Induced-charge Electrokinetics Research Articles

Liquid metal droplet-enabled electrocapillary flow in biased alternating electric fields: a theoretical analysis from the perspective of induced-charge electrokinetics

The phenomenon of Marangoni chaotic electroconvection of electrolytes induced by liquid metal droplets has been serving as a method of choice for pumping of microfluidics in the past ten years. Although an external additional alternating voltage, usually being sufficiently small, was invariably employed in practical experiments for stabilizing the unidirectional liquid motion caused by a preexisted direct-current electric field, its subtle effect on the superimposed non-uniform surface tension and the final fluid transport state is still theoretically unclear. From the physical perspective of induced-charge electrokinetics, we present herein an analytical criteria for the conditions under which a sessile metal drop can actuate unidirectional liquid motion passing across it affected by a biased alternating voltage signal, wherein a mechanical balance between the linear pump pressure and nonlinear bipolar stress components exerted on the conducting drop surface has been taken into account. For accuracy validation, a numerical model was developed, in which multiple frequency electrochemical polarization was calculated under the Debye–Huckel limit, and coupled to the mechanical problem via inserting the resultant surface tension gradient as a source term of electrocapillary stress at the droplet–medium interface. The analytical solution accords well with the simulation results, in terms of the threshold voltage amplitude for effective liquid transport along the static potential gradient. At last, we demonstrate that the effect of continuous electrowetting can be fully exploited for simultaneous pumping and extraction of microscale impurities in closed fluid loops embedding a network of three-dimensional floating electrode plates for invoking auxiliary dielectrophoretic attraction. These results prove invaluable for achieving a high degree of freedom manipulation of particle and liquid contents in modern microfluidic systems with flexible conducting microdroplets.

Asymmetric rectified electric fields generate flows that can dominate induced-charge electrokinetics

Extant theories for induced-charge electrokinetics (ICEK) sometimes fail to predict even the correct direction of flow. By combining the recently discovered phenomenon of asymmetric rectified electric fields with a generalized form of ICEK, this long-standing dilemma is explained.

Numerical analysis of non-uniform electric field effects on induced charge electrokinetics flow with application in micromixers

Induced charge electrokinetics (ICEK) has recently expanded its range of applications in biomedical engineering due to its high potential for being used in particle/cell manipulation platforms, micromixers, micropumps, and microvalves. In these devices, the generated microvortices around conducting hurdles/electrodes are used in order to achieve the desired operations. In the present study, the influences of non-uniform electric field on the generated microvortices around a conducting cylinder are analyzed in a micromixer. The electric field non-uniformities are produced either by pin-plate electrodes or by geometrical configuration. Results show that inhomogeneous electric fields lead to reduction of the induced zeta potential (IZP) and the microvortices velocity around the cylinder in the majority of the studied cases. However, imposing two electric fields with appropriate directions or positioning the cylinder in a suitable location enhances the IZP reductions. In this study, such electric fields are utilized in the ICEK micromixer for investigation of their effects on the mixing efficiency. Based on our findings, it is demonstrated that the generated electric field by two electrodes with angle of 30° increases the mixing efficiency from 69.07% to 96.89%. The present study provides new insights into the effects of non-uniform electric field on the IZP over a conducting hurdle and the generated microvortices in ICEK flows with an application in lab-on-a-chip (LOC) devices such as micromixers, microvalves, micropumps, and particle/cell separators.

Induced-charge electrokinetics in rotating electric fields: A linear asymptotic analysis

Concerning the electroconvective analyte manipulation in microfluidics, we describe the basic physics of fluid flow driven by rotating induced-charge electro-osmosis (ROT-ICEO), which occurs on the planar surface of a single floating electrode in an external rotating electric field. First, based on a linear asymptotic analysis, the dynamic flow stagnation line in ROT-ICEO induced on the bipolar electrode from a rotary Debye screening charge revolves synchronously with the applied rotating fields. A net hydrodynamic torque is then generated that acts on any fluid or particle sample to produce either a synchronous or asynchronous co-field rotation depending on the frequency of the ac signal. Next, from the synergy between the hydrodynamic and electrochemical ion relaxations, an analytical solution of the sample rotation rate subject to ROT-ICEO slipping on an ideally polarizable surface is obtained for different frequency ranges and determined by the transient nature of the rotating electro-osmotic flow oscillating at twice the field frequency. To visualize the flow field in ROT-ICEO, experiments were performed with fluorescent tracer nanoparticles; they exhibited concentric rotational behavior at the polarized phase interface. Formed like the arms of a nebula disk, the four twisted tails of nanoparticles can be arbitrarily directed under voltage-phase rectification. These experimental results are in good agreement with our mathematical simulations using the Debye–Hückel approximation on ROT-ICEO.

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.

Induced-charge electrokinetics, bipolar current, and concentration polarization in a microchannel-Nafion-membrane system.

The presence of a floating electrode array located within the depletion layer formed due to concentration polarization across a microchannel-membrane interface device may produce not only induced-charge electro-osmosis (ICEO) but also bipolar current resulting from the induced Faradaic reaction. It has been shown that there exists an optimal thickness of a thin dielectric coating that is sufficient to suppress bipolar currents but still enables ICEO vortices that stir the depletion layer, thereby affecting the system's current-voltage response. In addition, the use of alternating-current electro-osmosis by activating electrodes results in further enhancement of the fluid stirring and opens new routes for on-demand spatiotemporal control of the depletion layer length.

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.

An automated, high-throughput experimental system for induced charge electrokinetics

Recent experiments in induced charge electrokinetics (ICEK) have shown that the standard theory generally overpredicts experimentally observed velocities. Such discrepancies reduce the efficacy of practical ICEK devices, and highlight our incomplete understanding of electrokinetic phenomena. Here, we present an automated experimental system that allows for the rapid collection of ICEK data under a variety of conditions ( approximately 1000 per day) to help develop and constrain new theories. We demonstrate this system by studying the ICEK slip flows over electrodes that have been controllably "contaminated" with a dielectric layer, either SiO(2) or an alkanethiol self-assembled monolayer, of known thickness. We also develop a theory that accounts for the effects of the dielectric coatings surface chemistry that yields quantitative agreement with experiments over nearly a thousand distinct conditions in the SiO(2) system and present an additional three thousand experiments of flows over alkanethiol monolayers. Our experimental system allows the direct interrogation of the physico-chemical effects that influence ICEK flows and for the optimization of these flows in lab-on-a-chip systems.

Electrokinetic motion of particles and cells in microchannels

This paper provides an overview of the electrokinetic phenomena associated with particles and cells in microchannel systems. The most important phenomena covered include electrophoresis, dielectrophoresis, and induced-charge electrokinetics. The latest development of these electrokinetic techniques for particle or cell manipulations in microfluidic systems is reviewed, in terms of the basic theories, mathematical models, numerical and experimental methods, and the key results/findings from the published literatures in the most recent decades. Some of the limitations associated with the negative field effects are discussed and the perspectives for the future investigations are summarized.

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.

Continuous Flow Microfluidic Demixing of Electrolytes by Induced Charge Electrokinetics in Structured Electrode Arrays

A continuous flow microfluidic demixing process is realized. It utilizes high external electrical fields that are applied over electrically floating noble metal electrodes in an otherwise straight microchannel. The process converts axial electrical potential gradients into lateral molecular selective transport via a structure oriented ensemble of numerous electrodes. While the individual electrodes locally modify the electrolyte distribution by nonlinear electrokinetic effects and concentration polarization, the directed orientation of the electrode array combines the individual polarization zones to a dedicated molecular enrichment against the generated concentration gradient. A homogeneously concentrated electrolyte can be separated into arbitrarily shaped laminae of increased and depleted concentration by the presented microfluidic demixer.