation frequency.