Nonuniform Ac Field Research Articles

Behaviour prediction of closed-loop HTS coils in non-uniform AC fields

Field decay rate is the key characteristic of superconducting magnets based on closed-loop coils. However, in Maglev trains or rotating machines, closed-loop magnets work in external AC fields and will exhibit an evidently accelerated field decay resulting from dynamic resistances, which are usually much larger than joint resistance. Nevertheless, there has not been a numerical model capable of systematically studying this behaviour, which is the main topic of this work. The field decay curves of a closed-loop high-temperature-superconducting (HTS) coil in various AC fields are simulated based on H-formulation. A non-uniform external field generated by armature coils is considered. Reasonable consistence is found between experimental and simulation results. In our numerical model, the impact of current relaxation, which is a historical challenge, is analysed and subsequently eliminated with acceptable precision. Our simulation results suggest that most proportion of the field decay rate is from the innermost and outermost turns. Based on this observation, a magnetic shielding pattern is designed to reduce the field decay rate efficiently. This work has provided magnet designers with an effective method to predict the field decay rate of closed-loop HTS coils in external AC fields, and explore various shielding designs.

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.

(Invited) Electrostatic Field and Non-Thermal Plasma for Handling and Processing Bio-Molecules and Cells

Electrostatic field is an important tool for handling fine objects such as biological molecules and cells. Non-thermal plasma generates reactive radicals. Long life-time radicals can migrate in liquids to penetrate membranes of cells and tissues. Electrophoresis, or Coulombic force has been widely used in biology for separation of cells and molecules. Dielectrophoresis in non-uniform AC-field uses gradient force. Since AC voltage can be applied to conductive liquid with minimum generation of bubbles, dielectrophoresis is an important tool for handling living cells and micro-organisms. Due to polarization, gradient force appears on particles of neutral charge, and magnitude and direction of the gradient force depend on the frequency response of the objects. Gradient force can be generated using a focused laser without using fixed electrodes. Biological cells are pulled into the laser focus where E-field is high, and they are trapped. This is called laser tweezers, and is a convenient tool for handling microbes and fine objects with diameter range between 0.2 – 5 micro-meter. Laser tweezers cannot hold DNA molecules of random-coiled shape, because they are too thin (2 nm diameter). The globule transition of DNA molecules can be used to fold random-coiled DNA into a solid globule DNA. This transition is reversible, controlled by concentration of counter ions in medium. Globule DNAs, therefore, can be stretched again. Globule DNAs are manipulated by laser tweezers. Droplet-in-oil emulsion is a suitable container to be manipulated using electrostatic force or laser tweezers. This emulsion system can be used to conduct PCR amplification starting from single fragment of DNA. High electric field causes reversible or irreversible puncture of membrane of cells. This phenomenon can be used for cell fusion, infection of genes and chemicals, cell fusion, or sterilization. To punctuate cell membranes, reactive radicals are also effective. Radicals are generated in NTP and injected into liquid. Certain long-lived radicals migrate in the liquid and penetrate into cells through membrane or protein shell. E.coli or B. subtilis spore in liquid was exposed to NTP, either DBD(dielectric barrier discharge) or a plasma jet with Ar or He. Analysis using decay of GFP (green fluorescent protein) fluorescence proved that oxidative radicals are penetrating through the membrane into cells. Exposure of NTP to Bacteriophages indicated that protein shells are damaged at first, and that double stranded DNA is stronger than that of single stranded genes of the phages. In conclusion, electrostatic force and NTP are effective tools for handling and for processing biological particles including cells, viruses and molecules. Wide range of applications are still to be developed. Several examples will be reported of the application of electrostatic field and NTP in handling and processing of cells and single DNA molecules.

Solution pH change in non-uniform alternating current electric fields at frequencies above the electrode charging frequency.

AC Faradaic reactions have been reported as a mechanism inducing non-ideal phenomena such as flow reversal and cell deformation in electrokinetic microfluidic systems. Prior published work described experiments in parallel electrode arrays below the electrode charging frequency (fc ), the frequency for electrical double layer charging at the electrode. However, 2D spatially non-uniform AC electric fields are required for applications such as in plane AC electroosmosis, AC electrothermal pumps, and dielectrophoresis. Many microscale experimental applications utilize AC frequencies around or above fc . In this work, a pH sensitive fluorescein sodium salt dye was used to detect [H(+)] as an indicator of Faradaic reactions in aqueous solutions within non-uniform AC electric fields. Comparison experiments with (a) parallel (2D uniform fields) electrodes and (b) organic media were employed to deduce the electrode charging mechanism at 5 kHz (1.5fc ). Time dependency analysis illustrated that Faradaic reactions exist above the theoretically predicted electrode charging frequency. Spatial analysis showed [H(+)] varied spatially due to electric field non-uniformities and local pH changed at length scales greater than 50 μm away from the electrode surface. Thus, non-uniform AC fields yielded spatially varied pH gradients as a direct consequence of ion path length differences while uniform fields did not yield pH gradients; the latter is consistent with prior published data. Frequency dependence was examined from 5 kHz to 12 kHz at 5.5 Vpp potential, and voltage dependency was explored from 3.5 to 7.5 Vpp at 5 kHz. Results suggest that Faradaic reactions can still proceed within electrochemical systems in the absence of well-established electrical double layers. This work also illustrates that in microfluidic systems, spatial medium variations must be considered as a function of experiment time, initial medium conditions, electric signal potential, frequency, and spatial position.

AC-electrokinetic manipulation and controlled encapsulate release of surfactant based micelles

Effective manipulation and understanding of the dynamic responses of complex micelle nanocolloids under ac-electric fields is of great scientific and technological importance due to the emerging interest in using rich electrokinetic phenomena for medical diagnostics and drug delivery. In this work, we employ fluorescence correlation spectroscopy (FCS) to study the ac-electrokinetic behaviors of surfactant-based micelles encapsulated with varied fluorescence and model drug probes at a single micelle level. Under applied non-uniform ac-fields of constant voltage across two adjacent microelectrodes in a quadrupole configuration, probe-encapsulated micelles exhibit positive dielectrophoresis (DEP) characteristics at the ac-frequency range of 10–500 kHz, in contrast to negative or zero DEP at higher frequency of approximately greater than 500 kHz. The transition from positive to negative or zero DEP is indicated by the field-induced decrease of micelle concentration in the FCS focal volume located in the high field regions between two adjacent microelectrodes. The estimated DEP crossover frequency from ω-dependent micelle concentration profile agrees with advanced DEP theory, which predicts the scaling of crossover frequency with Stern layer conductance. At a lower frequency range of ω = 5–10 kHz, unique and surprising response in the measured fluorescence autocorrelation functions of charged probe-encapsulated micelles is observed, but not so with neutral encapsulated probes, suggesting ac-field induced micelle instability with the resulting release of interior charged probe encapsulates. Intriguingly, the release of encapsulates from micelles by non-uniform ac-electric fields shows strong dependence on ac-frequency and encapsulate chemistry.

Manipulating single annealed polyelectrolyte under alternating current electric fields: Collapse versus accumulation

Effective manipulation and understanding of the structural and dynamic behaviors of a single polyelectrolyte (PE) under alternating current (AC) electric fields are of great scientific and technological importance because of its intimate relevance to emerging bionanotechnology. In this work, we employ fluorescence correlation spectroscopy (FCS) to study the conformational and AC-electrokinetic behaviors of a model annealed PE, poly(2-vinyl pyridine) (P2VP) under both spatially uniform and non-uniform AC fields at a single molecule level. Under spatially uniform AC-fields, we observe a gradual and continuous coil-to-globule conformational transition (CGT) of single P2VP at varied AC-frequency when a critical AC-field strength is exceeded, in contrast to the pH-induced abrupt CGT in the absence of AC-fields. On the contrary, under spatially non-uniform AC-fields, we observe field-driven net flow and accumulation of P2VP near high AC-field regions due to combined AC electro-osmosis and dielectrophoresis but surprisingly no conformational change. Thus, distinct AC-electric polarization effect on single annealed PE subject to AC-field homogeneity is suggested.

Dispersant-assisted low frequency electrophoretically deposited TiO2 nanoparticles in non-aqueous suspensions for gas sensing applications

The effect of dispersant on deposition mechanism of TiO2 nanoparticles at 1Hz under non-uniform AC fields was investigated. It was found that by adding Dolapix to suspension, deposition pattern is drastically changed enabling particles to enter the gap leaving the electrodes intact. Using low frequency AC electrophoretic deposition technique in the presence of dispersant, we succeeded in fabricating gas sensor in less than 2min. Gas sensing measurements were performed in the temperature range of 450–550°C. The results explained that the sensor has good stability in time and repeatability performance toward high response. The maximum sensitivity of about 180 for the TiO2 nanoparticles sensor is observed with 47ppm NO2 gas and the response and recovery times is about 60–150s. The optimum temperature of the gas sensor was obtained in 450°C where sensor showed a linear trend up to 50ppm of NO2 gas. This sensing behavior in un-doped TiO2 as NO2 sensor can be mainly ascribed to the porous structure of the sensing film and its good contacts to the substrate and electrode assembly.

Qualitative and quantitative analysis of dynamic deformation of a cell in nonuniform alternating electric field

The present paper is dedicated to numerical modeling of the deformation of a cell in a nonuniform ac electric field. A two-fluid model is presented to characterize the flow mechanism of a cell suspending in a medium, in which the mechanical force of cell membranes and dielectrophoretic forces due to the nonuniform ac field are incorporated. The dielectrophoretic forces are then validated by comparing them with the dipole moment approximation. The simulation results demonstrate that the cell deformation depends on the frequency of the electric field. At low frequency, the cell is repelled away from the electrodes, and tilts clockwise. The lower part of the cell deforms thinner, whereas the upper part becomes fatter. At intermediate frequency, the cell moves almost along the centerline of the microchannel, and deforms slightly. The cell orientation oscillates with the average value of zero. At high frequency, however, the cell is attracted toward the electrodes, and tilts counterclockwise. The lower part of the cell becomes fatter, but the upper part deforms thinner. These behaviors are mainly attributed to the distribution of dielectrophoresis force strength generated by the nonuniform electric field.

Explorations of ABO‐Rh antigen expressions on erythrocyte dielectrophoresis: Changes in cross‐over frequency

A quadrupole dielectrophoretic microdevice was utilized to examine the ABO-Rh dependencies on erythrocyte polarizations. This important step toward medical microdevice technology would transform key clinical blood tests from the laboratory into the field. Previous work in dielectrophoretic microdevices demonstrated that the large number of ABO antigens on erythrocyte membranes impacts their dielectrophoretic signature at 1 MHz. This work explores the dielectrophoretic behavior of native human erythrocytes categorized by their ABO-Rh blood types and directly compares these responses to the same erythrocyte sample modified to remove the A and B antigens. A β(1-3)-galactosidase enzyme was utilized to cleave the ABO polysaccharide backbone at the galactosidase bonds. The enzymatic reaction was optimized by comparing agglutination of the native and modified blood cells in addition to UV-Vis and HPLC analysis of the reaction effluent for saccharide residues. Next, the dielectrophoretic behaviors of the native and modified erythrocytes were visually verified in a quadrupole electrode microdevice over a frequency range from 100 kHz to 80 MHz. The lower cross-over frequency (COF), which transitions from negative to positive dielectrophoresis, for ABO blood types tested (A+, A-, B+, B-, AB+, O+ and O-) differed over the range from 17 to 47 MHz. The COFs of the corresponding enzyme-modified erythrocytes were also determined and the range narrowed to 29-41 MHz. A second COF in the 70-80 MHz range was observed and was reduced in the presence of the transmembrane Rhesus factor. These results suggest that antigen expression on erythrocyte membrane surfaces influence cell polarizations in nonuniform AC fields.

Ac field enhanced protein crystallization

Uniform and nonuniform ac electric fields sustained by internal microfabricated electrodes are shown to sensitively affect protein crystallization by reducing the number of nucleation sites and enhancing the quality of the crystals formed. Under conditions when massive spontaneous nucleation occurs, a properly tuned ac field can desolvate the protein molecules to form a gel matrix with only a few nucleation sites. A dielectrophoretic force generated by the nonuniform ac field further consolidates these crystallites. Upon field removal, the gel transforms into crystals around these nucleation sites to form high-quality large crystals.

Double-layer effects on low frequency dielectrophoresis-induced colloidal assembly

Strong frequency dependence of particle aggregation in applied ac-electric fields is reported for colloidal assembly under positive dielectrophoresis. A curious low-frequency threshold ωth on the order of 104–105Hz is observed for the onset of collecting submicron latex particles at the high-field regions near coplanar electrodes. Similar to the dielectrophoretic crossover frequency, ωth is independent of medium conductivity, σm at low-salt concentration, yet scales with σm0.5 at high-salt concentrations. This intriguing salt effect suggests the distinct contribution from Stern and diffuse layer conductance to the interfacial polarization mechanism of latex particles in nonuniform ac fields.

Dielectrophoretic analysis of changes in cytoplasmic ion levels due to ion channel blocker action reveals underlying differences between drug-sensitive and multidrug-resistant leukaemic cells

Dielectrophoresis (DEP)—the motion of particles in non-uniform AC fields—has been used in the investigation of cell electrophysiology. The technique offers the advantages of rapid determination of the conductance and capacitance of membrane and cytoplasm. However, it is unable to directly determine the ionic strengths of individual cytoplasmic ions, which has potentially limited its application in assessing cell composition. In this paper, we demonstrate how dielectrophoresis can be used to investigate the cytoplasmic ion composition by using ion channel blocking agents. By blocking key ion transporters individually, it is possible to determine their overall contribution to the free ions in the cytoplasm. We use this technique to evaluate the relative contributions of chloride, potassium and calcium ions to the cytoplasmic conductivities of drug sensitive and resistant myelogenous leukaemic (K562) cells in order to determine the contributions of individual ion channel activity in mediating multi-drug resistance in cancer. Results indicate that whilst K+ and Ca2+ levels were extremely similar between sensitive and resistant lines, levels of Cl− were elevated by three times to that in the resistant line, implying increased chloride channel activity. This result is in line with current theories of MDR, and validates the use of ion channel blockers with DEP to investigate ion channel function.

Positioning of extended individual DNA molecules on electrodes by non-uniform ACelectric fields

Recent developments in the analysis and the application of DNA often require thestretching of individual DNA molecules to specific surfaces. We propose and demonstrate amethod for the positioning of unmodified extended DNA molecules. A local microscopiccircular flow is created by a non-uniform AC field and utilized to stretch theλ-DNA on a gold surface or between gold electrodes. The electrical-field amplitude andfrequency responses of DNA motion are studied. The method can be applied to positionthe DNA with accuracy on a microscopic scale while requiring no modification of the DNAfor terminal binding. With a diluted DNA solution, the number of DNA molecules acrossthe electrodes is controllable and the positioning of a single extended DNA acrosselectrodes is achievable.

Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. I. Experimental measurements

Under the influence of an ac electric field, electrolytes on planar microelectrodes exhibit fluid flow. The nonuniform electric field generated by the electrodes interacts with the suspending fluid through a number of mechanisms, giving rise to body forces and fluid flow. This paper presents the detailed experimental measurements of the velocity of fluid flow on microelectrodes at frequencies below the charge relaxation frequency of the electrolyte. The velocity of latex tracer particles was measured as a function of applied signal frequency and potential, electrolyte conductivity, and position on the electrode surface. The data are discussed in terms of a linear model of ac electroosmosis: the interaction of the nonuniform ac field and the induced electrical double layer.

Simple electromagnetic detection of variations in properties of metal surfaces

Recently, a significant improvement in the sensitivity of the nonuniform AC field measurement technique for the detection of cracks was reported. In this paper, the results of the application of the improved technique in the detection of heat-effected zones and ground welds are presented and briefly discussed. In such applications, the use of a long probe is essential.

Relationship between dough-raising activity of baker's yeast and the fraction of ‘vital’ cells as determined by methylene blue staining or by dielectrophoresis

Three methods of comparing the “vitility” of baker's yeast (Saccharomyces cerevisiae) were examined, and found to correlated well. The rate of dough-raising, the degree of visible staining by a reducible dye (methylene blue), and the rate of dielectrophoretic collection as the cells were subject to a nonuniform ac field all appear to agree well as indices of cell viability. The simplicity of the dielectrophoretic technique suggests its further use in cellular assays.

Electrochemistry of gas-particulate interfaces

The exchange of matter across gas-solid and gas-liquid interfaces has often been found to be accompanied by electrification of the solid or liquid and ionization of the gas. This holds especially for sorption and desorption of water. The existence of the effect is well established but little is known about its mechanism, and quantitative information is scarce. However, techniques for its study have been developed. A single particle is suspended in the gas and contained by a nonuniform ac field, and the charge of the particle is measured as a function of time. The thermodynamics of conventional electrochemistry, e.g., liquid junction potentials, applies to the effect.

The Temperature of Strongly Charged Water Droplets

Abstract The temperature of strongly charged 100μ water droplets, suspended in a non-uniform ac field, was estimated from measurements of the rate of loss of mass at various temperatures and from freezing experiments with water containing colloidal AgI. The observations were compared with information in the literature. It is concluded that such droplets exchange heat with the surrounding air much more slowly than do unchanged or weakly charged droplets.

INVESTIGATIONS OF CHARGED WATER DROPS

Charged water drops were suspended in a nonuniform a.c. field and observed through the microscope visually and by high-speed photography. The initial drop size was approximately 100µ. The drop shrank by evaporation and became unstable at approximately 40µ. The evaporation of stable drops and the behavior of unstable drops were studied. The rates of changes of mass and charge were determined at various temperatures, and the activation energy for the evaporation was determined. The evidence indicates that ions are ejected from an unstable drop, and that a burst of ions occurs in the evaporation of a stable drop under these conditions. The removal of water molecules from the evaporating drop is not rate-determining under these conditions. The drop evaporated rapidly at −26°C. and became unstable repeatedly without freezing.