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The editorial in Micromachines discusses the advancements in AC electrokinetics within microfluidic systems, highlighting its applications in manipulating particles and liquids for various fields like biotechnology and materials science. The Special Issue focuses on research articles exploring particle manipulation, fluid flow control, and electrolyte responses to AC electric fields in microsystems. The diverse studies presented underscore the growing significance of AC electrokinetics in microfluidic applications, showcasing its potential impact on scientific and industrial sectors. [Extracted from the article]

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. [ABSTRACT FROM AUTHOR]

We experimentally study the repulsion of charged microscopic particles with the channel walls during electrophoresis in microfluidic devices. For low frequencies of the electric fields (< 10 kHz), this repulsion is mainly due to the hydrodynamic interaction caused by the flow vortices that arise from the slip velocity induced by the electric field on the particle surface, as shown in a recent publication [Fernandez-Mateo et al., Phys. Rev. Lett. 128, 074501 (2022)]. The maximum slip velocity on the particle surface is inferred from measurements of wall-particle separation. Importantly, this procedure allows us to infer very small slip velocities that, otherwise, are too weak to be measured directly. Data at small electric field amplitudes ( E 0 ) agree with theoretical predictions using the model of Concentration Polarization Electro-osmosis (CPEO), which has recently been proposed as the mechanism behind the flow vortices on the surface of the particles. Data for higher electric fields show that the predictions of the CPEO theory for weak electric fields are not valid beyond E 0 ∼ 60 kV/m. Additionally, we also show that, for sufficiently strong electric fields, the quadrupolar flow structures become disrupted, leading to a weaker wall repulsion. [ABSTRACT FROM AUTHOR]

[Display omitted] While previous studies indicated the mobility reversal of an electrically-powered metallo-dielectric Janus particle (JP) with increasing frequency, here we report an intriguing second mobility reversal observed in optoelectronically-driven JPs. In contrast to the commonly used setup with parallel ITO-coated glass substrates to induce a uniform electric field orthogonal to the velocity direction, this setup incorporates a thin photoconductive layer deposited on the bottom ITO-coated glass substrate. We have found that the reversal is associated with the asymmetry of the bottom substrate's photoconductivity, localized underneath the JP, resulting from the self-shading effect of the metallic hemisphere under top optical illumination. Numerous control tests, including optical illumination from the bottom, along with numerical simulations, support this hypothesized mechanism. [ABSTRACT FROM AUTHOR]

This paper describes the behavior of particles in a deterministic lateral displacement (DLD) separation device with DC and AC electric fields applied orthogonal to the fluid flow. As proof of principle, we demonstrate tunable microparticle and nanoparticle separation and fractionation depending on both particle size and zeta potential. DLD is a microfluidic technique that performs size-based binary separation of particles in a continuous flow. Here, we explore how the application of both DC and AC electric fields (separate or together) can be used to improve separation in a DLD device. We show that particles significantly smaller than the critical diameter of the device can be efficiently separated by applying orthogonal electric fields. Following the application of a DC voltage, Faradaic processes at the electrodes cause local changes in medium conductivity. This conductivity change creates an electric field gradient across the channel that results in a nonuniform electrophoretic velocity orthogonal to the primary flow direction. This phenomenon causes particles to focus on tight bands as they flow along the channel countering the effect of particle diffusion. It is shown that the final lateral displacement of particles depends on both particle size and zeta potential. Experiments with six different types of negatively charged particles and five different sizes (from 100 nm to 3 μm) and different zeta potential demonstrate how a DC electric field combined with AC electric fields (that causes negative-dielectrophoresis particle deviation) could be used for fractionation of particles on the nanoscale in microscale devices. [ABSTRACT FROM AUTHOR]

We describe a novel particle separation technique that combines deterministic lateral displacement (DLD) with orthogonal electrokinetic forces. DLD is a microfluidic technique for continuous flow particle separation based on size. We describe new tunable devices that use a combination of AC electric fields with DLD to separate particles below the critical diameter. Planar electrodes were integrated into a classical DLD device to produce a force orthogonal to the fluid flow direction. Experiments with 3.0 μm, 1.0 μm and 500 nm diameter microspheres show that at low frequencies (up to 500 Hz) particles oscillate in the direction of the field due to electrophoretic (EP)/electroosmotic (EO) forces. As the frequency of the field increases, the amplitude of these oscillations vanishes and, eventually dielectrophoresis (DEP) becomes the dominant electrokinetic force on the particles (DEP arises from electric field inhomogeneities caused by the presence of the DLD posts). Both mechanisms alter the paths of the particles inside the DLD devices leading to enhanced sorting of particles below the critical diameter of the device. [ABSTRACT FROM AUTHOR]

This paper reviews both theory and experimental observation of the AC electrokinetic properties of conducting microparticles suspended in an aqueous electrolyte. Applied AC electric fields interact with the induced charge in the electrical double layer at the metal particle–electrolyte interface. In general, particle motion is governed by both the electric field interacting with the induced dipole on the particle and also the induced-charge electro-osmotic (ICEO) flow around the particle. The importance of the RC time for charging the double layer is highlighted. Experimental measurements of the AC electrokinetic behaviour of conducting particles (dielectrophoresis, electro-rotation and electro-orientation) are compared with theory, providing a comprehensive review of the relative importance of particle motion due to forces on the induced dipole compared with motion arising from induced-charge electro-osmotic flow. In addition, the electric-field driven assembly of conducting particles is reviewed in relation to their AC electrokinetic properties and behaviour. [ABSTRACT FROM AUTHOR]

We describe the reversible assembly of an aqueous suspension of metal nanowires into two different 2-dimensional stable configurations. The assembly is induced by an AC electric field of magnitude around 10 kV/m. It is known that single metal nanowires orientate parallel to the electric field for all values of applied frequency, according to two different mechanisms depending on the frequency. These different mechanisms also govern the mutual interaction between nanowires, which leads to directed-assembly into distinctive structures, the shape of which depends on the frequency of the applied field. We show that for frequencies higher than the typical frequency for charging the electrical double layer at the metal-electrolyte interface, dipole-dipole interaction leads to the formation of chains of nanowires. For lower frequencies, the nanowires form wavy bands perpendicular to the electric field direction. This behavior appears to be driven by the elec-troosmotic flow induced on the metal surface of the nanowires. Remarkably, no similar structures have been reported in previous studies of nanowires. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of DC-biased AC-electrokinetic (DC-biased ACEK) flow over a pair of symmetrical electrodes. The flow mechanism is based on a transverse conductivity gradient created through incipient Faradaic reactions occurring at the electrodes when a DC-bias is applied. The DC biased AC electric field acting on this gradient generates a fluid flow in the form of vortexes. To understand more in depth the DC-biased ACEK flow mechanism, a phenomenological model is developed to study the effects of voltage, conductivity ratio, channel width, depth, and aspect ratio on the induced flow characteristics. It was found that flow velocity on the order of mm/s can be produced at higher voltage and conductivity ratio. Such rapid flow velocity is one of the highest reported in microsystems technology using electrokinetics. [ABSTRACT FROM AUTHOR]

Abstract: AC electric fields are of increasing importance for the generation of fluid flows in microsystems. We analyse numerically the use of AC electric fields at microwave frequencies for electro-thermal actuation of water in microdevices. Water is heated because of its significant dielectric loss at microwave frequencies. Buoyancy and dielectric forces actuate in the liquid bulk, and the relative importance between them is studied. The microwave liquid actuation can be used for pure water as well as for water saline solutions, such as bio-fluids. Therefore, it is of interest for the Lab-on-a-Chip technology. [Copyright &y& Elsevier]

We present a theoretical study of the hydrodynamic and electrokinetic response of both metallic spherical polarized colloids as well as metallodielectic Janus particles, which are subjected to an arbitrary non-uniform ambient electric field (DC or AC forcing). The analysis is based on employing the linearized 'standard' model (Poisson–Nernst–Planck formulation) and on the assumptions of a 'weak' field and small Debye scale. In particular, we consider cases of linear and helical time-harmonic travelling-wave excitations and provide explicit expressions for the resulting dielectrophoretic and induced-charge electrophoretic forces and moments, exerted on freely suspended particles. The new analytic expressions thus derived for the linear and angular velocities of the initially uncharged polarizable particle are compared against some available solutions. We also analyze the levitation problem (including stability) of metallic and Janus particles placed in a cylindrical (insulating or conducting) pore near a powered electrode. [ABSTRACT FROM AUTHOR]

Advances in the miniaturization of microelectromechanical systems (MEMS) [[1]] are revolutionizing the possibilities of sample analysis. In this case, the particles are also deflected, and, significantly, the threshold electric field magnitude for particle deviation is much lower than for high frequencies (DEP deviation). 30912779 15 Calero V., Garcia-Sanchez P., Ramos A., Morgan H. Combining DC and AC electric fields with deterministic lateral displacement for micro-and nano-particle separation. [Extracted from the article]

Experimental determination of the critical axial electric field needed to hold a nonconducting liquid bridge of given slenderness is presented. The Plateau tank technique is used to minimize gravity forces on earth. Improvement of previous data is achieved by determining the gravitational Bond number. The results are compared to numerical calculations of the stability limits, showing a good agreement. [ABSTRACT FROM AUTHOR]

We study theoretically and numerically the electrokinetic behavior of metal microparticles immersed in aqueous electrolytes. We consider small particles subjected to non-homogeneous ac electric fields and we describe their motion as arising from the combination of electrical forces (dielectrophoresis) and the electroosmotic flows on the particle surface (induced-charge electrophoresis). The net particle motion is known as dipolophoresis. We also study the particle motion induced by travelling electric fields. We find analytical expressions for the dielectrophoresis and induced-charge electrophoresis of metal spheres and we compare them with numerical solutions. This validates our numerical method, which we also use to study the dipolophoresis of metal cylinders. [ABSTRACT FROM AUTHOR]

We study experimentally the electrorotation (ROT) of semiconducting microspheres. ZnO microspheres obtained by a hydrothermal synthesis method are dispersed in KCl aqueous solutions and subjected to rotating electric fields. Two ROT peaks are found in experiments: a counterfield peak and a cofield peak at somewhat higher frequencies. These observations are in accordance with recent theoretical predictions for semiconducting spheres. The counterfield rotation is originated by the charging of the electrical double layer at the particle-electrolyte interface, while the cofield rotation is due to the Maxwell-Wagner relaxation. Additionally, we also found that some microspheres in the sample behaved differently and only showed counterfield rotation. We show that the behavior of these particles can be described by the so-called shell model. The microstructure of the microspheres is analyzed with electron microscope techniques and related to the ROT measurements. [ABSTRACT FROM AUTHOR]

Conical points of a leaky dielectric drop surrounded by a dielectric gas in an external ac electric field are investigated. A novel class of steady conical tips depending on the permittivity ratio and applied signal frequency is presented. It is found that conical solutions with very small angles are possible (angles much smaller than the classical Taylor cone angle 49.3° for a conducting drop in a dc field); this result can be relevant to the observations of small cone angles in Chetwani, Maheshwari, and Chang experiments [N. Chetwani, S. Maheshwari, and H.-C. Chang. Phys. Rev. Lett. 101, 204501 (2008)]. [ABSTRACT FROM AUTHOR]

• Electrokinetics tunes Deterministic Lateral Displacement particle separation. • Scaling laws of AC electrokinetic particle separation in DLD devices were identified. • Simulations are in excellent agreement with the experimental data at high frequencies. • New insights on the particle low frequency AC electrokinetic behaviour are provided. Deterministic Lateral Displacement (DLD) is a microfluidic technique where arrays of micropillars within a microchannel deflect particles leading to size-based segregation. We recently demonstrated that applying AC electric fields orthogonal to the fluid flow increases the separation capabilities of these devices with a deflection angle that depends on the electric field magnitude and frequency. Particle deviation occurs in two distinct regimes depending on frequency. At high frequencies particles deviate due to negative dielectrophoresis (DEP). At low frequencies (below 1 kHz) particles oscillate perpendicular to the flow direction due to electrophoresis and are also deflected within the device. Significantly, the threshold electric field magnitude for the low frequency deviation is much lower than for deflection at high frequencies by DEP. In order to characterize the enhanced separation at low frequencies, the induced deviation was compared between the two frequency ranges. For high frequencies, we develop both theoretically and experimentally scaling laws for the dependence of particle deviation on several parameters, namely the amplitude of the applied voltage, particle size and liquid velocity where DEP forces compete with viscous drag. A novel theoretical framework is presented that enables simulation of particle trajectories subjected to DEP forces in DLD devices. Deviation angles predicted by simulations are in very good agreement with experimental data. At low frequencies (below 1 kHz), particles follow the same scaling law, but with much lower voltages. This indicates that electrokinetic phenomena other than DEP play an important role in driving particle behaviour. Experiments show that at low frequencies, particle motion is affected by quadrupolar electrohydrodynamic flows around the insulating pillars of the DLD array. We quantify the difference between the two frequency regimes and show that an electrokinetic model based only on DEP forces is limited to frequencies of 1 kHz and above. [ABSTRACT FROM AUTHOR]

We study theoretically the rotation induced on an uncharged metal nanocylinder immersed in an electrolyte by AC electric fields. We consider the rotation of the cylinder when subjected to a rotating electric field (electrorotation) and the orientation of the cylinder in an AC field with constant direction (electro-orientation). The cylinder rotation is due to two mechanisms: the electric field interaction with the induced dipole on the particle and the hydrodynamic stress on the particle originated by the induced-charge electro-osmotic (ICEO) flow around the particle. The cylinder rotation induced by the ICEO mechanism can be calculated by using the Lorentz reciprocal theorem, while the rotation due to the induced dipole is calculated from the cylinder polarizability. We employ 3D numerical computations using finite elements for the general case as well as analytical methods for slender cylinders. Both calculations use the thin-double-layer approximation. We compare the results for slender cylinders of both methods showing good agreement. The electro-orientation (EOr) due to dipole torque aligns the axis of slender cylinders with the applied field, but aligns the axis of short cylinders perpendicularly to the field. The EOr due to ICEO torque always aligns the axis of cylinders with the field. The rotation induced by ICEO torque tends to disappear for frequencies of the applied field much greater than the characteristic frequency for charging the double-layer capacitance of the metal-electrolyte interface. [ABSTRACT FROM AUTHOR]

Abstract In this work, we review the physics of the liquid ejection by the application of electric fields, paying special attention to the steady cone-jet mode of electrospray. We aim to provide a comprehensive view on the role of electrohydrodynamics effects, and how the full electrokinetic equations can be reduced or simplified into the Taylor-Melcher leaky dielectric model. We provide an extensive review of the steady Taylor cone-jet mode, considering both its predicting scaling laws and limits of operation. Coaxial Taylor cone-jets are also briefly reviewed. Finally, we outline a revision of AC electrohydrodynamics and electrospraying. Highlights • We review the physics of liquid ejection by application of electric fields. • We review the onset of electrospray to understand charge relaxation issues. • Steady cone-jets, their models, scaling laws and limits of operation are revised. • Coaxial Taylor cone-jets are briefly reviewed. • We outline a revision on AC electrohydrodynamics and electrospraying. [ABSTRACT FROM AUTHOR]

Deterministic Lateral Displacement (DLD) exploits periodic arrays of pillars inside microfluidic channels for high-precision sorting of micro- and nano-particles. Previously we demonstrated how DLD separation can be significantly improved by the addition of AC electrokinetic forces, increasing the tunability of the technique and expanding the range of applications. At high frequencies of the electric field (>1 kHz) the behaviour of such systems is dominated by Dielectrophoresis (DEP), whereas at low frequencies the particle behaviour is much richer and more complex. In this article, we present a detailed numerical analysis of the mechanisms governing particle motion in a DLD micropillar array in the presence of a low-frequency AC electric field. We show how a combination of Electrophoresis (EP) and Concentration-Polarisation Electroosmosis (CPEO) driven wall-particle repulsion account for the observed experimental behaviour of particles, and demonstrate how this complete model can predict conditions that lead to electrically induced deviation of particles much smaller than the critical size of the DLD array. • Low frequency electrokinetics improves Deterministic Lateral Displacement separation. • CPEO driven wall-repulsion plays a major role in this mechanism. • Combination of Electrophoresis and CPEO fully explains the low frequency separation. • The mechanism of Electrokinetic biased DLD separation has been fully characterized. • This model enables numerical optimization of future electrokinetic DLD devices. [ABSTRACT FROM AUTHOR]

The physical mechanisms responsible for the electrical orientation and electrical rotation of metal nanowires suspended in an electrolyte as a function of frequency of the applied ac electric field are examined theoretically and experimentally. The alignment of a nanowire in an ac field with a fixed direction is called electro-orientation. The induced constant rotation of a nanowire in a rotating electric field is called electrorotation. In both situations, the applied electric field interacts with the induced charge in the electrical double layer at the metal-electrolyte interface, causing rotation due to the torque on the induced dipole, and also from induced-charge electro-osmotic flow around the particle. First, we describe the dipole theory that describes electro-orientation and electrorotation of perfectly polarizable metal rods. Second, based on a slender approximation, an analytical theory that describes induced-charge electro-orientation and electrorotation of metal nanowires is provided. Finally, experimental measurements of the electro-orientation and electrorotation of metal nanowires are presented and compared with theory, providing a comprehensive study of the relative importance between induced-dipole rotation and induced-charge electro-osmotic rotation. [ABSTRACT FROM AUTHOR]

We describe the physical mechanism responsible for the rotation of Brownian metal nanowires suspended in an electrolyte exposed to a rotating electric field. The electric field interacts with the induced charge in the electrical double layer at the metal-electrolyte interface, causing rotation due to the torque on the induced dipole and to the induced-charge electro-osmotic flow around the particle. Experiments demonstrate that the primary driving mechanism is the former of these two. Our analysis contrasts with previous work describing the electrical manipulation of metallic particles with electric fields, which neglected the electrical double layer. Theoretical values for the rotation speed are calculated and good agreement with experiments is found. [ABSTRACT FROM AUTHOR]

Electrophoresis describes the motion of charged particles suspended in electrolytes when subjected to an external electric field. Previous experiments have shown that particles undergoing electrophoresis are repelled from nearby channel walls, contrary to the standard description of electrophoresis that predicts no hydrodynamic repulsion. Dielectrophoretic (DEP) repulsive forces have been commonly invoked as the cause of this wall repulsion. We show that DEP forces can only account for this wall repulsion at high frequencies of applied electric field. In the presence of a low-frequency field, quadrupolar electro-osmotic flows are observed around the particles. We experimentally demonstrate that these hydrodynamic flows are the cause of the widely observed particle-wall interaction. This hydrodynamic wall repulsion should be considered in the design and application of electric-field-driven manipulation of particles in microfluidic devices. [ABSTRACT FROM AUTHOR]