AC Electroosmotic Flow Research Articles

Electric field gradient focusing with electro-osmotic flow to reduce analyte dispersion: Concept and numerical investigation

In proteomics, the need to precisely examine the protein compounds of small samples, requires sensitive analytical methods which can separate and enrich compounds with high precision. Current techniques require a minimal analysis time to obtain satisfactory compound separation where longer analysis time means better separation of compounds. But, molecular diffusion will create broadening of the separated compound bands over time, increasing the peak width, and thus reducing the resolution and the enrichment.Electric field gradient focusing (EFGF) is a separation technique, in which proteins are simultaneously separated and enriched by balancing a gradient electrostatic force with a constant hydrodynamic drag force. Because of this balance, analytes are continuously pushed back to their focusing point, limiting the time-dependent peak broadening due to molecular diffusion. Current EFGF techniques are however still suffering from peak broadening because of flow-profile inhomogeneities.In this paper, we propose to use AC electro-osmotic flow (AC EOF) to create a homogeneous flow in EFGF. The interference between the electric field gradient and the AC EOF was thoroughly analysed and the concept was validated using numerical simulations. The results show that a plug flow is obtained on top of a small, distorted boundary layer. While applying different DC electric fields in the electrolyte, a constant flow velocity can be obtained by including a DC offset to the electrodes generating the AC EOF. The plug flow is then maintained over the whole separation channel length, while an electric field gradient is applied. This way, the flow-induced contribution to peak broadening can be minimized in EFGF devices. By modelling the separation of green fluorescent protein (GFP) and R-Phycoerythrin (R-PE), it was shown that the peak width of separated compounds can be reduced and that the separation resolution can be improved, compared to current EFGF methods.

Real-Time Underwater Nanoplastic Detection beyond the Diffusion Limit and Low Raman Scattering Cross-Section via Electro-Photonic Tweezers.

Emerging as substantial concerns in the ecosystem, submicron plastics have attracted much attention for their considerable hazards. However, their effect and even amount in the environment remain unclear. Establishing a substantive analytic platform is essential to expand the understanding of nanoplastics. However, the issues of diffusion and detection limit that arise from ultradiluted concentration and extremely small scales of nanoplastics leave significant technical hurdles to analyze the nanoplastic pollutants. In this study, we obtain effective Raman signals in real time from underwater nanoplastics with ultralow concentrations via AC electro-osmotic flows and dielectrophoretic tweezing. This enables the field-induced active collection of nanoplastics toward the optical sensing area from remote areas in a rapid manner, integrating conventional technical skills of preconcentration, separation, and identification in a single process. A step further, synergetic combination with plasmonic nanorods, accomplishes the highest on-site detection performance so far.

AC electroosmosis micromixing on a lab-on-a-foil electric microfluidic device

Efficient mixing of fluids in lab-on-a-chip devices is very important for many biomedical and biochemical applications. Lab-on-a-foil as a novel concept provides a method for fast prototyping or mass production of microfluidic devices based on thin and flexible films materials. In this article, electroosmosis micromixing is conducted in a lab-on-a-foil microfluidic device. With the electroosmotic flow (EOF), an efficient micromixing is realized inside a microchannel by tooth-shaped planar electrodes. The mixing performance is evaluated based on intensity measurement, and frequency sweeping is used to identify optimal performance. Furthermore, according to local intensity profiles, the EOF pattern is analyzed to provide a deep understanding on the influence of frequency and flow rate. The amplitude of voltage and the number of pairs of electrode tooth are also investigated to find the optimal conditions of the device. To the best of our knowledge, this is the first demonstration of the AC EOF in a lab-on-a-foil electric device and the exploration of EOF pattern vertically and horizontally in the microchannels. This study provides a method to optimize mixing performance in EOF-based micromixer. Furthermore, the fabrication method cast the potential for mass production of low-cost flexible electric microfluidic devices.

Passive solute separation in AC electroosmosis including surface charge-coupled hydrodynamic slip effects.

Separation of electrically neutral, mutually noninteracting passive solutes via AC electroosmotic slit channel flows is investigated for general asymmetric wall surface zeta potentials and apparent hydrodynamic slip lengths. We consider the nontrivial coupling between the surface potentials (or charge densities) and the apparent slip lengths, and focus our attention on the occurrence of a so called "crossover phenomenon" for separating out the slow diffusers when both slow and fast diffusers are present. Results show that regardless of the potential-slip coupling, wider bandwidths become available for crossover phenomenon to occur when the electroosmotic velocity gradient (magnitude) is greater. Contrarily, plug-like velocity profiles inhibit crossover phenomenon, and the potential-slip parametric combinations leading to such profiles can be easily identified by the conditions for minimal transport enhancement reported in recent literature. When separating out the slow diffuser or crossover phenomenon is desired, we recommend incorporating significant asymmetry in the surface potential and apparent slip boundary conditions such that the operating frequency and flow oscillation amplitude may be lowered to more practical values. Our results also agree with and strengthen the physical picture for explaining crossover phenomenon in macroscopic pressure-driven oscillatory flows.

Numerical Simulation of the Photobleaching Process in Laser-Induced Fluorescence Photobleaching Anemometer

At present, a novel flow diagnostic technique for micro/nanofluidics velocity measurement—laser-induced fluorescence photobleaching anemometer (LIFPA)—has been developed and successfully applied in broad areas, e.g., electrokinetic turbulence in micromixers and AC electroosmotic flow. Nevertheless, in previous investigations, to qualitatively reveal the dynamics of the photobleaching process of LIFPA, an approximation of uniform laser distribution was applied. This is different from the actual condition where the laser power density distribution is normally Gaussian. In this investigation, we numerically studied the photobleaching process of fluorescent dye in the laser focus region, according to the convection–diffusion reaction equation. The profiles of effective dye concentration and fluorescence were elucidated. The relationship between the commonly used photobleaching time constant obtained by experiments and the photochemical reaction coefficient is revealed. With the established model, we further discuss the effective spatial resolution of LIFPA and study the influence of the detection region of fluorescence on the performance of the LIFPA system. It is found that at sufficiently high excitation laser power density, LIFPA can even achieve a super-resolution that breaks the limit of optical diffraction. We hope the current investigation can reveal the photobleaching process of fluorescent dye under high laser power density illumination, to enhance our understanding of fluorescent dynamics and photochemistry and develop more powerful photobleaching-related flow diagnostic techniques.

Self-powered AC electrokinetic microfluidic system based on triboelectric nanogenerator

The AC electrokinetic technology, such as AC electroosmosis, induced-charged electroosmosis, dielectrophoresis, etc., can precisely manipulate trace fluids and has been widely used in lab on a chip. However, the power supplies used in AC electrokinetic experiments are mostly function generators or other large-scare instruments, which is an obstacle to portability. Encouragingly, triboelectric nanogenerator (TENG) with the characteristics of portable, green, and simple structure is a potential opportunity for AC electrokinetic microfluidic operation. We have designed a self-powered AC electrokinetic microfluidic system based on vertical contact-separation mode TENG for the first time. The TENG, as a power supply, costs only 2 cents and maintains excellent output performance after millions of working cycles. Through the combination of TENG and microfluidic chip, this system successfully realizes AC electroosmotic flow and induced-charged electroosmotic flow in the microchannel and completes the effective mixing of two fluids at low frequencies (<10 Hz). Furthermore, we have achieved precise control of particle behavior, including focusing, deflection, and separation under manual operation. Therefore, the self-powered AC electrokinetic microfluidic system designed in this work not only expands the application fields of TENG but also provides a new power solution for AC electrokinetic microfluidic operation. This system is expected to get rid of the need for large-scale power supply in traditional methods and has broad application prospects in the fields of portable detection, micro-reaction, and micro-bioanalysis.

Inducing AC-electroosmotic flow using electric field manipulation with insulators.

Classically, the configuration of electrodes (conductors) is used as a means to determine AC-electroosmotic flow patterns. In this paper, we use the configuration of insulator materials to achieve AC-electroosmotic flow patterning in a novel approach. We apply AC electric fields between parallel electrodes situated on the top and bottom of a microfluidic channel and separated by an insulating material. Channels of various cross-sectional shapes (e.g. rectangular and parallelogram) were fabricated by shaping the insulating material between the electrodes. We found that vortex flow patterns are induced depending on the cross-sectional shape of the channel. A bell-shaped design with non-orthogonal corners gave rise to 2 vortices, whereas in a channel with a parallelogram shaped cross-section, only a single vortex was observed. The vortices were experimentally observed by analysing the 3D trajectories of fluorescent microparticles. From a theoretical analysis, we conclude that flow shaping is primarily caused by shaping the electrical field lines in the channel.

Asymmetric temporal variation of oscillating AC electroosmosis with a steady pressure-driven flow

In most electrokinetic flows in micro-/nanofluidics, electroosmotic flow (EOF) is always a fundamental phenomenon. When an external AC electric field is applied, there will be a transient EOF velocity, which is crucial for understanding the mechanism of the unsteady AC EOF. However, the transient flow velocity cannot be experimentally measured with sufficiently and simultaneously temporal and spatial resolution. Here we report that under a strong AC electric field, an asymmetric temporal variation of oscillating EOF (OEOF) can be generated near the electric double layer (EDL) on the bottom wall of a microchannel. The flow is measured by laser-induced fluorescence photobleaching anemometer (LIFPA). Both the flow velocity of the OEOF and the linearly related internal electric field in the microchannel are theoretically governed by an inviscid Burgers’ equations with an external forcing term and controlled by two coefficients which represent the magnitudes of linear ( $${Z}_{l}$$ ) and nonlinear ( $${Z}_{nl}$$ ) terms. These two coefficients are separately determined by five dimensionless parameters, including Stokes number ( $${St}$$ ), aspect ratio ( $$\gamma$$ ), Schmidt number ( $${Sc}$$ ), Peclet number of mean flow velocity ( $${P}_{e, U}$$ ) and Peclet number of electrophoresis ( $${P}_{e, {EP}}$$ ). In this work, the influence of $${P}_{e, U}$$ and $${P}_{e, {EP}}$$ which are tightly related to the local mean flow velocity and the amplitude of AC electric field is investigated. When the electric field is low, $${P}_{e, U}\gg {P}_{e,{EP}}$$ and accordingly $${Z}_{l}\gg {Z}_{nl}$$ , a temporal symmetry of the time series of velocity is observed. However, when the electric field is high, $${P}_{e, U}\ll {P}_{e, {EP}}$$ and $${Z}_{l}\ll {Z}_{nl}$$ , then the periodicity and temporal symmetry of OEOF can be significantly broken and a temporal asymmetry of the time series of velocity is resulted in, especially under the square wave external electric field. We also find increased mean flow velocity and saturation of velocity fluctuations with the increasing electric field. This work provides a new methodology to study unsteady electrokinetic flows.

Reduction of Taylor-Aris dispersion by lateral mixing for chromatographic applications.

Chromatographic columns are suffering from Taylor-Aris dispersion, especially for slowly diffusing molecules such as proteins. Since downscaling the channel size to reduce Taylor-Aris dispersion meets fundamental pressure limitations, new strategies are needed to further improve chromatography beyond its current limits. In this work we demonstrate a method to reduce Taylor-Aris dispersion by lateral mixing in a newly designed silicon AC-electroosmotic flow mixer. We obtained a reduction in κaris by a factor of three in a 40 μm × 20 μm microchannel, corresponding to a plate height gain of 2 to 3 under unretained conditions at low to high Pe values. We also demonstrate an improvement of a reverse-phase chromatographic separation of coumarins.

Multifrequency Induced-Charge Electroosmosis.

We present herein a unique concept of multifrequency induced-charge electroosmosis (MICEO) actuated directly on driving electrode arrays, for highly-efficient simultaneous transport and convective mixing of fluidic samples in microscale ducts. MICEO delicately combines transversal AC electroosmotic vortex flow, and axial traveling-wave electroosmotic pump motion under external dual-Fourier-mode AC electric fields. The synthetic flow field associated with MICEO is mathematically analyzed under thin layer limit, and the particle tracing experiment with a special powering technique validates the effectiveness of this physical phenomenon. Meanwhile, the simulation results with a full-scale 3D computation model demonstrate its robust dual-functionality in inducing fully-automated analyte transport and chaotic stirring in a straight fluidic channel embedding double-sided quarter-phase discrete electrode arrays. Our physical demonstration with multifrequency signal control on nonlinear electroosmosis provides invaluable references for innovative designs of multifunctional on-chip analytical platforms in modern microfluidic systems.

Analytical study of AC electroosmotic mixing in 2-dimensional microchannel with time periodic surface potential.

This work reported an analytic study of AC electroosmotic flows with a view to control the degree of mixing in a rectangular microchannel. Only with spatially non-uniform zeta potential distribution, fluid particles travel back and forth along a vortical flow field developed inside a microchannel. Although complex patterns of electroosmotic vortical flows can be obtained by various types of non-uniform zeta potential distributions, fluid particles always follow regular paths due to a laminar flow limit. To further facilitate the mixing of sample fluid, we propose a scheme that the zeta potential distribution was temporally non-uniform as well. General solutions for both the double layer potential distribution and the AC electroosmotic flow field are analytically determined by solving the unsteady Stokes equation with an electrostatic body force. As an illustrative example, we consider a case where two different types of non-uniform zeta potential distributions alternate with each other and the effects of both the AC frequency and the frequency of the alternation of the two zeta potential distributions on flow characteristics are examined using the Poincaré sections. Conclusively, one can either enhance or prevent mixing compared to a static electroosmotic flow, which is in line with previously demonstrated experimental works. Thus, the results presented would be an effective mean for controllable electroosmotic flow in a microfluidic platform.

High efficiency micromixing technique using periodic induced charge electroosmotic flow: A numerical study

Micromixers due to their prominence in Lab-On-a-Chip (LOC), biomanufacturing and chemical engineering applications are of great significances and emphases. In this paper, a 3D transient single phase solver has been developed in the open source code package, OpenFOAM, to investigate mixing efficiency and mixing length of a T-junction micromixer based on AC electroosmotic flow for both Newtonian and non-Newtonian fluids. The mathematical model and the developed solver are validated with existing data in the literature and the effects of substantial parameters such as AC amplitude, period of oscillation and conducting edges are investigated and the optimum range of each parameter are proposed to obtain simultaneously a high mixing efficiency and a relatively short mixing length. Present study shows that by using conducting edges, due to their capability of creating strong vortices, a high mixing efficiency and a short mixing length is achievable.Results show that via implementation of AC electric field as well as conducting edges, the mixing efficiency of 99% in a considerably short mixing length i.e. 20μm even in relatively high flow rates is obtainable. Since the mixing length is depended on the height of microchannel, in order to attain the capability of comparing achieved mixing length with findings of previous studies, a dimensionless parameter, Lm*, is proposed. Comparing of the value of this parameter for the present work and previous studies revealed that the presented micromixer can reduce the dimensionless mixing length up to 60%.

Rapid mixing by turbulent-like electrokinetic microflow

Rapid mixing has been achieved in a pressure-driven microflow which is forced under AC electric field. In our previous investigations, the mixing on large scale at the centerline of microchannel has been studied. Here, we show that the evaluation on mixing effect by flow visualization could be misleading, if its temporal resolution is not sufficiently high. By using single-point laser induced fluorescence (LIF) method, the mixing on both large and small scales can be investigated with high spatiotemporal resolution. It is found fast mixing is not only achieved at the centerline, but also in the majority of cross-sectional area, even near the bottom wall region. This is resulted from the large scale secondary flow due to unbalanced AC electroosmotic flow near the bottom wall. The temporal −5/3 spectrum of concentration measured with LIF in our previous investigations is also supported by the spatial spectrum of concentration. The physical process of mixing on small scales is further investigated by the flatness of concentration gradients.

In-situ AC electroosmotic and thermal perturbation effects for wide range of ionic strength

AC electrokinetic flow is promising in designing microfluidic chips for manipulation of biological and chemical samples toward clinical diagnostics. Four pieces of electrodes are optimized to enhance mixing effect inside a straight microchannel. In this research, the mixing dependency on the ionic strength of solutions is investigated. AC electroosmotic secondary flow is responsible for the mixing at low ionic strength (σ 5 mS m–1). The electrode-electrolyte impedance analysis is employed to facilitate the in-situation mixing process by choosing appropriate electrical excitation parameters for the electrodes.

Long-wave interface instabilities of a two-layer system under periodic excitation for thin films

The stability of a system of two thin liquid films under AC electroosmotic flow is studied using linear stability analysis for long-wave disturbances. The system is bounded by two rigid plates which act as substrate. Boltzmann charge distribution is assumed for the two electrolyte solutions. The effect of van der Waals interactions in these thin films is incorporated in the momentum equations through the disjoining pressure. The base-state velocity profile from the present study is compared with simple experiments and other analytical results. Parametric study involving various electrochemical factors is performed and the stability behaviour is analysed using growth rate, marginal stability, critical amplitude and maximum growth rate in phase space. An increase in the disjoining pressure is found to decrease stability of the system. On the other hand, increasing the frequency of the applied electric field is found to stabilize the system. However, the dependence of the stability on parameters such as viscosity ratio, permittivity ratio, interface zeta potential and interface charge depends not only on the value of individual parameters but also on the rest of the parameters. Design of experiments (DOE) is used to observe the general trend of stability with different parameters.

Microarray-integrated optoelectrofluidic immunoassay system.

A microarray-based analytical platform has been utilized as a powerful tool in biological assay fields. However, an analyte depletion problem due to the slow mass transport based on molecular diffusion causes low reaction efficiency, resulting in a limitation for practical applications. This paper presents a novel method to improve the efficiency of microarray-based immunoassay via an optically induced electrokinetic phenomenon by integrating an optoelectrofluidic device with a conventional glass slide-based microarray format. A sample droplet was loaded between the microarray slide and the optoelectrofluidic device on which a photoconductive layer was deposited. Under the application of an AC voltage, optically induced AC electroosmotic flows caused by a microarray-patterned light actively enhanced the mass transport of target molecules at the multiple assay spots of the microarray simultaneously, which reduced tedious reaction time from more than 30 min to 10 min. Based on this enhancing effect, a heterogeneous immunoassay with a tiny volume of sample (5 μl) was successfully performed in the microarray-integrated optoelectrofluidic system using immunoglobulin G (IgG) and anti-IgG, resulting in improved efficiency compared to the static environment. Furthermore, the application of multiplex assays was also demonstrated by multiple protein detection.

Aligned Immobilization of Proteins Using AC Electric Fields.

Protein molecules are aligned and immobilized from solution by AC electric fields. In a single-step experiment, the enhanced green fluorescent proteins are immobilized on the surface as well as at the edges of planar nanoelectrodes. Alignment is found to follow the molecules' geometrical shape with their longitudinal axes parallel to the electric field. Simultaneous dielectrophoretic attraction and AC electroosmotic flow are identified as the dominant forces causing protein movement and alignment. Molecular orientation is determined by fluorescence microscopy based on polarized excitation of the proteins' chromophores. The chromophores' orientation with respect to the whole molecule supports X-ray crystal data.

An ultrasensitive label-free electrochemical impedimetric DNA biosensing chip integrated with a DC-biased AC electroosmotic vortex

A label-free electrochemical impedance spectroscopy (EIS)-based DNA biosensing chip is integrated with DC-biased AC electroosmotic (ACEO) stirring using concentric double ring-single disk electrodes to greatly promote the hybridization efficiency of 20-base target DNA (tDNA) fragments. The +0.7V-biased ACEO flow of 3Vpp and 380Hz applied at the outer ring electrode (ORE) and inner ring electrode (IRE) can drive the tDNA-containing 1mM Tris solution from the ORE to the probe's DNA-modified disk electrode (DE) to achieve 90% saturation hybridization within 141s. The DNA biosensor used Pd-deposited ORE as the pseudo-reference electrode and IRE as the counter electrode for on-chip-type three-electrode EIS measurement, and presented good linearity and repeatability in the range of 1aM–10pM and exhibited an ultrasensitive detection limit of 0.5aM. The label-free EIS-based DNA sensing chips with integrated DC-biased ACEO vortex can achieve rapid hybridization, high selectivity and ultrasensitive detection for different tDNA samples.

Transient AC Electro-Osmotic Flow of Generalized Maxwell Fluids through Microchannels

In this paper, we represent analytical solutions of transient velocity for electroosmotic flow (EOF) of generalized Maxwell fluids through both micro-parallel channel and micro-tube using the method of Laplace transform. We solve the problem including the linearized Poisson-Boltzmann equation, the Cauchy momentum equation and generalized Maxwell constitutive equation. By numerical calculation, the results show that the EOF velocity is greatly depends on oscillating Reynolds number and normalized relaxation time.

Validated numerical analysis of vortical structures in 3D AC electro-osmotic flows

This paper presented an experimental validation of a numerical study on the vortical structures in AC electro-osmotic (ACEO) flows. First, the 3D velocity field of ACEO vortices above the symmetric electrodes was experimentally investigated using astigmatism microparticle tracking velocimetry. The experimentally obtained velocities were used to validate an extended nonlinear Gouy–Chapman–Stern model accounting for the surface conduction effect. A qualitative agreement between the simulations and experiments was found for the velocity field when changing AC voltage (from 0.5 to 2 V) and the frequency (from 50 to 3,000 Hz). However, the predicted magnitude of the velocity profiles was much higher than the experimentally obtained ones, except in some cases at low frequency. For frequencies higher than 200 Hz, a correction factor was introduced to make the numerical results quantitatively comparable to the experimental ones. In addition, the primary circulation, given in terms of the spanwise component of vorticity, was numerically and experimentally analyzed as function of frequency and amplitude of the AC voltage. The outline of the vortex boundary was determined via the eigenvalues of the strain-rate tensor estimated from the velocity field. It revealed that the experimental circulation was frequency dependent, tending to zero at both low and high frequency and the maximum changing from around 600 Hz for 1 V to 300 Hz for 2 V. The variation in the predicted vortex circulation as function of frequency and voltage, after using the above correction factor, was in good correspondence with the experiments. These results yield first insights into the characteristics of 3D ACEO flows and the ability of current numerical models to adequately describe them.