Special Issue "AC Electrokinetics in Microfluidic Devices"

A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "C:Chemistry".

Deadline for manuscript submissions: closed (31 January 2019)

Special Issue Editors

Guest Editor
Prof. Antonio Ramos

Departamento de Electrónica y Electromagnetismo, Facultad de Física, Universidad de Sevilla, Seville 41012, Spain
Website 1 | Website 2 | E-Mail
Interests: electrohydrodynamics; electrokinetics; microfluidics; dielectrophoresis
Guest Editor
Prof. Pablo García-Sánchez

Departamento de Electrónica y Electromagnetismo, Facultad de Física, Universidad de Sevilla, Seville 41012, Spain
Website 1 | Website 2 | E-Mail
Interests: electrokinetics; electrowetting; microfluidics; dielectrophoresis

Special Issue Information

Dear Colleagues,

The use of AC electric fields for manipulating and/or characterizing liquids and small particles in suspension is well-known. Owing to miniaturization, several applications in microsystems have appeared over the last few decades in multiple research fields, such as colloidal science, microelectronics and biotechnology. For example, dielectrophoretic (DEP) forces can be used for manipulation and separation of a great variety of particles, such as biological cells, semiconductor nanowires or tiny metal colloids. DEP combined with electrokinetic-induced fluid flows can be leveraged for particle concentration in microfluidic devices. Additionally, application of AC fields gives rise to particle–particle interactions that lead to self-assembly patterns, a common bottom-up approach for the fabrication of engineered microstructures. A number of electric field-induced fluid flows occur in microelectrode structures; these flows can be used for standard liquid manipulation such as pumping and mixing. In addition, the electrical control of the substrate wettability can be achieved by the electrowetting effect, allowing for fine tuning of contact angle and droplet manipulation within microsystems. Besides particle manipulation, AC electrokinetics has also been used to characterize the dielectric properties of particles through DEP, as well as to assist other measurement techniques such as fluorescent spectroscopy or electrical impedance spectroscopy by pre-concentrating the particles.

This Special Issue seeks to showcase research papers, short communications, and review articles that focus on all aspects of the application of AC electrokinetic methods in microfluidics and the lab-on-a-chip technologies.

Prof. Antonio Ramos
Prof. Pablo García-Sánchez
Guest Editors

Manuscript Submission Information

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Keywords

  • AC Electrokinetics
  • Dielectrophoresis
  • AC Electroosmosis
  • Induced-charge Electroosmosis
  • Microfluidics
  • Electrowetting
  • Electrorotation
  • Lab-on-a-Chip

Published Papers (11 papers)

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Editorial

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Open AccessEditorial
Editorial for the Special Issue on AC Electrokinetics in Microfluidic Devices
Micromachines 2019, 10(5), 345; https://doi.org/10.3390/mi10050345
Received: 21 May 2019 / Accepted: 21 May 2019 / Published: 25 May 2019
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Abstract
The use of AC electric fields for manipulating and/or characterizing liquids and small particles in suspension is well-known [...] Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)

Research

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Open AccessArticle
Defining Cell Cluster Size by Dielectrophoretic Capture at an Array of Wireless Electrodes of Several Distinct Lengths
Micromachines 2019, 10(4), 271; https://doi.org/10.3390/mi10040271
Received: 9 March 2019 / Revised: 16 April 2019 / Accepted: 18 April 2019 / Published: 23 April 2019
Cited by 1 | PDF Full-text (4146 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Clusters of biological cells play an important role in normal and disease states, such as in the release of insulin from pancreatic islets and in the enhanced spread of cancer by clusters of circulating tumor cells. We report a method to pattern cells [...] Read more.
Clusters of biological cells play an important role in normal and disease states, such as in the release of insulin from pancreatic islets and in the enhanced spread of cancer by clusters of circulating tumor cells. We report a method to pattern cells into clusters having sizes correlated to the dimensions of each electrode in an array of wireless bipolar electrodes (BPEs). The cells are captured by dielectrophoresis (DEP), which confers selectivity, and patterns cells without the need for physical barriers or adhesive interactions that can alter cell function. Our findings demonstrate that this approach readily achieves fine control of cell cluster size over a broader range set by other experimental parameters. These parameters include the magnitude of the voltage applied externally to drive capture at the BPE array, the rate of fluid flow, and the time allowed for DEP-based cell capture. Therefore, the reported method is anticipated to allow the influence of cluster size on cell function to be more fully investigated. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Graphical abstract

Open AccessFeature PaperArticle
A Continuous Flow-through Microfluidic Device for Electrical Lysis of Cells
Micromachines 2019, 10(4), 247; https://doi.org/10.3390/mi10040247
Received: 31 March 2019 / Revised: 10 April 2019 / Accepted: 11 April 2019 / Published: 13 April 2019
Cited by 1 | PDF Full-text (21166 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
In contrast to the delicate 3D electrodes in the literature, a simple flow-through device is proposed here for continuous and massive lysis of cells using electricity. The device is essentially a rectangular microchannel with a planar electrode array built on its bottom wall, [...] Read more.
In contrast to the delicate 3D electrodes in the literature, a simple flow-through device is proposed here for continuous and massive lysis of cells using electricity. The device is essentially a rectangular microchannel with a planar electrode array built on its bottom wall, actuated by alternating current (AC) voltages between neighboring electrodes, and can be incorporated easily into other biomedical systems. Human whole blood diluted 10 times with phosphate-buffered saline (about 6 × 108 cells per mL) was pumped through the device, and the cells were completely lysed within 7 s after the application of a 20 V peak-to-peak voltage at 1 MHz, up to 400 μL/hr. Electric field and Maxwell stress were calculated for assessing electrical lysis. Only the lower half-channel was exposed to an electric field exceeding the irreversible threshold value of cell electroporation (Eth2), suggesting that a cross flow, proposed here primarily as the electro-thermally induced flow, was responsible for bringing the cells in the upper half-channel downward to the lower half-channel. The Maxwell shear stress associated with Eth2 was one order of magnitude less than the threshold mechanical stresses for lysis, implying that an applied moderate mechanical stress could aid electrical lysis. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Open AccessArticle
Characterization of Chaotic Electroconvection near Flat Inert Electrodes under Oscillatory Voltages
Micromachines 2019, 10(3), 161; https://doi.org/10.3390/mi10030161
Received: 1 February 2019 / Revised: 20 February 2019 / Accepted: 20 February 2019 / Published: 26 February 2019
Cited by 1 | PDF Full-text (4705 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The onset of electroconvective instability in an aqueous binary electrolyte under external oscillatory electric fields at a single constant frequency is investigated in a 2D parallel flat electrode setup. Direct numerical simulations (DNS) of the Poisson–Nernst–Planck equations coupled with the Navier–Stokes equations at [...] Read more.
The onset of electroconvective instability in an aqueous binary electrolyte under external oscillatory electric fields at a single constant frequency is investigated in a 2D parallel flat electrode setup. Direct numerical simulations (DNS) of the Poisson–Nernst–Planck equations coupled with the Navier–Stokes equations at a low Reynolds number are carried out. Previous studies show that direct current (DC) electric field can create electroconvection near ion-selecting membranes in microfluidic devices. In this study, we show that electroconvection can be generated near flat inert electrodes when the applied electric field is oscillatory in time. A range of applied voltage, the oscillation frequency and the ratio of ionic diffusivities is examined to characterize the regime in which electroconvection takes place. Similar to electroconvection under DC voltages, AC electroconvection occurs at sufficiently high applied voltages in units of thermal volts and is characterized by transverse instabilities, physically manifested by an array of counter-rotating vortices near the electrode surfaces. The oscillating external electric field periodically generate and destroy such unsteady vortical structures. As the oscillation frequency is reduced to O ( 10 1 ) of the intrinsic resistor–capacitor (RC) frequency of electrolyte, electroconvective instability is considerably amplified. This is accompanied by severe depletion of ionic species outside the thin electric double layer and by vigorous convective transport involving a wide range of scales including those comparable to the distance L between the parallel electrodes. The underlying mechanisms are distinctly nonlinear and multi-dimensional. However, at higher frequencies of order of the RC frequency, the electrolyte response becomes linear, and the present DNS prediction closely resembles those explained by 1D asymptotic studies. Electroconvective instability supports increased electric current across the system. Increasing anion diffusivity results in stronger amplification of electroconvection over all oscillation frequencies examined in this study. Such asymmetry in ionic diffusivity, however, does not yield consistent changes in statistics and energy spectrum at all wall-normal locations and frequencies, implying more complex dynamics and different scaling for electrolytes with unequal diffusivities. Electric current is substantially amplified beyond the ohmic current at high oscillation frequencies. Also, it is found that anion diffusivity higher than cation has stronger impact on smaller-scale motions ( 0.1 L). Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Open AccessArticle
Modeling the AC Electrokinetic Behavior of Semiconducting Spheres
Micromachines 2019, 10(2), 100; https://doi.org/10.3390/mi10020100
Received: 3 January 2019 / Revised: 23 January 2019 / Accepted: 25 January 2019 / Published: 29 January 2019
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Abstract
We study theoretically the dielectrophoresis and electrorotation of a semiconducting microsphere immersed in an aqueous electrolyte. To this end, the particle polarizability is calculated from first principles for arbitrary thickness of the Debye layers in liquid and semiconductor. We show that the polarizability [...] Read more.
We study theoretically the dielectrophoresis and electrorotation of a semiconducting microsphere immersed in an aqueous electrolyte. To this end, the particle polarizability is calculated from first principles for arbitrary thickness of the Debye layers in liquid and semiconductor. We show that the polarizability dispersion arises from the combination of two relaxation interfacial phenomena: charging of the electrical double layer and the Maxwell–Wagner relaxation. We also calculate the particle polarizability in the limit of thin electrical double layers, which greatly simplifies the analytical calculations. Finally, we show the model predictions for two relevant materials (ZnO and doped silicon) and discuss the limits of validity of the thin double layer approximation. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Open AccessArticle
Highlighting the Role of Dielectric Thickness and Surface Topography on Electrospreading Dynamics
Micromachines 2019, 10(2), 93; https://doi.org/10.3390/mi10020093
Received: 8 January 2019 / Revised: 21 January 2019 / Accepted: 22 January 2019 / Published: 28 January 2019
Cited by 1 | PDF Full-text (2036 KB) | HTML Full-text | XML Full-text
Abstract
The electrospreading behavior of a liquid drop on a solid surface is of fundamental interest in many technological processes. Here we study the effect of the solid topography as well as the dielectric thickness on the dynamics of electrostatically-induced spreading by performing experiments [...] Read more.
The electrospreading behavior of a liquid drop on a solid surface is of fundamental interest in many technological processes. Here we study the effect of the solid topography as well as the dielectric thickness on the dynamics of electrostatically-induced spreading by performing experiments and simulations. In particular, we use an efficient continuum-level modeling approach which accounts for the solid substrate and the electric field distribution coupled with the liquid interfacial shape. Although spreading dynamics depend on the solid surface topography, when voltage is applied electrospreading is independent of the geometric details of the substrate but highly depends on the solid dielectric thickness. In particular, electrospreading dynamics are accelerated with thicker dielectrics. The latter comes to be added to our recent work by Kavousanakis et al., Langmuir, 2018, which also highlights the key role of the dielectric thickness on electrowetting-related phenomena. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Open AccessArticle
Simultaneous Pumping and Mixing of Biological Fluids in a Double-Array Electrothermal Microfluidic Device
Micromachines 2019, 10(2), 92; https://doi.org/10.3390/mi10020092
Received: 8 December 2018 / Revised: 18 January 2019 / Accepted: 25 January 2019 / Published: 28 January 2019
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Abstract
Transport and mixing of minute amounts of biological fluids are significantly important in lab-on-a-chip devices. It has been shown that the electrothermal technique is a suitable candidate for applications involving high-conductivity biofluids, such as blood, saliva, and urine. Here, we introduce a double-array [...] Read more.
Transport and mixing of minute amounts of biological fluids are significantly important in lab-on-a-chip devices. It has been shown that the electrothermal technique is a suitable candidate for applications involving high-conductivity biofluids, such as blood, saliva, and urine. Here, we introduce a double-array AC electrothermal (ACET) device consisting of two opposing microelectrode arrays, which can be used for simultaneous mixing and pumping. First, in a 2D simulation, an optimum electrode-pair configuration capable of achieving fast transverse mixing at a microfluidic channel cross-section is identified by comparing different electrode geometries. The results show that by adjusting the applied voltage pattern and position of the asymmetrical microelectrodes in the two arrays, due to the resultant circular flow streamlines, the time it takes for the analytes to be convected across the channel cross-section is reduced by 95% compared to a diffusion-only-based transport regime, and by 80% compared to a conventional two-layer ACET device. Using a 3D simulation, the fluid transport (pumping and mixing) capabilities of such an electrode pair placed at different angles longitudinally relative to the channel was studied. It was found that an asymmetrical electrode configuration placed at an angle in the range of 30 ° θ 45 ° can significantly increase transversal mixing efficiency while generating strong longitudinal net flow. These findings are of interest for lab-on-a-chip applications, especially for biosensors and immunoassays, where mixing analyte solutions while simultaneously moving them through a microchannel can greatly enhance the sensing efficiency. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Open AccessArticle
AC Electrokinetics of Polarizable Tri-Axial Ellipsoidal Nano-Antennas and Quantum Dot Manipulation
Micromachines 2019, 10(2), 83; https://doi.org/10.3390/mi10020083
Received: 29 October 2018 / Revised: 11 January 2019 / Accepted: 17 January 2019 / Published: 24 January 2019
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Abstract
By realizing the advantages of using a tri-axial ellipsoidal nano-antenna (NA) surrounded by a solute for enhancing light emission of near-by dye molecules, we analyze the possibility of controlling and manipulating the location of quantum dots (similar to optical tweezers) placed near NA [...] Read more.
By realizing the advantages of using a tri-axial ellipsoidal nano-antenna (NA) surrounded by a solute for enhancing light emission of near-by dye molecules, we analyze the possibility of controlling and manipulating the location of quantum dots (similar to optical tweezers) placed near NA stagnation points, by means of prevalent AC electric forcing techniques. First, we consider the nonlinear electrokinetic problem of a freely suspended, uncharged, polarized ellipsoidal nanoparticle immersed in a symmetric unbounded electrolyte which is subjected to a uniform AC ambient electric field. Under the assumption of small Peclet and Reynolds numbers, thin Debye layer and ‘weak-field’, we solve the corresponding electrostatic and hydrodynamic problems. Explicit expressions for the induced velocity, pressure, and vorticity fields in the solute are then found in terms of the Lamé functions by solving the non-homogeneous Stokes equation forced by the Coulombic density term. The particular axisymmetric quadrupole-type flow for a conducting sphere is also found as a limiting case. It is finally demonstrated that stable or equilibrium (saddle-like) positions of a single molecule can indeed be achieved near stagnation points, depending on the directions of the electric forcing and the induced hydrodynamic (electroosmotic) and dielectrophoretic dynamical effects. The precise position of a fluorophore next to an ellipsoidal NA, can thus be simply controlled by adjusting the frequency of the ambient AC electric field. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Open AccessArticle
Numerical Solution of the Electrokinetic Equations for Multi-ionic Electrolytes Including Different Ionic Size Related Effects
Micromachines 2018, 9(12), 647; https://doi.org/10.3390/mi9120647
Received: 8 November 2018 / Revised: 26 November 2018 / Accepted: 5 December 2018 / Published: 7 December 2018
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Abstract
One of the main assumptions of the standard electrokinetic model is that ions behave as point-like entities. In a previous work (López-García, et al., 2015) we removed this assumption and analyzed the influence of finite ionic size on the dielectric and electrokinetic properties [...] Read more.
One of the main assumptions of the standard electrokinetic model is that ions behave as point-like entities. In a previous work (López-García, et al., 2015) we removed this assumption and analyzed the influence of finite ionic size on the dielectric and electrokinetic properties of colloidal suspensions using both the Bikerman and the Carnahan–Starling equations for the steric interactions. It was shown that these interactions improved upon the standard model predictions so that the surface potential, electrophoretic mobility, and the conductivity and permittivity increment values were increased. In the present study, we extend our preceding works to systems made of three or more ionic species with different ionic sizes. Under these conditions, the Bikerman and Carnahan–Starling expressions cease to be valid since they were deduced for single-size spheres. Fortunately, the Carnahan–Starling expression has been extended to mixtures of spheres of unequal size, namely the “Boublik–Mansoori–Carnahan–Starling–Leland” (BMCSL) equation of state, making it possible to analyze the most general case. It is shown that the BMCSL expression leads to results that differ qualitatively and quantitatively from the standard electrokinetic model. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Open AccessArticle
Asymmetrical Induced Charge Electroosmotic Flow on a Herringbone Floating Electrode and Its Application in a Micromixer
Micromachines 2018, 9(8), 391; https://doi.org/10.3390/mi9080391
Received: 12 July 2018 / Revised: 29 July 2018 / Accepted: 1 August 2018 / Published: 7 August 2018
Cited by 1 | PDF Full-text (3796 KB) | HTML Full-text | XML Full-text
Abstract
Enhancing mixing is of significant importance in microfluidic devices characterized by laminar flows and low Reynolds numbers. An asymmetrical induced charge electroosmotic (ICEO) vortex pair generated on the herringbone floating electrode can disturb the interface of two-phase fluids and deliver the fluid transversely, [...] Read more.
Enhancing mixing is of significant importance in microfluidic devices characterized by laminar flows and low Reynolds numbers. An asymmetrical induced charge electroosmotic (ICEO) vortex pair generated on the herringbone floating electrode can disturb the interface of two-phase fluids and deliver the fluid transversely, which could be exploited to accomplish fluid mixing between two neighbouring fluids in a microscale system. Herein we present a micromixer based on an asymmetrical ICEO flow induced above the herringbone floating electrode array surface. We investigate the average transverse ICEO slip velocity on the Ridge/Vee/herringbone floating electrode and find that the microvortex generated on the herringbone electrode surface has good potential for mixing the miscible liquids in microfluidic systems. In addition, we explore the effect of applied frequencies and bulk conductivity on the slip velocity above the herringbone floating electrode surface. The high dependence of mixing performance on the floating electrode pair numbers is analysed simultaneously. Finally, we investigate systematically voltage intensity, applied frequencies, inlet fluid velocity and liquid conductivity on the mixing performance of the proposed device. The microfluidic micromixer put forward herein offers great opportunity for fluid mixing in the field of micro total analysis systems. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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Review

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Open AccessReview
Liquid Crystals-Enabled AC Electrokinetics
Micromachines 2019, 10(1), 45; https://doi.org/10.3390/mi10010045
Received: 12 December 2018 / Revised: 1 January 2019 / Accepted: 1 January 2019 / Published: 10 January 2019
Cited by 1 | PDF Full-text (8234 KB) | HTML Full-text | XML Full-text
Abstract
Phenomena of electrically driven fluid flows, known as electro-osmosis, and particle transport in a liquid electrolyte, known as electrophoresis, collectively form a subject of electrokinetics. Electrokinetics shows a great potential in microscopic manipulation of matter for various scientific and technological applications. Electrokinetics is [...] Read more.
Phenomena of electrically driven fluid flows, known as electro-osmosis, and particle transport in a liquid electrolyte, known as electrophoresis, collectively form a subject of electrokinetics. Electrokinetics shows a great potential in microscopic manipulation of matter for various scientific and technological applications. Electrokinetics is usually studied for isotropic electrolytes. Recently it has been demonstrated that replacement of an isotropic electrolyte with an anisotropic, or liquid crystal (LC), electrolyte, brings about entirely new mechanisms of spatial charge formation and electrokinetic effects. This review presents the main features of liquid crystal-enabled electrokinetics (LCEK) rooted in the field-assisted separation of electric charges at deformations of the director that describes local molecular orientation of the LC. Since the electric field separates the charges and then drives the charges, the resulting electro-osmotic and electrophoretic velocities grow as the square of the applied electric field. We describe a number of related phenomena, such as alternating current (AC) LC-enabled electrophoresis of colloidal solid particles and fluid droplets in uniform and spatially-patterned LCs, swarming of colloids guided by photoactivated surface patterns, control of LCEK polarity through the material properties of the LC electrolyte, LCEK-assisted mixing at microscale, separation and sorting of small particles. LC-enabled electrokinetics brings a new dimension to our ability to manipulate dynamics of matter at small scales and holds a major promise for future technologies of microfluidics, pumping, mixing, sensing, and diagnostics. Full article
(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices)
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