Frequency Of Ac Electric Field Research Articles

Enhanced mixing quality of ring-type electroosmotic micromixer using baffles

In this study, an attempt has been taken to enhance the mixing efficiency in a micromixer of two different concentration fluids using electrokinetic effects such as electroosmotic flow. Four microelectrodes are located at specific locations on the outer surface of the one ring-shaped micromixer chamber. In addition, rigid baffles at the inlet side of the microchannel have been considered to enhance the mixing quality. The computation is performed using the finite element-based solver. The influences of different parameters such as average inlet fluid velocity (U0), frequency of the alternating current (f), the voltage amplitude of the alternating current (V), angle of inclination of the baffles (α), and the orientation fashion of the baffles at the inlet side have been investigated to achieve best mixing quality. This technique produces rapid mixing in the microchamber. Higher mixing quality can be achieved by considering the baffles in a specific orientation. The best mixing quality (99.67%) is obtained at the low inlet velocity of 0.05 mm/s. On the contrary, the lowest mixing quality (15.78%) is concerned with the higher inlet velocity with a high frequency of AC electric field.

EHD augmented heat transfer during upward bubbly flow boiling for low to medium frequency AC electric fields

The effect of a non-uniform AC electric field on the flow regime and heat transfer performance of upward flow boiling has been investigated experimentally using HFE-7000 as the working fluid in a tubular transparent heat exchanger. The non-uniform electric field was established by a concentric electrode connected to a high-voltage power supply and an Indium Tin Oxide (ITO) film on the tube outer wall. High-speed videography was employed to visualize the flow regimes. The experiments were performed for the initially bubbly flow regime and inlet pressure of 1.2 bars, subcooling of 2 K, and mass flux of 100 kg/m2s. Of key interest is the influence of low to medium AC frequency, which was varied between 1 Hz ≤ f ≤ 100 Hz for a square waveform of 10 kV peak-to-peak amplitude. One very striking observation was the strong electrophoretic interaction with the phases. At lower frequencies, the electrophoretic force interaction was clear from bubbles being attracted to the electrode, opposite to the direction of the dielectrophoretic force. For heat fluxes up to 10 kW/m2, the initially bubbly flow heat transfer coefficient was enhanced up to 3.5-fold and optimal frequency of f = 10 Hz. This research underpins enhancement and potential solid-state control of vertical two-phase heat exchangers, as well as two-phase heat exchangers in microgravity conditions, where similar flow regimes exist.

Coalescence and Break-Up Behaviors of Nanodroplets under AC Electric Field.

Water must be separated from water-in-oil (W/O) emulsion because of the corrosion it brings to the relative equipment in the process of transportation and storage. It is an effective method to apply external electric field to achieve high performance of separating small, dispersed water droplets from W/O emulsion; however, the coalescing micromechanism of such small salty droplets under AC electric field is unclear. In this paper, molecular dynamics simulation was adopted to investigate the coalescence and separation process of two NaCl-aqueous droplets under AC electric field and discuss the effect of AC electric field frequency, as well as the time required for contacting, the critical electric field strength, the dynamic coalescence process and the stability of the final merged droplet. The results show that the critical electric field strength of the droplet coalescence increases with the increase of frequency, while the time required for droplet contacting becomes shorter. The shrinkage function curve was applied to characterize the droplet coalescence effect and it was found that the droplets coalescence and form a nearly spherical droplet under the AC electric field with a frequency of 1.25 GHz and strength of 0.5 V/nm. When the electric field frequency is 10 GHZ, the merged droplet presents a periodic fluctuation with the same period as the AC electric field, which mainly depends on the periodic movement of cations and anions under the AC electric field. The results can provide theoretical basis for the practical application of electrostatic demulsification technology in the petroleum or chemical industry from the microscopic perspective.

Electrocatalytic Degradation of Rhodamine B Using Li-Doped ZnO Nanoparticles: Novel Approach.

In this paper, we discuss the preparation of Li-doped ZnO nanostructures through combustion and report on their structural, morphological, optical, and electrocatalysis properties. X-ray diffraction analyses show that the samples have a structure crystallized into the usual hexagonal wurtzite ZnO structure according to the P63mc space group. The scanning electron microscope images conceal all samples' nanosphere bundles and aggregates. The reflectance spectra analysis showed that the direct bandgap values varied from 3.273 eV (for pure ZnO, i.e., ZnL1) to 3.256 eV (for high Li-doped ZnO). The measured capacitance concerning frequency has estimated the variation of dielectric constant, dielectric loss, and AC conductivity against AC electric field frequency. The dielectric constant variations and AC conductivity are analyzed and discussed by well-known models such as Koop's phenomenological theory and Jonscher's law. The Raman spectra have been recorded and examined for the prepared samples. Rhodamine B was electro-catalytically degraded in all prepared samples, with the fastest time for ZnL5 being 3 min.

Microscopic mechanism for nanoparticle-laden droplet–droplet electrocoalescence: A molecular dynamics study

Electrocoalescence is an energy-efficient and environmentally friendly process for separating water-in-oil emulsions. In this study, nanoparticle-laden droplet–droplet electrocoalescence behaviors under external electric fields were numerically investigated using molecular dynamics (MD) methods. Good agreement was obtained between numerical results and experimental validation work. The influences of electric field strength, droplet diameter, nanoparticle (NP) concentration, NP size, and electric frequency were systematically examined, analyzed, and discussed from the perspective of intermolecular interactions. The NPs migrated to the liquid bridge to form nanoparticle shell under electric fields, and the NP brown motion increased the breakup probabilities, leading to partial electrocoalescence, which is undesirable in oil–water separation process. As to unequal droplets (droplet pairs of 6&4 nm, 6&6 nm, and 6&8 nm), the daughter droplets were always ejected from the apex of the small droplet. The NP concentrations influence not only the film coalescence process but also the droplet approach process, and high concentrations (0.15, 0.22 and 0.29 mol L−1) resulted in partial coalescence. The NP size also played significant roles during electrocoalescence. At large NP sizes (0.86 & 1.16 nm), the electrostatic repulsion between NPs played the dominant roles in the overall system, accounting for the PC mode with increasing NP size. In addition, applying high frequency (10 & 40 GHz) AC electric fields was found to be a time-saving and high-efficiency method. The results of this work will be potentially useful for optimizing the design of compact and efficient oil–water separators.

Experimental study on the performance of an electric field enhanced separator for crude oil production fluid

As most of the oil fields gradually come to the middle and late exploitation period, the crude oil production becomes more complex, stable and water cut is also very high. This has brought challenges to oil–water separation in onshore and offshore production trains especially for the demulsification and dehydration of crude oil. In particularly, the pre-separation efficiency of conventional three-phase separators installed at the front position of the crude oil processing systems are severely affected. In this paper, we explored the demulsification performance of W/O emulsion in an electric field-enhanced separator with insulated electrode assembly inside by static demulsification experiment and dynamic demulsification experiment separately. Focused beam reflectance measurement (FBRM) was used to make real-time online measurements of water droplet sizes in the emulsion. The average water droplet diameters, distribution of droplets within certain size and water content in oil outlet of the separator are set as indicators for evaluating the dehydration performance. The effect of electric voltage, frequency, water content and inlet flowrate on the demulsification performance were both investigated. Experimental results showed that the high frequency AC electric field with insulated electrode can promote the growth of small water droplets and thus contribute to the demulsification. The dehydration performance is deteriorated drastically with the increase of inlet water content or flowrate. The electrical field parameters such as electrical voltage and frequency both have obvious effect on the diameter distribution of dispersed water droplets and water contents at oil outlet of the separator. For certain inlet water content 20% and inlet flowrate 0.3 m3/h, the optimal electrical voltage is 5 kV and frequency is 3 kHz. The research results can provide guidance for the optimization and performance improvement of the electric field-enhanced separator.

Time periodic electroosmotic flow in a pH-regulated parallel-plate nanochannel

In this paper, the separation of variables method is applied to investigate the effects of solution pH, background salt concentration and AC electric field frequency on time periodic electroosmotic flow in a pH-regulated parallel-plate nanochannel. The surface charge is generated by the protonation and deprotonation of the functional group SiOH. The background salt is KCl. The pH value of the solution is adjusted by HCl and KOH. Analytical and semi-analytical solutions for electric potential and velocity distributions are obtained. The results show that the electric potential caused by the electric double layer depends greatly on the solution pH and background salt concentration. The amplitudes of the velocity and flow rate of the time periodic electroosmotic flow decrease with the background salt concentration and increase with the deviation of the solution pH from the isoelectric point. In a nanochannel having a height less than 100 nm, the electroosmotic velocity amplitude is not affected by the AC electric field frequency because the oscillating Reynolds number is much less than unity.

Ehd Augmented Heat Transfer During Upward Bubbly Flow Boiling for Low to Medium Frequency Ac Electric Fields

Ehd Augmented Heat Transfer During Upward Bubbly Flow Boiling for Low to Medium Frequency Ac Electric Fields

Concentration-Polarization Electroosmosis near Insulating Constrictions within Microfluidic Channels.

Electric fields are commonly used to trap and separate micro- and nanoparticles near channel constrictions in microfluidic devices. The trapping mechanism is attributed to the electrical forces arising from the nonhomogeneous electric field caused by the constrictions, and the phenomenon is known as insulator-based-dielectrophoresis (iDEP). In this paper, we describe stationary electroosmotic flows of electrolytes around insulating constrictions induced by low frequency AC electric fields (below 10 kHz). Experimental characterization of the flows is described for two different channel heights (50 and 10 μm), together with numerical simulations based on an electrokinetic model that considers the modification of the local ionic concentration due to surface conductance on charged insulating walls. We term this phenomenon concentration–polarization electroosmosis (CPEO). The observed flow characteristics are in qualitative agreement with the predictions of this model. However, for shallow channels (10 μm), trapping of the particles on both sides of the constrictions is also observed. This particle and fluid behavior could play a major role in iDEP and could be easily misinterpreted as a dielectrophoretic force.

Characterization of interactive force acting on colloidal particles near an electrode in presence of a high-frequency (>10 kHz) AC electric field using particle diffusometry

Colloidal particles like polystyrene beads and metallic micro and nanoparticles are known to assemble in crystal-like structures near an electrode surface under both DC and AC electric fields. Various studies have shown that this self-assembly is governed by a balance between an attractive electrohydrodynamic (EHD) force and an induced dipole-dipole repulsion (Trau et al., 1997). The EHD force originates from electrolyte flow caused by interaction between the electric field and the polarized double layers of both the particles and the electrode surface. The particles are found to either aggregate or repel from each other on application of electric field depending on the mobility of the ions in the electrolyte (Woehl et al., 2014). The particle motion in the electrode plane is studied well under various conditions however, not as many references are available in the literature that discuss the effects of the AC electric field on their out-of-plane motion, especially at high frequencies (>10 kHz). Haughey and Earnshaw (1998), and Fagan et al. (2005) have studied the particle motion perpendicular to the electrode plane and their average height from the electrode mostly in presence of DC or low frequency AC (<1 kHz) electric field. However, these studies do not provide enough insight towards the effects of high frequency (>10 kHz) electric field on the particles’ motion perpendicular to the electrode plane.

Nonperturbative quasiclassical theory of graphene photoconductivity

We present a nonperturbative quasi-classical theory of graphene photoconductivity. We consider the influence of low-frequency (microwave, terahertz, mid-infrared) radiation on the static conductivity of a uniform graphene layer and calculate its photoconductivity as a function of frequency, polarization and strength of the external ac electric field, as well as on the material properties (electron density, scattering time) and temperature. The theory is valid at frequencies $\hbar\omega\lesssim 2E_F$ and at arbitrarily strong ac electric fields. We compare our results with those of the third-order perturbation theory and determine the applicability range of the perturbative solutions.

Anisotropic colloidal interactions & assembly in AC electric fields.

We match experimental and simulated configurations of anisotropic epoxy colloidal particles in high frequency AC electric fields by identifying analytical potentials for dipole-field and dipole-dipole interactions. We report an inverse Monte Carlo simulation algorithm to determine optimal fits of analytical potentials by matching simulated and experimental distribution functions for non-uniform liquid, liquid crystal, and crystal microstructures in varying amplitude electric fields. Two potentials that include accurate particle volume and dimensions along with a concentration dependent prefactor quantitatively capture experimental observations. At low concentrations, an effective ellipsoidal point dipole potential works well, whereas a novel stretched point dipole potential is found to be suitable at all concentrations, field amplitudes, and degrees of ordering. The simplicity, accuracy, and adjustability of the stretched point dipole potential suggest it can be applied to model field mediated microstructures and assembly of systematically varying anisotropic particle shapes.

Piezoelectric Properties of Pb1-xLax(Zr0.52Ti0.48)1-x/4O3 Thin Films Studied by In Situ X-ray Diffraction.

The piezoelectric properties of lanthanum-modified lead zirconate titanate Pb1−xLax(Zr0.52Ti0.48)1−x/4O3 thin films, with x = 0, 3 and 12 mol% La, were studied by in situ synchrotron X-ray diffraction under direct (DC) and alternating (AC) electric fields, with AC frequencies covering more than four orders of magnitude. The Bragg reflections for thin films with low lanthanum concentration exhibit a double-peak structure, indicating two contributions, whereas thin films with 12% La possess a well-defined Bragg peak with a single component. In addition, built-in electric fields are revealed for low La concentrations, while they are absent for thin films with 12% of La. For static and low frequency AC electric fields, all lanthanum-modified lead zirconate titanate thin films exhibit butterfly loops, whereas linear piezoelectric behavior is found for AC frequencies larger than 1 Hz.

Effects of AC/DC Electric Fields on Stretching DNA Molecules

The effects of AC/DC electric fields on stretching DNA molecules were discussed in this work. In the experiments of stretching DNA molecules with AC/DC electric fields, the voltage range was changed from 0[Formula: see text]V to 10[Formula: see text]V, and the frequency of AC electric field was kept at 50[Formula: see text]kHz. An atomic force microscope (AFM) was used to obtain DNA distributions under different electric fields. DNA molecules were curved and randomly distributed in solution if there was not any force applied to them. When an AC electric field was applied to the DNA sample, the curvature of DNA molecules was decreased gradually, and the stretching result was more obvious with the increase of voltage from 0.1[Formula: see text]V to 5[Formula: see text]V. The DNA molecules were broken when the voltage was increased to 6[Formula: see text]V. However, under the DC electric field, the stretching result of DNA molecules reached to their optimum state when the voltage was 2[Formula: see text]V, and they kept their steady state even though larger electric field intensities applied to the electrodes. The results can be used for the study of DNA–DNA, protein–DNA and quantum dot–DNA interactions and for the exploration of DNA biophysical properties.

Spatially varying colloidal phase behavior on multi-dimensional energy landscapes.

A method is reported to determine equilibrium concentration profiles and local phase behavior of colloids on multi-dimensional energy landscapes. A general expression is derived based on local particle concentration and osmotic pressure differences that are balanced by forces on colloids due to energy landscape gradients. This analysis is applied to colloidal particles in high frequency AC electric fields within octupolar electrodes, where the energy landscape can be shaped in two dimensions. These results are also directly applicable to any particles having induced dipoles in spatially non-uniform electromagnetic fields. Predictions based on modeling colloids with an effective hard disk equation of state indicate inhomogeneous solid and fluid states coexisting on different shaped energy landscapes including multiple minima. Model predictions show excellent agreement with time-averaged Brownian dynamic simulations at equilibrium. Findings demonstrate a general approach to understand colloidal phase behavior on energy landscapes due to external fields, which could enable control of colloidal microstructures on morphing energy landscapes and the inverse design of fields to assemble hierarchically structured colloidal materials.

RDNA-01. TUBULIN AND MICROTUBULES AS MOLECULAR TARGETS FOR TTFIELD THERAPY

Abstract TTField (Tumor-treating field) therapy utilizes low intensity intermediate frequency AC electric fields to reduce the spread of cancer. While it has attained FDA approval for the treatment of recurrent glioblastoma multiforme, the exact molecular targets of TTField therapy are not well understood. Microtubules are pipe-like polymers of the highly charged (–31 e) and strongly dipolar (dipole moment 1666 D) protein, α, β- tubulin. Studies on the electrical properties of microtubules have recently gained interest, with them being modelled as molecular targets of TTFields. Here, we experimentally show that while tubulin polymerized into microtubules leads to an increase in solution capacitance, unpolymerized tubulin has no appreciable effect. To the best of our knowledge, we present the first experimental quantification of the capacitance of a 20 μm-long microtubule. Using these results, we calculate the resonant frequency of a microtubule meshwork in a cell-like environment to be in the TTField regime. Our results utilize high ionic strength solutions and cell-like concentrations of tubulin to show the potential of microtubules as the targets of TTField action and as intracellular charge-storage devices. We conclude with a hypothesis of an electrically-tunable cell, where the dielectric properties of the cytoskeleton alter local and global charge storage and transport.

Influence of alkali concentration, electric waveform, and frequency on the critical electric field strength of droplet–interface partial coalescence

During the process of electro-dehydration, the electric field strength should be set below a critical value (Ecrit) to inhibit droplet–interface partial coalescence. The alkali present in alkaline–surfactant–polymer flooding may affect Ecrit because they change the electrical properties of produced liquids. In view of this, this study investigated the effects of Na2CO3 solution concentration, electric field waveform, and frequency on the root mean square (RMS) values of Ecrit. The results show that elevated Na2CO3 concentrations increase Ecrit, owing to the enhanced solution conductivity which may affect the charge transfer during coalescence. The convection of ions is associated with penetration of fluid into the bulk phase and an increase in Na2CO3 concentration decreases the penetration velocity. Moreover, increasing the AC or pulsed electric field frequency can increase Ecrit, and the optimal waveform (i.e., with the largest Ecrit) varies with frequency. These findings will be useful for optimizing electro-coalescence systems.

Continuous Cell Characterization and Separation by Microfluidic Alternating Current Dielectrophoresis.

A novel alternating current (ac)-dielectrophoretic (DEP) microfluidic chip for continuous cell characterization and separation is presented in this paper. To generate DEP forces, two electrode-pads are embedded in a set of asymmetric orifices on the opposite sidewalls to produce the nonuniform electric fields. In the vicinity of a small orifice, the cells experience the strongest nonuniform gradient and are drawn toward it by the positive DEP forces, while the cells experiencing a negative DEP force are repelled away and move toward the large orifice. The DEP behaviors of yeast cells in suspending media with different ionic concentrations, i.e., different electrical conductivities, and over a large range of the ac electric field frequency were investigated. Furthermore, the lateral migrations of yeast cells as a function of the ac frequency were measured. The trends of measured lateral migrations of yeast cells are similar to the corresponding Clausius-Mossotti (CM) factors. In addition, by adjusting the frequency and strength of the ac electric field, the continuous separation of live and dead yeast cells as well as the yeast cells with targeted diameter and dielectric property can be easily achieved. This is the first time that the measurement of ac-DEP lateral migration of yeast cells in solutions with different electrical conductivities as a function of the applied frequency in a microfluidic chip was reported. This ac-DEP system provides a method to characterize the crossover frequency of the specific cells and manipulate the targeted cells.

AC Electrokinetics of Polarizable Tri-Axial Ellipsoidal Nano-Antennas and Quantum Dot Manipulation.

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.

Study of Electromagnetic Electron Cyclotron Waves for Kappa Distribution with AC Field in the Magnetosphere of Saturn

Parallel propagating electromagnetic electron cyclotron (EMEC) waves in the extended plasma sheet (~12RS) and in the outer magnetosphere (~18RS) of Saturn have been studied. A dispersion relation for parallel propagating relativistic EMEC waves has been applied to the magnetosphere of Saturn, and comparisons have been made with the data of Voyager 1 at these radial distances. The detailed investigations for EMEC waves have been done in the presence of the perpendicular AC electric field, using the kappa distribution function. The relativistic temporal growth rate is calculated by the method of characteristic solution with the data provided by Voyager 1. The effect of the suprathermal electron density, temperature anisotropy, frequency of AC electric field, thermal energy of ions, and relativistic factor on the temporal growth rate of EMEC wave emission has been studied. The simulation results show that the growth of parallel propagating EMEC waves is significantly affected by variations in the temperature anisotropy, electron density, ion thermal energy, and relativistic factor in both the extended plasma sheet and the outer magnetosphere of Saturn. The temperature anisotropy (T⊥/T║), ion thermal energy (KBT║i), and electron density (n0) have been found to be a major source of free energy for parallel propagating EMEC waves in both regions.