AC Electrokinetics Research Articles

A novel microfluidic driver via AC electrokinetics

A novel ac electrokinetic microfluidic driver based on alternating current electro-osmosis flow induced by asymmetrically capacitance/chemistry-modulated microelectrode arrays has been successfully developed and demonstrated. Asymmetric capacitance modulation (ACM) is made of comb electrode arrays and parts of individual electrode surfaces are modulated/deposited with a SiO(2) dielectric layer. This proposed design can be utilized to shift the optimal operation frequency of maximum velocity to a higher frequency 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 microm s(-1) 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 microl h(-1). This is the first successful demonstration regarding bubble-free ac electrokinetic micropumping via such 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 the proposed novel bubble-free ac electrokinetic microfluidic driver.

Interactions of electrical fields with fluids:laboratory-on-a-chip applications

The area of 'laboratory-on-a-chip', miniaturised or microfluidic analysis systems, is a rapidly developing field. At the microscale, electrokinetic processes become enhanced, and the advent of AC electrokinetics (EK) in recent years further promotes the development of electrokinetic devices for microfluidics. ACEK has demonstrated to manipulate fluids and polarisable particles at low voltages without some of the disadvantages from DCEK, such as electrochemical reactions and the limitation of low ionic strength fluids. The three major mechanisms of ACEK, that is, dielectrophoresis, AC electro-osmosis and AC electrothermal effect, provide versatility and flexibility to interface with many current methods and technologies in multiple biological, chemical and physical disciplines. This paper gives an overview of ACEK and its applications, with an emphasis on fluid manipulation by electric fields.

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.

Electrokinetic mixing in microfluidic systems

The applications of electrokinetics in the development of microfluidic devices have been widely attractive in the past decade. Electrokinetic devices generally require no external mechanical moving parts and can be made portable by replacing the power supply by small battery. Therefore, electrokinetic-based microfluidic systems can serve as a viable tool in creating a lab-on-a-chip (LOC) or micro-total analysis system (μTAS) for use in biological and chemical assays. Mixing of analytes and reagents is a critical step in realizing lab-on-a-chip. This step is difficult due to the low Reynolds numbers flows in microscale devices. Hence, various schemes to enhance micro-mixing have been proposed in the past years. This review reports recent developments in the micro-mixing schemes based on DC and AC electrokinetics, including electrowetting-on-dielectric (EWOD), dielectrophoresis (DEP), and electroosmosis (EO). These electrokinetic-based mixing approaches are generally categorized as either active or passive in nature. Active mixers either use time-dependent (AC or DC field switching) or time-independent (DC field) external electric fields to achieve mixing, while passive mixers achieve mixing in DC fields simply by virtue of their geometric topology and surface properties, or electrokinetic instability flows. Typically, chaotic mixing can be achieved in some ways and is helpful to mixing under large Peclet number regimes. The overview given in this article provides a potential user or researcher of electrokinetic-based technology to select the most favorable mixing scheme for applications in the field of micro-total analysis systems.

AC electrothermal manipulation of conductive fluids and particles for lab-chip applications

AC electrokinetics has shown great potential for microfluidic functions such as pumping, mixing and concentrating particles. So far, electrokinetics are typically applied on fluids that are not too conductive (<0.02 S/m), which excludes most biofluidic applications. To solve this problem, this paper seeks to apply AC electrothermal (ACET) effect to manipulate conductive fluids and particles within. ACET generates temperature gradients in the fluids, and consequently induces space charges that move in electric fields and produce microflows. This paper reports two new ACET devices, a parallel plate particle trap and an asymmetric electrode micropump. Preliminary experiments were performed on fluids with conductivity at 0.224 S/m. Particle trapping and micropumping were demonstrated at low voltages, reaching approximately 100 microm/s for no more than 8 Vrms at 200 kHz. The fluid velocity was found to depend on the applied voltage as V(4), and the maxima were observed to be approximately 20 microm above the electrodes.

AC electrokinetic manipulation of DNA

The controlled manipulation of molecules is a crucial prerequisite for the emerging field of molecular nanotechnology. AC electrokinetics provide a powerful mechanism for both positioning and inducing conformational changes in molecules. In this paper, we investigate the elongation of fluorescently-labelled DNA strands, which are covalently tethered by one end to gold microelectrodes arranged in an opposing-finger geometry, when exposed to strong ac electric fields. We found that the elongation of the DNA molecules is restricted by the geometry of the gap, and that the observed contour of the elongated DNA molecules coincides with the electric field line pattern. Further, we discuss a potential elongation mechanism and provide evidence that the major contribution to the elongation originates from the ac electrokinetic torque, which is supplemented by a small bias force provided by the electric-field-induced fluid flow.

Multiphase electrodes for microbead control applications: Integration of DEP and electrokinetics for bio-particle positioning

Advances in microfabrication have introduced new possibilities for automated, high-throughput biomedical investigations and analysis. Physical effects such as dielectrophoresis (DEP) and AC electrokinetics can be used to manipulate particles in solution to coordinate a sequence of bioanalytical processing steps. DEP is accomplished with non-uniform electric fields that can polarize particles (microbeads, cells, viruses, DNA, proteins, etc.) in suspension causing translational or rotational movement. AC electrokinetics is another phenomena involved with movement of particles in suspension with electric fields and is comprised of both electro-thermal and electro-osmotic effects. This paper investigates single layer electrodes that are effective for particle localization and clustering based on DEP and AC electrokinetic effects. We demonstrate a novel multi-electrode setup capable of clustering particles into an array of discrete bands using activated and electrically floating electrodes. These bands shift to adjacent regions on the electrode surface by altering the electrode activation scheme. The predictability of particle placement to specific locations provides new opportunities for integration and coordination with raster scanning lasers or a charge coupled device (CCD) for advanced biomedical diagnostic devices, and more sophisticated optical interrogation techniques.

The influence of the molecular structure of lipid membranes on the electric field distribution and energy absorption

We consider the influence of the molecular structure of phospholipid membranes on their dielectric properties in the radio frequency range. Membranes have a stratified dielectric structure on the submolecular level, with the lipid chains forming a central hydrophobic layer enclosed by the polar headgroups (HGs) and bound water layers. In our numerical model, isotropic permittivities of 2.2 and 48.8 were assigned to the lipid chain and bound water layers, respectively. The HG region was assumed to possess an anisotropic static permittivity with 142.2 and 30.2 in the tangential and normal directions, respectively. The permittivities of the HG and bound water regions have been assumed to disperse at frequencies around 51 and 345 MHz to become 2.2 and 1.8, respectively, in both the normal and tangential directions. Electric field distribution and absorption were calculated for phospholipid vesicles with 75 nm radius as an example. Significant absorption has been obtained in the HG and bound water regions. Averaging the membrane absorption over the layers resulted in a decreased absorption below 1 GHz but a more than 10-fold increase above 1 GHz, compared to a model with a homogeneous membrane of averaged properties. We propose single particle dielectric spectroscopy by AC electrokinetics at low-bulk medium conductivities for an experimental verification of our model.

On-chip micromanipulation and assembly of colloidal particles by electric fields

We overview the ways in which electric fields can be used for on-chip manipulation and assembly of colloidal particles. Particles suspended in water readily respond to alternating (AC) or direct current (DC) electric fields. Charged particles in DC fields are moved towards oppositely charged electrodes by electrophoresis. Dielectrophoresis, particle mobility in AC fields, allows precise manipulation of particles through a range of parameters including field strength and frequency and electrode geometry. Simultaneously, DC or AC electrokinetics may drive liquid flows inside the experimental cells, which also leads to transport and redistribution of the suspended particles. Examples of dielectrophoretic manipulation and assembly of nanoparticles and microparticles by planar on-chip electrodes are presented. The structures assembled include conductive microwires from metallic nanoparticles and switchable two-dimensional crystals from polymer microspheres. We also discuss how dielectrophoresis and AC electrokinetics can be used in droplet-based microfluidic chips, biosensors, and devices for collection of particles from diluted suspensions.

Experimental analysis of particle and fluid motion in ac electrokinetics

An ac electric field is applied to induce particle and fluid motion in a wedge-shaped microchannel. Micron-resolution particle image velocimetry (μ-PIV) is used to determine spatially resolved particle velocity and fluid velocity fields. Under steady-state conditions, the particles experience a balance between dielectrophoretic forces induced by the nonuniform electric field and Stokes’ drag forces due to viscous interactions with the fluid. The particle velocity is therefore different from the fluid velocity because of dielectrophoresis. A variant of μ-PIV, two-color μ-PIV, is developed and used to uniquely determine the fluid velocity from the observation of particles without a priori knowledge of the electrical properties. This technique is used to explore ac electrokinetically generated fluid motion. A series of voltage levels at fixed frequency are applied to the wedge-shaped electrodes. The dependency of fluid velocity on applied voltage at different regions in the flow is obtained by fitting power-law curves. This is used to determine the underlying physical phenomena associated with ac electrokinetics. We found that both electrothermal effects and ac electroosmosis are important for the current experimental conditions. However, the electrothermal effect is dominant in the bulk fluid.

Analysis of dielectrophoretic electrode arrays for nanoparticle manipulation

A typical microelectrode structure, interdigitated bar electrodes, has been developed for the selective manipulation and separation of nanoparticles using traveling field dielectrophoresis effects under AC electrokinetics. This paper presents meshless numerical solutions of traveling wave dielectrophoresis for such interdigitated electrode array energized with four-phase signal. This meshless method is based on a truncated 2D Taylor series expansion with the first three orders together with a weighted least-square approximation procedure. Dielectrophoretic forces and traveling wave forces are calculated and their applications for nanoparticle manipulation and separation are discussed.

Manipulation of bioparticles using traveling wave dielectrophoresis: numerical approach

A typical microelectrode structure, interdigitated bar electrodes, has been developed for the selective manipulation and separation of bioparticles using traveling field dielectrophoresis effects under AC electrokinetics. This paper presents meshless numerical solutions of traveling wave dielectrophoresis for such interdigitated electrode array energized with 4-phase signal. This meshless method is based on a truncated 2D Taylor series expansion with the first three orders together with a weighted least-square approximation procedure. Dielectrophoretic forces and traveling wave forces are calculated and their applications for bioparticle manipulation and separation are discussed.

Building structured biomaterials using AC electrokinetics.

In this article, the application of alternating current (ac) electrokinetics in the formation of multicellular structured biomaterials from cells is reviewed. The formation of bacterial biofilms with defined internal architectures is investigated. Investigating such interactions could not only help in understanding how biofilms operate, but biofilms could also be used as a primitive model for cell interactions and development in higher organisms such as animals and plants. A discussion on how the same techniques can be used to create multicellular systems from animal and plant cells is presented.

Construction of biofilms with defined internal architecture using dielectrophoresis and flocculation.

A novel approach was developed for the construction of biofilms with defined internal architecture using AC electrokinetics and flocculation. Artificial structured microbial consortia (ASMC) consisting of localized layered microcolonies of different cell types were formed by sequentially attracting different cell types to high field regions near microelectrodes using dielectrophoresis. Stabilization of the microbial consortia on the electrode surface was achieved by crosslinking the cells using the flocculant polyethyleneimine (PEI). Consortia of Escherichia coli, Micrococcus luteus, and Saccharomyces cerevisiae were made as model systems. Also, more natural consortia were made of the bacteria Pseudomonas putida, Clavibacter michiganense, and Methylobacterium mesophilum, which are found together in consortia during biodegradation of metal-cutting waste fluids.

Numerical solution of the dielectrophoretic and travelling wave forces for interdigitated electrode arrays using the finite element method

AC electrokinetics is the study of the movement of polarisable particles under the influence of AC electric fields. The fields are applied to a suspension of particles by planar microelectrode structures and one particular design, the interdigitated bar electrode has been used in both dielectrophoretic (DEP) field flow fractionation and travelling wave dielectrophoresis. This paper presents, numerical solutions of the DEP and travelling wave forces for an interdigitated electrode array energised with either a 2- or 4-phase signal. The electrorotational torque experienced by the particle in the 4-phase travelling wave array is also calculated. The solutions are compared with previous results.

AC electrokinetics: applications for nanotechnology

The phenomena of dielectrophoresis and electrorotation,collectively referred to as AC electrokinetics, have been used formany years to study, manipulate and separate particles on the cellular(1 µm or more) scale. However, the technique has much to offer theexpanding field of nanotechnology, that is the precise manipulation ofparticles on the nanometre scale. In this paper we present theprinciples of AC electrokinetics for particle manipulation, review thecurrent state of AC electrokinetic techniques for the manipulation ofparticles on the nanometre scale, and consider how these principlesmay be applied to nanotechnology.

Ac electrokinetics: a survey of sub-micrometre particle dynamics

Particles suspended in fluid exhibit motion when subjected to ac electric fields. The applied field results in forces on both the particles and the fluid, the study of which is referred to as ac electrokinetics. The ac electrokinetic techniques can be used for the controlled manipulation and characterization of particles, and the separation of mixtures. For sub-micrometre particles, Brownian motion is important and strong electric fields are required to overcome these effects. Planar micro-electrode arrays, fabricated using semiconductor manufacturing processes, can generate electric fields of the required strength from low potentials over a wide range of frequencies. This paper reviews and discusses sub-micrometre particle dynamics under the influence of dielectrophoretic and electrohydrodynamic forces. New experimental observations of the movement of sub-micrometre particles are also presented.

Determination of rock properties by low‐frequency AC electrokinetics

In brine‐saturated rock the existence of a mobile space charge at the fluid/solid interface leads to the electrokinetic phenomena of electroosmotic pressure and streaming potential. The coupling coefficients of these electrokinetic effects, when combined with the conductivity of the brine‐saturated rock, determine the brine permeability of rock exactly. A sensitive low‐frequency AC technique has been used to measure electrokinetic response of a collection of eight rock and four glass bead samples saturated with NaCl brine as a function of salt concentration (fluid conductivity of 0.5 to 6.38 S/m); the response of four of the original 12 samples has also been measured as a function of temperature from 0° to 50°C. All data verify the predicted permeability relationship. Additionally, the frequency response of the electroosmotic pressure signal alone can also be used to determine the permeability, given knowledge of experimental parameters. The concentration and temperature dependence of electroosmosis and streaming potential is found to mostly conform to the predictions of a simple model based on the Helmholtz‐Smoluchowski equation, the Stern model of the electrochemical double layer, and an elementary theory of ionic conduction.

Low-frequency AC electrokinetics

By using a low-frequency AC method based on lock-in amplification, electrokinetic response in porous rock can be accurately measured with high sensitivity. With this method, we have measured streaming potentials as small as 300 nV and electroosmotic pressures as low as 10 Pa. We describe our technique, and discuss the results from a suite of 12 brine-saturated samples, consisting of six sandstones, two carbonates, and four fused glass bead specimens. We show that the permeability of the rock may be derived exactly from the electrokinetic transport coefficients and sample conductivity. This result is verified experimentally for all NaCl concentrations. The electrokinetic response as a function of concentration and temperature has been measured and is compared with predictions from the Debye–Hückel model and Helmholtz–Smoluchowski equation in the thin-layer limit.

Ac electrokinetics: a review of forces in microelectrode structures

Ac electrokinetics is concerned with the study of the movement and behaviour of particles in suspension when they are subjected to ac electrical fields. The development of new microfabricated electrode structures has meant that particles down to the size of macromolecules have been manipulated, but on this scale forces other than electrokinetic affect particles behaviour. The high electrical fields, which are required to produce sufficient force to move a particle, result in heat dissipation in the medium. This in turn produces thermal gradients, which may give rise to fluid motion through buoyancy, and electrothermal forces. In this paper, the frequency dependency and magnitude of electrothermally induced fluid flow are discussed. A new type of fluid flow is identified for low frequencies (up to 500 kHz). Our preliminary observations indicate that it has its origin in the action of a tangential electrical field on the diffuse double layer of the microfabricated electrodes. The effects of Brownian motion, diffusion and the buoyancy force are discussed in the context of the controlled manipulation of sub-micrometre particles. The orders of magnitude of the various forces experienced by a sub-micrometre latex particle in a model electrode structure are calculated. The results are compared with experiment and the relative influence of each type of force on the overall behaviour of particles is described.