Induced-charge Electrokinetic Research Articles

Investigating the influence of induced charge and surface charge heterogeneity on electrokinetic micromixing: A comparative analysis

This paper investigates the performance of a micromixer by considering the combined effects of surface charge heterogeneity and the induced charge electrokinetic (ICEK) phenomenon. With the applied electric field, surface charge heterogeneity generates a recirculation zone within the microchannel, while conductive links induce micro-vortices. The main objective is to explore the influence of various control parameters on the mixing efficiency, including zeta potential, the number and orientation of conductive links, and applied electrical potential. The generated recirculation zone and induced micro vortices introduce chaotic perturbations within the microchannel, improving mixing performance. The results demonstrate that increasing the zeta potential on heterogeneously charged patches enhances the mixing performance, albeit at the expense of a reduced average outlet velocity. Additionally, a higher number of conductive links leads to increased mixing efficiency. The combination of mounting a conductive link inside the channel and applying a heterogeneous zeta potential patch on the wall substantially increased mixing efficiency. The influence of fluid rheology on mixing performance is also investigated, revealing that pseudoplastic fluids exhibit better mixing than dilatant fluids. Finally, the combined effects of all parameters are evaluated using the Taguchi method, resulting in an optimized mixing performance for the proposed micromixer. Micromixing efficiency exceeding 98% is achieved for all types of non-Newtonian fluids under optimal parametric conditions.

A Novel Electrokinetic-Based Technique for the Isolation of Circulating Tumor Cells.

The separation of rare cells from complex biofluids has attracted attention in biological research and clinical applications, especially for cancer detection and treatment. In particular, various technologies and methods have been developed for the isolation of circulating tumor cells (CTCs) in the blood. Among them, the induced-charge electrokinetic (ICEK) flow method has shown its high efficacy for cell manipulation where micro-vortices (MVs), generated as a result of induced charges on a polarizable surface, can effectively manipulate particles and cells in complex fluids. While the majority of MVs have been induced by AC electric fields, these vortices have also been observed under a DC electric field generated around a polarizable hurdle. In the present numerical work, the capability of MVs for the manipulation of CTCs and their entrapment in the DC electric field is investigated. First, the numerical results are verified against the available data in the literature. Then, various hurdle geometries are employed to find the most effective geometry for MV-based particle entrapment. The effects of electric field strength (EFS), wall zeta potential magnitude, and the particles' diameter on the trapping efficacy are further investigated. The results demonstrated that the MVs generated around only the rectangular hurdle are capable of trapping particles as large as the size of CTCs. An EFS of about 75 V/cm was shown to be effective for the entrapment of above 90% of CTCs in the MVs. In addition, an EFS of 85 V/cm demonstrated a capability for isolating particles larger than 8 µm from a suspension of particles/cells 1-25 µm in diameter, useful for the enrichment of cancer cells and potentially for the real-time and non-invasive monitoring of drug effectiveness on circulating cancer cells in blood circulation.

Dielectric polarization-mediated efficient solute mixing: Effect of the geometrical configuration of polarizing blocks.

We propose a novel technique, consistent with the induced charge electrokinetic (ICEK) phenomenon, for the efficient mixing of solute species at a microfluidic scale. A nonuniform bipolar electric double layer develops in the presence of an external electric field over a polarizable object is better known as the ICEK phenomenon. This ICEK is one of the most favorable techniques preferred for enhanced solute mixing in on-chip microfluidic platforms. In the purview of the ICEK phenomenon, instead of using perfectly conducting polarizable objects, for the first time in this study, we employ polarizable dielectric objects of different sizes and shapes for efficient mixing of solute species. We show that different types of vortices developed in the flow pathway adjacent to the polarizable dielectric blocks help in yielding efficient mixing in the proposed configuration. The novelty of our work is embellished in two different perspectives, that is, first, concentrating on the influences of the physical properties of the polarizable dielectric block on the underlying mixing, and, second, focusing on their sizes, shapes, and the arrangements in tuning the underlying mixingphenomena.

Novel Self-Periodic Motion due to Induced Charge Electrokinetic Phenomena in Porous Materials

A nonlinear oscillator has attracted much attention as a representative nonequilibrium dynamics. We here report a novel self-periodic motion in a porous material. That is, by using a nonlinear elec...

Continuous-Flow Nanoparticle Trapping Driven by Hybrid Electrokinetics in Microfluidics.

We introduce herein an efficient microfluidic approach for continuous transport and localized collection of nanoparticles via hybrid electrokinetics, which delicately combines linear and nonlinear electrokinetics driven by a composite DC-biased AC voltage signal. The proposed technique utilizes a simple geometrical structure, in which one or a series of metal strips serving as floating electrode (FE) are attached to the substrate surface and arranged in parallel between a pair of coplanar driving electrodes (DE) in a straight microchannel. On application of a DC-biased AC electric field across the channel, nanoparticles can be transported continuously by DC bulk electroosmotic flow, and then trapped selectively onto the metal strips due to AC-field induced-charge electrokinetic (ICEK) phenomenon, which behaves as counter-rotating micro-vortices around the ideally polarizable surfaces of FE. Finite-element simulation is carried out by coupling the dual-frequency electric field, flow field and sample mass transfer in sequence, for guiding a practical design of the microfluidic nanoparticle concentrator. With the optimal device geometry, the actual performance of the technique is investigated with respect to DC bias, AC voltage amplitude, and field frequency by using both latex nanospheres (∼500nm) and BSA molecules (∼10nm). Our experimental observation indicates nanoparticles are always enriched into a narrow bright band on the surface of each FE, and a horizontal concentration gradient even emerges in the presence of multiple metal strips, which therefore permits localized analyte enrichment. The proposed trapping method is supposed to guide an elaborate design of flexible electrokinetic frameworks embedding FE for continuous-flow analyte manipulation in modern microfluidic systems.

A three-dimensional numerical investigation of an induced-charge electrokinetic micromixer equipped with fully polarizable particle

Improvement of the mixaing index of low Reynolds number flows in microdimensional devices is essential. This paper investigates the enhancement of the mixing index in a three-dimensional micromixer using an electrically conductive microparticle. The governing equations of the induced-charged electrokinetics based on the thin electric double layer (EDL) assumption are numerically solved using the finite element-based COMSOL Multiphysics (Version 5.3a) software. The effects of various parameters on the mixing process were studied. These parameters include: particle size, chamber height size, position of the particle (initial and equilibrium condition), diameter of inlet and outlet and the Zeta potential distribution over the particle. The three-dimensional simulation improves the comparisons for shape of the chamber as compared with two-dimensional. Therefore, the effect of various shape microchamber (spherical, cylindrical and rectangular-cubic) was investigated. The results indicate that for achieving the perfect mixing index, it is necessary to place the conductive particle at the equilibrium condition. Also, present computational simulations show that the spherical chamber is the appropriate choice which can improve the mixing within the microchannel.

Transient characteristics of electric double layer charging and the associated induced-charge electrokinetic flow

This paper presents a detailed numerical analysis of the transient characteristics of electric double layer (EDL) charging and the associated induced-charge electrokinetic (ICEK) flow around an ideally polarizable cylinder. To this end, we solved numerically the coupled Poisson-Nernst-Planck and Navier-Stokes equations with the finite element method. The numerical simulation provides an unprecedented full-field (including the EDL region) characterization of the transient evolutions of ion transport, electric potential, and fluid flow during the EDL charging. The simulation results show that the EDL charging is driven by the electric current normal to the cylinder surface. With EDL being charged, the charge density in the EDL counteracts the local external electric field on the cylinder surface to reduce the electric current, which then leads to the slowing down of the EDL charging. At the steady state, the EDL becomes fully charged and the charge density in EDL exactly counteracts the external electric field, and then the EDL charging stops. During the EDL charging, the interaction of the external electric field with the charge density in the EDL drives the liquid in the EDL to move first, and then as time evolves, the liquid in the bulk electrolyte sets in motion because of the momentum transfer between the EDL and the bulk. These findings are conducive to the understanding of the transient dynamics of ICEK phenomena around polarizable objects.

On Developing Field-Effect-Tunable Nanofluidic Ion Diodes with Bipolar, Induced-Charge Electrokinetics.

We introduce herein the induced-charge electrokinetic phenomenon to nanometer fluidic systems; the design of the nanofluidic ion diode for field-effect ionic current control of the nanometer dimension is developed by enhancing internal ion concentration polarization through electrochemical transport of inhomogeneous inducing-counterions resulting from double gate terminals mounted on top of a thin dielectric layer, which covers the nanochannel connected to microfluidic reservoirs on both sides. A mathematical model based on the fully-coupled Poisson-Nernst-Plank-Navier-Stokes equations is developed to study the feasibility of this structural configuration causing effective ionic current rectification. The effect of various physiochemical and geometrical parameters, such as the native surface charge density on the nanochannel sidewalls, the number of gate electrodes (GE), the gate voltage magnitude, and the solution conductivity, permittivity, and thickness of the dielectric coating, as well as the size and position of the GE pair of opposite gate polarity, on the resulted rectification performance of the presented nanoscale ionic device is numerically analyzed by using a commercial software package, COMSOL Multiphysics (version 5.2). Three types of electrohydrodynamic flow, including electroosmosis of 1st kind, induced-charge electroosmosis, and electroosmosis of 2nd kind that were originated by the Coulomb force within three distinct charge layers coexist in the micro/nanofluidic hybrid network and are shown to simultaneously influence the output current flux in a complex manner. The rectification factor of a contrast between the ‘on’ and ‘off’ working states can even exceed one thousand-fold in the case of choosing a suitable combination of several key parameters. Our demonstration of field-effect-tunable nanofluidic ion diodes of double external gate electrodes proves invaluable for the construction of a flexible electrokinetic platform for ionic current control and may help transform the field of smart, on-chip, integrated circuits.

Continuous-flow trapping and localized enrichment of micro- and nano-particles using induced-charge electrokinetics.

In this work, we report an effective microfluidic technique for continuous-flow trapping and localized enrichment of micro- and nano-particles by using induced-charge electrokinetic (ICEK) phenomena. The proposed technique utilizes a simple microfluidic device that consists of a straight microchannel and a conducting strip attached to the bottom wall of the microchannel. Upon application of the electric field along the microchannel, the conducting strip becomes polarized to introduce two types of ICEK phenomena, the ICEK flow vortex and particle dielectrophoresis, and they are identified by a theoretical model formulated in this study to be jointly responsible for the trapping of particles over the edge of the conducting strip. Our experiments showed that successful trapping requires an AC/DC combined electric field: the DC component is mainly to induce electroosmotic flow for transporting particles to the trapping location; the AC component induces ICEK phenomena over the edge of the conducting strip for particle trapping. The performance of the technique is examined with respect to the applied electric voltage, AC frequency and the particle size. We observed that the trapped particles form a narrow band (nearly a straight line) defined by the edge of the conducting strip, thereby allowing localized particle enrichment. For instance, we found that under certain conditions a high particle enrichment ratio of 200 was achieved within 30 seconds. We also demonstrated that the proposed technique was able to trap particles from several microns down to several tens of nanometer. We believe that the proposed ICEK trapping would have great flexibility that the trapping location can be readily varied by controlling the location of the patterned conducting strip and multiple-location trapping can be expected with the use of multiple conducting strips.

AC electric field induced dielectrophoretic assembly behavior of gold nanoparticles in a wide frequency range

In this work, we focus on frequency-dependence of pearl chain formations (PCF) of gold nanoparticles driven by AC dielectrophoresis (DEP), especially in a low field-frequency range, where induced double-layer charging effect at ideally polarizable surfaces on particle DEP behavior and surrounding liquid motion need not be negligible. As field frequency varies, grown features of DEP assembly structures ranging from low-frequency non-bridged gap to high-frequency single gold nanoparticle-made nanowires bridging the electrodes are demonstrated experimentally. Specifically, at 10kHz, a kind of novel channel-like structure with parallel opposing banks is formed at the center of interelectrode gap. In stark contrast, at 1MHz, thin PCF with diameter of 100nm is created along the shortest distance of the isolation spacing. Moreover, a particular conductive path of nanoparticle chains is produced at 1MHz in a DEP device embedded with multiple floating electrodes. A theoretical framework taking into account field-induced double-layer polarization at both the particle/electrolyte and electrode/electrolyte interface is developed to correlate these experimental observations with induced-charge electrokinetic (ICEK) phenomenon. And a RC circuit model is helpful in accounting for the formation of this particular non-bridged channel-like structure induced by a low-frequency AC voltage. As compared to thin PCF formed at high field frequency that effectively short circuits the electrode pair, though it is difficult for complete PCF bridging to occur at low frequency, the non-bridged conducting microstructure has potential to further miniaturize the size of electrode gap fabricated by standard micromachining process and may find useful application in biochemical sensing.

AC electric field for rapid assembly of nanostructured polyaniline onto microsized gap for sensor devices.

Interconnected network of nanostructured polyaniline (PANI) is giving strong potential for enhancing device performances than bulk PANI counterparts. For nanostructured device processing, the main challenge is to get prototypes on large area by requiring precision, low cost and high rate assembly. Among processes meeting these requests, the alternate current electric fields are often used for nanostructure assembling. For the first time, we show the assembly of nanostructured PANI onto large electrode gaps (30-60 μm width) by applying alternate current electric fields, at low frequencies, to PANI particles dispersed in acetonitrile (ACN). An important advantage is the short assembly time, limited to 5-10 s, although electrode gaps are microsized. That encouraging result is due to a combination of forces, such as dielectrophoresis (DEP), induced-charge electrokinetic (ICEK) flow and alternate current electroosmotic (ACEO) flow, which speed up the assembly process when low frequencies and large electrode gaps are used. The main achievement of the present study is the development of ammonia sensors created by direct assembling of nanostructured PANI onto electrodes. Sensors exhibit high sensitivity to low gas concentrations as well as excellent reversibility at room temperature, even after storage in air.

Induced-charge electrokinetics in a conducting nanochannel with broken geometric symmetry: Towards a flexible control of ionic transport

In the literature, conventional electrokinetics is widely used as a principle of operating nanofluidic devices. Different from the conventional electrokinetics involving nonpolarizable solid surfaces with fixed surface charge, induced-charge electrokinetic (ICEK) phenomena deal with polarizable surfaces with the ability of surface charge modulation through electric polarization under external electric fields. Because of several advantages, ICEK phenomena have drawn a great deal of attention in microfluidic community. Herein, we propose the first effort of extending the ICEK phenomena from microfluidics to nanofluidics. In particular, we report a numerical model for the ICEK phenomena in a tapered nanochannel with conducting (ideally polarizable) walls. It is shown that due to the broken geometric symmetry of the nanochannel, induced-charge electroosmotic flow inside the nanochannel exhibits a flow rectification such that electrolyte solution always flows from the narrow end of the nanochannel to the wide end for either a forward electric bias (electric field from the narrow to wide ends) or a reverse electric bias (electric field from the wide to narrow ends). In addition, we demonstrate that the ion selectivity of such tapered conducting nanochannel can be actively tuned to be cation-selective with a forward bias and anion-selective with a reverse bias. Promisingly, conducting nanochannels with broken geometric symmetry could be potentially used for constructing nanofluidic pumps with the unidirectional pumping capacity and ion selectors with the tuneable ionic selection.

AC field induced‐charge electroosmosis over leaky dielectric blocks embedded in a microchannel

An effective electrical boundary condition is formulated to describe AC field-driven induced-charge electrokinetic (ICEK) phenomena at the interface between a liquid and a leaky dielectric solid. Since most materials in reality possess finite dielectric and conductive properties, i.e. leaky dielectric, the present boundary condition can be used to describe the induced zeta potential on a leaky dielectric surface with consideration of both bond charges (due to polarization) and free charges (due to conduction). Two well-known limiting cases, i.e. the perfectly dielectric and the perfectly conducting wall boundary conditions can be recovered from the present formulation. Utilizing the derived boundary condition, we obtain analytical solutions in closed form for the AC field-driven induced-charge electroosmosis (ICEO) over two symmetric leaky dielectric blocks embedded in the walls of an infinitely long microchannel. Two important factors for the induced zeta potential are identified to respectively account for the polarization charges and the free charges, and their effects on AC field-driven ICEO oscillating flow patterns are analyzed. It is found that the flow patterns exhibit two counter-rotating vortices, which can be deformed, relocated, eliminated and even reverse their rotating directions. It is very promising that such temporary evolution of flow patterns can possibly induce chaotic advection which can enhance microfluidic mixing.

RETRACTED ARTICLE: Induced-charge electrokinetic phenomena

The induced-charge electrokinetic (ICEK) phenomena are relatively new area of research in microfluidics and nanofluidics. Different from the traditional electrokinetic phenomena which are based on the interactions between applied electric field and the electrostatic charge, the ICEK phenomena result from the interaction of the applied electric field and the induced charge on polarisable surfaces. Because of the different underline physics, ICEK phenomena have many unique characteristics that may lead to new applications in microfluidics and nanofluidics. In this paper, we review the major advancement of research in the field of ICEK phenomena, discuss the applications and the limitations, and suggest some future research directions.

Induced-charge electrokinetics: fundamental challenges and opportunities

Induced-charge electrokinetic (ICEK) phenomena occur when an applied electric field induces an ionic double-layer over a polarizable surface, then forces that induced double-layer into electro-osmotic flow. The nonlinear character of ICEK phenomena enable steady flows to be driven using small AC potentials, which in turn reduces or eliminates electrochemical reactions. As such, ICEK holds promise as a mechanism by which low-voltage, high-pressure pumps may be developed to enable portable, self-contained microfluidic manipulation. Here we review the basic physics of induced-charge electrokinetic phenomena and the advantages they hold for Lab-on-a-Chip devices, in addition to the opportunities they present for fundamental science. In particular, these systems are unique in that all aspects of the (measurable) ICEK flows can be predicted in advance using standard electrokinetic theories, which can then be compared with experimental data. Such comparisons have revealed a number of striking discrepancies between theory and experiment, thus indicating the standard model is missing key physical or chemical ingredients. We discuss specific discrepancies-which remain to be understood-and the challenges they pose for widespread implementation of ICEK in practical Lab-on-a-Chip devices. We propose and present a variety of challenges and opportunities-theoretical and experimental, fundamental and applied-which must be addressed.