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2 pages, 132 KiB

Open AccessEditorial

Editorial for the Special Issue on AC Electrokinetics in Microfluidic Devices, Volume II

by Antonio Ramos, Pablo García-Sánchez and Raúl Fernández-Mateo

Micromachines 2024, 15(10), 1229; https://doi.org/10.3390/mi15101229 - 30 Sep 2024

Viewed by 654

Abstract

The use of AC electric fields in manipulating and characterizing liquids and suspended particles in microfluidic systems continues to drive innovation in several fields, such as colloidal science, microelectronics, and biotechnology [...] Full article

(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices, Volume II)

Research

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22 pages, 7732 KiB

Open AccessArticle

Trajectories and Forces in Four-Electrode Chambers Operated in Object-Shift, Dielectrophoresis and Field-Cage Modes—Considerations from the System’s Point of View

by Jan Gimsa and Michal M. Radai

Micromachines 2023, 14(11), 2042; https://doi.org/10.3390/mi14112042 - 31 Oct 2023

Cited by 1 | Viewed by 984

Abstract

In two previous papers, we calculated the dielectrophoresis (DEP) force and corresponding trajectories of high- and low-conductance 200-µm 2D spheres in a square 1 × 1-mm chamber with plane-versus-pointed, plane-versus-plane and pointed-versus-pointed electrode configurations by applying the law of maximum entropy production (LMEP) [...] Read more.

In two previous papers, we calculated the dielectrophoresis (DEP) force and corresponding trajectories of high- and low-conductance 200-µm 2D spheres in a square 1 × 1-mm chamber with plane-versus-pointed, plane-versus-plane and pointed-versus-pointed electrode configurations by applying the law of maximum entropy production (LMEP) to the system. Here, we complete these considerations for configurations with four-pointed electrodes centered on the chamber edges. The four electrodes were operated in either object-shift mode (two adjacent electrodes opposite the other two adjacent electrodes), DEP mode (one electrode versus the other three electrodes), or field-cage mode (two electrodes on opposite edges versus the two electrodes on the other two opposite edges). As in previous work, we have assumed DC properties for the object and the external media for simplicity. Nevertheless, every possible polarization ratio of the two media can be modeled this way. The trajectories of the spherical centers and the corresponding DEP forces were calculated from the gradients of the system’s total energy dissipation, described by numerically-derived conductance fields. In each of the three drive modes, very high attractive and repulsive forces were found in front of pointed electrodes for the high and low-conductance spheres, respectively. The conductance fields predict bifurcation points, watersheds, and trajectories with multiple endpoints. The high and low-conductance spheres usually follow similar trajectories, albeit with reversed orientations. In DEP drive mode, the four-point electrode chamber provides a similar area for DEP measurements as the classical plane-versus-pointed electrode chamber. Full article

(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices, Volume II)

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16 pages, 3486 KiB

Open AccessArticle

pH Gradients in Spatially Non-Uniform AC Electric Fields around the Charging Frequency; A Study of Two Different Geometries and Electrode Passivation

by Azade Tahmasebi, Sanaz Habibi, Jeana L. Collins, Ran An, Esmaeil Dehdashti and Adrienne Robyn Minerick

Micromachines 2023, 14(9), 1655; https://doi.org/10.3390/mi14091655 - 23 Aug 2023

Cited by 2 | Viewed by 1286

Abstract

Dielectrophoresis (DEP), a precision nonlinear electrokinetic tool utilized within microfluidic devices, can induce bioparticle polarization that manifests as motion in the electric field; this phenomenon has been leveraged for phenotypic cellular and biomolecular detection, making DEP invaluable for diagnostic applications. As device operation [...] Read more.

Dielectrophoresis (DEP), a precision nonlinear electrokinetic tool utilized within microfluidic devices, can induce bioparticle polarization that manifests as motion in the electric field; this phenomenon has been leveraged for phenotypic cellular and biomolecular detection, making DEP invaluable for diagnostic applications. As device operation times lengthen, reproducibility and precision decrease, which has been postulated to be caused by ion gradients within the supporting electrolyte medium. This research focuses on characterizing pH gradients above, at, and below the electrode charging frequency (0.2–1.4 times charging frequency) in an aqueous electrolyte solution in order to extend the parameter space for which microdevice-imposed artifacts on cells in clinical diagnostic devices have been characterized. The nonlinear alternating current (AC) electric fields (0.07 V_pp/μm) required for DEP were generated via planar T-shaped and star-shaped microelectrodes overlaid by a 70 μm high microfluidic chamber. The experiments were designed to quantify pH changes temporally and spatially in the two microelectrode geometries. In parallel, a 50 nm hafnium oxide (HfO₂) thin film on the microelectrodes was tested to provide insights into the role of Faradaic surface reactions on the pH. Electric field simulations were conducted to provide insights into the gradient shape within the microelectrode geometries. Frequency dependence was also examined to ascertain ion electromigration effects above, at, and below the electrode charging frequency. The results revealed Faradaic reactions above, at, and below the electrode charging frequency. Comparison experiments further demonstrated that pH changes caused by Faradaic reactions increased inversely with frequency and were more pronounced in the star-shaped geometry. Finally, HfO₂ films demonstrated frequency-dependent properties, impeding Faradaic reactions. Full article

(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices, Volume II)

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12 pages, 3539 KiB

Open AccessArticle

Multiphase Actuation of AC Electrothermal Micropump

by Stirling Cenaiko, Thomas Lijnse and Colin Dalton

Micromachines 2023, 14(4), 758; https://doi.org/10.3390/mi14040758 - 29 Mar 2023

Cited by 3 | Viewed by 1247

Abstract

Electrothermal micropumps apply an AC electric field to a conductive fluid within the range of 10 kHz–1 MHz to generate fluid flow. In this frequency range, coulombic forces dominate fluid interactions over opposing dielectric forces, resulting in high flow rates (~50–100 μm/s). To [...] Read more.

Electrothermal micropumps apply an AC electric field to a conductive fluid within the range of 10 kHz–1 MHz to generate fluid flow. In this frequency range, coulombic forces dominate fluid interactions over opposing dielectric forces, resulting in high flow rates (~50–100 μm/s). To date, the electrothermal effect—using asymmetrical electrodes—has been tested only with single-phase and 2-phase actuation, while dielectrophoretic micropumps have shown improved flow rates with 3- and 4-phase actuation. Simulating muti-phase signals in COMSOL Multiphysics requires additional modules and a more involved implementation to accurately represent the electrothermal effect in a micropump. Here, we report detailed simulations of the electrothermal effect under multi-phase conditions, including single-phase, 2-phase, 3-phase and 4-phase actuation patterns. These computational models indicate that 2-phase actuation leads to the highest flow rate, with 3-phase resulting in a 5% reduced flow rate and 4-phase resulting in an 11% reduced flow rate compared to 2-phase. With these modifications to the simulation, various actuation patterns can later be tested in COMSOL for a range of electrokinetic techniques. Full article

(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices, Volume II)

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17 pages, 1806 KiB

Open AccessArticle

Impedance-Frequency Response of Closed Electrolytic Cells

by José Juan López-García, José Horno and Constantino Grosse

Micromachines 2023, 14(2), 368; https://doi.org/10.3390/mi14020368 - 31 Jan 2023

Cited by 2 | Viewed by 1215

Abstract

The electric AC response of electrolytic cells with DC bias is analyzed solving numerically the Poisson–Nernst–Planck equations and avoiding the commonly used infinite solution approximation. The results show the presence of an additional low-frequency dispersion process associated with the finite spacing of the [...] Read more.

The electric AC response of electrolytic cells with DC bias is analyzed solving numerically the Poisson–Nernst–Planck equations and avoiding the commonly used infinite solution approximation. The results show the presence of an additional low-frequency dispersion process associated with the finite spacing of the electrodes. Moreover, we find that the condition of fixed ionic content inside the electrolytic cell has a strong bearing on both the steady-state and the frequency response. For example: the characteristic frequency of the high-frequency dispersion decreases when the DC potential increases and/or the electrode spacing decreases in the closed cell case, while it remains essentially insensitive on these changes for open cells. Finally, approximate analytic expressions for the dependences of the main parameters of both dispersion processes are also presented. Full article

(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices, Volume II)

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14 pages, 1773 KiB

Open AccessArticle

Scattering of Metal Colloids by a Circular Post under Electric Fields

by José Eladio Flores-Mena, Pablo García-Sánchez and Antonio Ramos

Micromachines 2023, 14(1), 23; https://doi.org/10.3390/mi14010023 - 22 Dec 2022

Cited by 2 | Viewed by 1379

Abstract

We consider the scattering of metal colloids in aqueous solutions by an insulating circular post under the action of an AC electric field. We analyze the effects on the particle of several forces of electrical origin: the repulsion between the induced dipole of [...] Read more.

We consider the scattering of metal colloids in aqueous solutions by an insulating circular post under the action of an AC electric field. We analyze the effects on the particle of several forces of electrical origin: the repulsion between the induced dipole of the particle and its image dipole in the post, the hydrodynamic interaction with the post due to the induced-charge electroosmotic (ICEO) flow around the particle, and the dielectrophoresis arising from the distortion of the applied electric field around the post. The relative influence of these forces is discussed as a function of frequency of the AC field, particle size and distance to the post. We perform numerical simulations of the scattering of the metal colloid by the insulating circular post flowing in a microchannel and subjected to alternating current electric fields. Our simulation results show that the maximum particle deviation is found for an applied electric field parallel to the flow direction. The deviation is also greater at low electric field frequencies, corresponding to the regime in which the ICEO’s interaction with the post is predominant over other mechanisms. Full article

(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices, Volume II)

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8 pages, 3079 KiB

Open AccessArticle

Oscillatory Motion of Water Droplets Both in Oil and on Superhydrophobic Surface under Corona Discharge

by Qiang Tang, Zongtang Zhang, Jia-Han Zhang, Feiran Tang, Chengjun Wang and Xiaxia Cui

Micromachines 2022, 13(12), 2229; https://doi.org/10.3390/mi13122229 - 15 Dec 2022

Cited by 6 | Viewed by 1619

Abstract

Charged droplets driven by Coulomb force are an important part of a droplet-based micro reactor. In this study, we realized the rapid oscillatory motion of droplets both in oil and on superhydrophobic surface by injecting charges through corona discharge. Distinct from the oscillatory [...] Read more.

Charged droplets driven by Coulomb force are an important part of a droplet-based micro reactor. In this study, we realized the rapid oscillatory motion of droplets both in oil and on superhydrophobic surface by injecting charges through corona discharge. Distinct from the oscillatory motion of water droplets under a DC electric field, charge injection can make the motion of water droplets more flexible. A droplet in the oil layer can move up and down regularly under the action of corona discharge, and the discharge voltage can control the movement period and height of the droplet. In addition, the left–right translation of droplets on a superhydrophobic surface can be achieved by injecting charges into the hydrophobic film surface through corona discharge. Two kinds of droplet motion behaviors are systematically analyzed, and the mechanism of droplet motion is explained. The present results could help establish new approaches to designing efficient machines in microfluidics and micromechanical equipment. Full article

(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices, Volume II)

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20 pages, 1045 KiB

Open AccessArticle

Travelling-Wave Electrophoresis, Electro-Hydrodynamics, Electro-Rotation, and Symmetry-Breaking of a Polarizable Dimer in Non-Uniform Fields

by Touvia Miloh and Eldad J. Avital

Micromachines 2022, 13(8), 1173; https://doi.org/10.3390/mi13081173 - 25 Jul 2022

Cited by 2 | Viewed by 1587

Abstract

A theoretical framework is presented for calculating the polarization, electro-rotation, travelling-wave dielectrophoresis, electro-hydrodynamics and induced-charge electroosmotic flow fields around a freely suspended conducting dimer (two touching spheres) exposed to non-uniform direct current (DC) or alternating current (AC) electric fields. The analysis is based [...] Read more.

A theoretical framework is presented for calculating the polarization, electro-rotation, travelling-wave dielectrophoresis, electro-hydrodynamics and induced-charge electroosmotic flow fields around a freely suspended conducting dimer (two touching spheres) exposed to non-uniform direct current (DC) or alternating current (AC) electric fields. The analysis is based on employing the classical (linearized) Poisson–Nernst–Planck (PNP) formulation under the standard linearized ‘weak-field’ assumption and using the tangent-sphere coordinate system. Explicit expressions are first derived for the axisymmetric AC electric potential governed by the Robin (mixed) boundary condition applied on the dimer surface depending on the resistance–capacitance circuit (RC) forcing frequency. Dimer electro-rotation due to two orthogonal (out-of-phase) uniform AC fields and the corresponding mobility problem of a polarizable dimer exposed to a travelling-wave electric excitation are also analyzed. We present an explicit solution for the non-linear induced-charge electroosmotic (ICEO) flow problem of a free polarized dimer in terms of the corresponding Stokes stream function determined by the Helmholtz–Smoluchowski velocity slip. Next, we demonstrate how the same framework can be used to obtain an exact solution for the electro-hydrodynamic (EHD) problem of a polarizable sphere lying next to a conducting planar electrode. Finally, we present a new solution for the induced-charge mobility of a Janus dimer composed of two fused spherical colloids, one perfectly conducting and one dielectrically coated. So far, most of the available electrokinetic theoretical studies involving polarizable nano/micro shapes dealt with convex configurations (e.g., spheres, spheroids, ellipsoids) and as such the newly obtained electrostatic AC solution for a dimer provides a useful extension for similar concave colloids and engineered particles. Full article

(This article belongs to the Special Issue AC Electrokinetics in Microfluidic Devices, Volume II)

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