Non-uniform AC Electric Field Research Articles

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.

The Generation and Migration of Bubbles in Oil-Pressboard Insulation Needle-Plate System

Bubbles in transformer oil can easily lead to partial discharge (PD), which can deteriorate the transformer oil and even breakdown the transformer insulation. To clarify the migration process and the characteristics of bubbles generated in an oil-immersed power transformer exposed to an extremely uneven electric field, we experimentally monitor these phenomena under an extremely nonuniform ac electric field and numerically simulate the migration distance and the migration speed of bubbles with different initial positions and sizes. The results show that the streamer discharge channel formed by a PD in oil is gasified into a bubble channel. After it collides with the surface of the pressboard, its morphology is transformed into approximately spherical bubbles due to the surface tension of the gas–liquid interface. After bubbles are generated in the oil, they move away from areas with a strong electric field due to the electric-field force and gradually approach the oil surface due to the buoyancy force. The experimental results are consistent with the simulation results, which verify the rationality of the simulation model.

Induction and suppression of cell lysis in an electrokinetic microfluidic system.

The ability to strategically induce or suppress cell lysis is critical for many cellular-level diagnostic and therapeutic applications conducted within electrokinetic microfluidic platforms. The chemical and structural integrity of sub-cellular components is important when inducing cell lysis. However, metal electrodes and electrolytes participate in undesirable electrochemical reactions that alter solution composition and potentially damage protein, RNA, and DNA integrity within device microenvironments. For many biomedical applications, cell viability must be maintained even when device-imposed cell-stressing stimuli (e.g., electrochemical reaction byproducts) are present. In this work, we explored a novel and tunable method to accurately induce or suppress device-imposed artifacts on human red blood cell (RBC) lysis in non-uniform AC electric fields. For precise tunability, a dielectric hafnium oxide (HfO2 ) layer was used to prevent electron transfer between the electrodes and the electric double layer and thus reduce harmful electrochemical reactions. Additionally, a low concentration of Triton X-100 surfactant was explored as a tool to stabilize cell membrane integrity. The extent of hemolysis was studied as a function of time, electrode configuration (T-shaped and star-shaped), cell position, applied non-uniform AC electric field, with uncoated and HfO2 coated electrodes (50nm), and absence and presence of Triton X-100 (70µM). Tangible outcomes include a parametric analysis relying upon literature and this work to design, tune, and operate electrokinetic microdevices to intentionally induce or suppress cellular lysis without altering intracellular components. Implications are that devices can be engineered to leverage or minimize device-imposed biological artefacts extending the versatility and utility of electrokinetic diagnostics.

Formation of water chain under the non-uniform AC electric field

Electrocoalescence is widely used to separate water from crude oil in petroleum industry. However, the electric field often forces water droplets distributed in chain shape, thus improving the danger of short circuit by bridging electrodes. To understand the mechanism of water chain phenomenon, an in-situ observation experiment were conducted to investigate the effect of voltage, frequency, water content and surfactant concentration. The results showed that the water chain length and growth rate of droplet diameter varied synchronously with the voltage and frequency. Additionally, the water chain length and growth rate of droplet diameter increased firstly and then decreased with the initial water content. The initial droplet diameter decreased with the surfactant concentration, which inhibit formation of water chains and coalescence. As the surfactant concentration reached a critical value, the initial droplet diameter remained constant, while the inhabitation of micelle for the coalescence strengthened with the surfactant concentration. These conclusions are helpful to understand the mechanism of formation of water chains.

Effect of bubble flow on partial discharge under non-uniform AC electric fields in FC-72

The focus of this study is on thermal bubbles which limits the application of phase-change cooling technology in high electric fields. In order to investigate the impact of the boil-induced bubble flow on the partial discharge (PD), a PD test chamber is set up which is modified to adjust the intensity of the bubble flow. PD signals and characteristics of the bubble flow are recorded under different heating conditions and electric fields. It is found that thermal bubbles have negligible impact on the PD characteristics under a non-uniform field, but the discharge frequency decreases as the boiling intensity increases. A deviation occurs in the trajectory of bubbles, which makes discharge in bubbles impossible. It is also found that the heat induced fluid affects the density of dielectric liquid and ionization coefficient of the discharge environment, which reduces PD frequency.

Continuous ES/Feeder Cell-Sorting Device Using Dielectrophoresis and Controlled Fluid Flow

Pluripotent stem cells (PSCs) are considered as being an important cell source for regenerative medicine. The culture of PSCs usually requires a feeder cell layer or cell adhesive matrix coating such as Matrigel, laminin, and gelatin. Although a feeder-free culture using a matrix coating has been popular, the on-feeder culture is still an effective method for the fundamental study of regenerative medicine and stem cell biology. To culture PSCs on feeder cell layers, the elimination of feeder cells is required for biological or gene analysis and for cell passage. Therefore, a simple and cost-effective cell sorting technology is required. There are several commercialized cell-sorting methods, such as FACS or MACS. However, these methods require cell labeling by fluorescent dye or magnetic antibodies with complicated processes. To resolve these problems, we focused on dielectrophoresis (DEP) phenomena for cell separation because these do not require any fluorescent or magnetic dyes or antibodies. DEP imposes an electric force on living cells under a non-uniform AC electric field. The direction and magnitude of the DEP force depend on the electric property and size of the cell. Therefore, DEP is considered as a promising approach for sorting PSCs from feeder cells. In this study, we developed a simple continuous cell-sorting device using the DEP force and fluid-induced shear force. As a result, mouse embryonic stem cells (mESCs) were purified from a mixed-cell suspension containing mESCs and mouse embryonic fibroblasts (MEFs) using our DEP cell-sorting device.

Serial Separation of Microalgae in a Microfluidic Chip Under Inertial and Dielectrophoretic Forces

There are a large number and variety of microalgae in the ocean, which are important natural resources for extensive applications. However, how to identify and separate microalgae quickly and effectively remains a major challenge for investigation and utilization of microalgae resources. There are many methods for cell separation, while each of them has pros and cons. Therefore, it is difficult to meet the particular requirements of separating complex samples, such as microalgae, solely with a single method. Herein, a two-stage microfluidic separation chip was designed by combining inertial and dielectrophoretic (DEP) forces, which can achieve rapid and accurate separation of three common microalgae cells, including Platymonas , Closterium and Chlorella , with separation efficiency exceeding 90%. In the first-stage, high-throughput inertial separation technique was used to separate microalgae with their different equilibrium positions. Then, the DEP force was used to separate microalgae cells which were not separated in the first-stage through their dielectric properties. To the best of our knowledge, there is no prior reports of microalgae separation by combining DEP and inertial techniques. In addition, the DEP characteristics of microalgae cells under a non-uniform AC electric field were firstly reported in this study, and the DEP responses of each cell type were measured against AC electrical frequency changes. These findings may contribute to our understanding of the dielectric properties of microalgae, as well as the development of novel techniques for microalgae separation, detection, and marine environmental monitor and analysis.

Controlled manipulation of CNTs in glass/epoxy composites with cut-outs using non-uniform electric field

In this work, an effective methodology to incorporate aligned carbon nanotubes (CNTs) in long fiber composites in the presence of geometric discontinuities has been studied. Unidirectional (UD) glass/CNT-epoxy composites containing circular cut-outs were used as representative of a common discontinuity. Multi-walled (MW) CNTs were introduced in the composites via the epoxy matrix and aligned using low-frequency non-uniform AC electric field. This work discusses the methodology to control CNT alignment around the cut-out with the objective of enhancing the stiffness locally. The effectiveness of CNT alignment was assessed indirectly through the mapping of electrical resistance and polarized Raman spectroscopy. A strong correlation was found between the targeted CNT alignment state and the measured resistance values around the cut-out. As a precursor to developing glass/CNT-epoxy hierarchical composites, the response of MW-CNTs in the epoxy resin system was assessed at various input voltage levels in correlation with the weight fraction (wt.%) of CNTs. While a change in electrical resistance by ~3 orders of magnitude was found along the direction of CNT alignment in the hierarchical composites, a change in electrical resistance by ~5 orders of magnitude was observed in the CNT-epoxy composites in comparison with that of the respective control samples.

Demulsification of Water-in-Oil Emulsions under the Action of Nonuniform AC Electric Field

We present an experimental approach using the microfluidic techniques, optical microscopy, and high-speed imaging to study the demulsification of water-in-oil emulsions under the action of a nonuniform alternating electric field. The parameters characterizing the demulsification dynamics are estimated. The approach can be used to develop effective methods for water-in-oil emulsions treatment, including in combination with other methods of exposure, for example, chemical, acoustic, thermal, etc.

Co-deposition and electrokinetic behavior of TiO2–WO3 nanoparticles under non-uniform AC field

AbstractThe electrokinetic behavior of TiO2–WO3 nanoparticles while applying non-uniform AC electric fields at a frequency of 10 kHz was investigated using planar parallel electrodes. For this purpose, WO3 and TiO2 nanoparticles, which exhibited a different response to the applied electric field, were deposited simultaneously at 10 kHz. Characterization of the obtained patterns by optical and scanning electron microscopy revealed that the TiO2 nanoparticles, which were unable to enter the gap, moved toward it and got deposited there in the presence of WO3 nanoparticles. Hence, a composite layer consisting of TiO2 and WO3 nanoparticles was formed in the gap. A hypothesis for the change in the deposition and electric behavior of TiO2 nanoparticles is presented in this paper using the multi-shell model. Furthermore, a possible method for the fabrication of a TiO2–WO3based gas sensor has been provided.

Motion observation of metal nanoparticles in ionic liquid under nonuniform AC electric field

Motion observation of metal nanoparticles in ionic liquid under nonuniform AC electric field

Soap-film flow induced by electric fields in asymmetric frames.

Net fluid flow of soap films induced by (ac or dc) electric fields in asymmetric frames is presented. Previous experiments of controllable soap film flow required the simultaneous use of an electrical current passing through the film and an external electric field or the use of nonuniform ac electric fields. Here a single voltage difference generates both the electrical current going through the film and the electric field that actuates on the charge induced on the film. The film is set into global motion due to the broken symmetry that appears by the use of asymmetric frames. If symmetric frames are used, the film flow is not steady but time dependent and irregular. Finally, we study numerically these film flows by employing the model of charge induction in ohmic liquids.

Measurement of Anisotropic Particle Interactions with Nonuniform ac Electric Fields.

Optical microscopy measurements are reported for single anisotropic polymer particles interacting with nonuniform ac electric fields. The present study is limited to conditions where gravity confines particles with their long axis parallel to the substrate such that particles can be treated using quasi-2D analysis. Field parameters are investigated that result in particles residing at either electric field maxima or minima and with long axes oriented either parallel or perpendicular to the electric field direction. By nonintrusively observing thermally sampled positions and orientations at different field frequencies and amplitudes, a Boltzmann inversion of the time-averaged probability of states yields kT-scale energy landscapes (including dipole-field, particle-substrate, and gravitational potentials). The measured energy landscapes show agreement with theoretical potentials using particle conductivity as the sole adjustable material property. Understanding anisotropic particle-field energy landscapes vs field parameters enables quantitative control of local forces and torques on single anisotropic particles to manipulate their position and orientation within nonuniform fields.

Energy landscapes for ellipsoids in non-uniform AC electric fields.

We report a closed-form analytical model for energy landscapes of ellipsoidal particles in non-uniform high-frequency AC electric fields to identify all possible particle positions and orientations. Three-dimensional equilibrium positions and orientations of prolate (rx = ry < rz), oblate (rx = rz > ry), and scalene (rx≠ry≠rz) ellipsoids are reported vs. field frequency and amplitude, which are determined from energy landscape minima. For ellipsoids within non-uniform electric fields between co-planar parallel electrodes, the number of configurations of position and orientation is 6 for prolate, 5 for oblate, and 9 for scalene ellipsoids. In addition, for coplanar electrodes, conditions are identified when particles can be treated using a quasi-2D analysis in the plane of their most probable elevation near an underlying surface. The reported expressions are valid for time-averaged interactions of ellipsoid particles in arbitrary AC electric field configurations, such that our results are applicable to electromagnetic tweezers interacting with particles having an appropriate material property contrast with the medium in the frequency range of interest.

High-throughput separation of cells by dielectrophoresis enhanced with 3D gradient AC electric field.

We propose a novel, high-performance dielectrophoretic (DEP) cell-separation flow chamber with a parallel-plate channel geometry. The flow chamber, consisting of a planar electrode on the top and an interdigitated-pair electrode array at the bottom, was developed to facilitate the separation of cells by creating a nonuniform AC electric field throughout the volume of the flow chamber. The operation and performance of the device were evaluated using live and dead human epithermal breast (MCF10A) cells. The separation dynamics of the cell suspension in the flow chamber was also investigated by numerically simulating the trajectories of individual cells. A theoretical model to describe the dynamic cell behavior under the action of DEP, including dipole-dipole interparticle, viscous, and gravitational forces, was developed. The results demonstrated that the live cells traveling through the flow chamber congregated into sites where the electric field gradient was minimal, in the middle of the flow stream slightly above the centerlines of the grounded electrodes at the bottom. Meanwhile, the dead cells were trapped on the edges of the high-voltage electrodes at the bottom. Cells were thus successfully separated with a remarkably high separation ratio (∼98%) at the appropriately tuned field frequency and applied voltage. The numerically predicted behavior and spatial distribution of the cells during separation also showed good agreement with those observed experimentally.

Concentration-tuned interactions and structures in flowing suspensions under a gradient AC electric field

In this study, numerical simulations of suspensions of Al2O3 particles in corn oil at various volume fractions were performed to explore the mechanisms involved in the electro-mechanical behavior of positive polarized particles suspended in an insulated suspending medium traveling through a rotating flow channel while exposed to a nonuniform AC electric field. A theoretical model was developed to simulate the trajectories of individual particles under the action of dielectrophoretic (DEP), dipole-dipole interparticle, viscous, and gravitational forces in a laminar flow. The results demonstrated that particles began to aggregate along the edges of electrodes with electric field application. New particles coming from more distant regions then gradually joined the vertices of particle aggregates, leading to the growth of chain-like clusters along the electric field gradient. Predictions showed good agreement with the results observed in a previous experiment. The new findings of this study are that the DEP force always tends to confine particles near the electrode edges, whereas the clustering of particles on the electrodes arises from the competition between particle interaction due to the DEP force and the long-range dipole-dipole interparticle interaction with neighboring particles. This process became more apparent with higher volume fractions of particle.

Dielectrophoretic motion and deformation of a liquid drop in an axisymmetric non-uniform AC electric field

Use of dielectrophoresis as a tool to maneuver particles (biological, rigid, fluid, etc.) or their carrier drops is prevalent in various microfluidics and biotechnological applications. Employing AC electric field gives frequency as an additional attribute to manipulate the particles or drops in such applications. In the present work we experimentally studied the dielectrophoretic behavior of a freely suspended drop in another fluid in the presence of AC electric field. The response of a conducting drop dispersed in a dielectric medium to the non-uniform AC electric field with up-down asymmetry is observed to be distinctly different than DC dielectrophoresis in the same fluid system. The effect of frequency of the applied sinusoidal field on the motion and shape evolution of a drop is thoroughly investigated. We also used analytical theory to predict the periodic dielectrophoretic motion of the drop between electrodes. The orientation of molecular dipoles, instantaneously upon application of the electric field, is a major contributor to the overall phase lag between the applied field and the drop's response in addition to the lag due to charge dynamics. The periodic motion and shape of a dielectrophoretically moving drop are found to be governed by different parameters in addition to the frequency of applied field.

Enhancement of continuous-flow separation of viable/nonviable yeast cells using a nonuniform alternating current electric field with complex spatial distribution.

The variability in cell response to AC electric fields is selective enough to separate not only the cell types but also the activation states of similar cells. In this work, we use dielectrophoresis (DEP), which exploits the differences in the dielectric properties of cells, to separate nonviable and viable cells. A parallel-plate DEP device consisting of a bottom face with an array of micro-fabricated interdigitated electrodes and a top face with a plane electrode was proposed to facilitate the separation of cells by creating a nonuniform electric field throughout the flow channel. The operation and performance of the device were evaluated using live and dead yeast cells as model biological particles. Further, numerical simulations were conducted for the cell suspensions flowing in a channel with a nonuniform AC electric field, modeled on the basis of the equation of motion of particles, to characterize the separation efficiency by changing the frequency of applied AC voltage. Results demonstrated that dead cells traveling through the channel were focused onto a site around the minimum electric field gradient in the middle of the flow stream, while live cells were trapped on the bottom face. Cells were thus successfully separated under the appropriately tuned frequency of 1 MHz. Predictions showed good agreement with the observation. The proposed DEP device provides a new approach to, for instance, hematological analysis or the separation of different cancer cells for application in circulating tumor cell identification.

Combined AC electroosmosis and dielectrophoresis for controlled rotation of microparticles.

Electrorotation is widely used for characterization of biological cells and materials using a rotating electric field. Generally, multiphase AC electric fields and quadrupolar electrode configuration are needed to create a rotating electric field for electrorotation. In this study, we demonstrate a simple method to rotate dielectrophoretically trapped microparticles using a stationary AC electric field. Coplanar interdigitated electrodes are used to create a linearly polarized nonuniform AC electric field. This nonuniform electric field is employed for dielectrophoretic trapping of microparticles as well as for generating electroosmotic flow in the vicinity of the electrodes resulting in rotation of microparticles in a microfluidic device. The rotation of barium titanate microparticles is observed in 2-propanol and methanol solvent at a frequency below 1 kHz. A particle rotation rate as high as 240 revolutions per minute is observed. It is demonstrated that precise manipulation (both rotation rate and equilibrium position) of the particles is possible by controlling the frequency of the applied electric field. At low frequency range, the equilibrium positions of the microparticles are observed between the electrode edge and electrode center. This method of particle manipulation is different from electrorotation as it uses induced AC electroosmosis instead of electric torque as in the case of electrorotation. Moreover, it has been shown that a microparticle can be rotated along its own axis without any translational motion.

Rapid and sensitive detection of small biomolecule by capacitive sensing and low field AC electrothermal effect

Conventional affinity biosensor typically relies on passive diffusion of analytes for binding reaction, which in many cases leads to long response time and lack of sensitivity. Recent research showed that directed particle motion towards sensor electrodes could be induced in sample matrix by applying a non-uniform AC electric field, often with AC dielectrophoresis as the responsible mechanism. As a result, shorter assay time and higher sensitivity can be achieved. Previously, we demonstrated a rapid and sensitive AC capacitive affinity sensor, which integrates low voltage AC dielectrophoresis into label-free capacitive measurement to achieve a single-step operation without any wash steps for clinical samples. However, dielectrophoretic force is rather short-ranged, and is also proportional to the size of target biomolecules/particles. Therefore, to detect target molecule at diluted concentrations or small molecule, improvement in sensitivity by dielectrophoresis could be quite limited. Alternatively, AC electric field can also produce microfluidic movement to carry biomolecules to sensors, which is of long range and size independent. This work demonstrates the use of low voltage AC electrothermal effect to enhance and accelerate the detection of low abundance and small target molecules by AC capacitive sensing with simultaneous AC electrokinetic enrichment. Electrode designs were studied for their effectiveness in AC electrothermal capacitive sensing. Electrodes with larger characteristic length were found to be more amenable to inducing AC electrothermal convection and were successfully used to detect low abundance protein and femto-molar level small molecules.