Levitation Condition Research Articles

Observation of ice nucleation in acoustically levitated water drops

The supercooling and nucleation of acoustically levitated water drops were investigated at two different sound pressure levels (SPL). These water drops were supercooled by 13to16K at the low SPL of 160.6dB, whereas their supercoolings varied from 5to11K at the high SPL of 164.4dB. The maximum supercooling obtained in the experiments is 32K. Statistical analyses based on the classical nucleation theory reveal that the occurrence of ice nucleation in water drops is mainly confined to the surface region under acoustic levitation conditions and the enlargement of drop surface area caused by the acoustic radiation pressure reduces water supercoolability remarkably. A comparison of the nucleation rates at the two SPLs indicates that the sound pressure can strengthen the surface-dominated nucleation of water drops. The acoustic stream around levitated water drops and the cavitation effect associated with ultrasonic field are the main factors that induce surface-dominated nucleation.

Stable dust structures in non-self-sustained gas discharge under atmospheric pressure

The formation of stable dust structures in electron-beam-controlled non-self-sustained discharge plasma was predicted and experimentally observed. To determine the conditions for dust-particle levitation, the self-consisted one-dimensional simulation of a non-self-sustained gas discharge was carried out using a nonlocal model of charged plasma-particle transport with allowance for electron diffusion. It is shown that, in the cathode layer of a non-self-sustained gas discharge, a strong electric field arises in the Thomson regime, which, in conjunction with the gravity force, forms a potential well where the dust particles undergo levitation.

Heat Transfer in Water Under Strong Gradient Magnetic Fields

Heat transfer in water was visually investigated using a liquid-crystal sheet with thermochromism in a hybrid magnet. Upward thermal convection was significantly suppressed by gradient magnetic fields, however, it remained even under magnetic levitation conditions with a gradient field of B(dB/dz)=-1360 T/sup 2/ m/sup -1/. A thermal conduction state without convection was realized under a stronger gradient field of B(dB/dz)=-2880 T/sup 2/m/sup -1/ at 35-40/spl deg/C, and surprisingly the downward magnetic convection was observed under the same field at 40-45/spl deg/C. The temperature dependence of the magnetic susceptibility of water is responsible for the convection behavior in the gradient fields.

Magnetic orientation of paraffin in a magnetic levitation furnace

Containerless melting of paraffin under a magnetic levitation condition has been performed by using a cryogen-free hybrid magnet and two kinds of laser furnaces. One is local irradiation of CO 2 laser light at the top of the sample. The other is homogeneous irradiation of YAG laser light with a concave ring mirror. In the latter case, reduction of the Marangoni convection on the surface of the sample and the magnetic orientation of paraffin molecules were observed. The magnetic anisotropy of the spherical sample was confirmed by the measurement of magnetization and X-ray diffraction.

Dynamic characteristics and finite element analysis of a magnetic levitation system using aYBCO bulk superconductor

We have been developing a magnetic levitating device with two-dimensional movement, namely a‘levitating X–Y transporter’. For the real design of a levitatingX–Y transporter, it is necessary to clarify the levitation characteristics, such as the lift, thelevitation height and the stability against mechanical disturbances. Furthermore variouskinds of force may be applied to the levitating part and cause mechanical oscillation.Therefore the characteristics of oscillation are also important factors in the dynamicstability of such a levitation system. In this paper, we examine experimentally the lift andthe restoring force and develop a new simulation code based on the three-dimensionalhybrid finite and boundary element method to analyse the dynamic electromagneticbehaviour of the HTS bulk. We have investigated a suitable permanent-magnetarrangement to enhance the levitation characteristics through experiment and numericalsimulation. We can then determine the suitable conditions for stable levitation from thoseresults.

Temperature dependence of single-axis acoustic levitation

The temperature-dependent physical conditions for single-axis acoustic levitation are theoretically analyzed with consideration of the deviation of the actual acoustic field from the plane standing wave approximation. The effects of temperature variation on the resonant conditions, levitation force and threshold pressures pm (the minimum entrapping pressure) and pM (the maximum pressure to keep the integration of a liquid drop) are discussed by assuming a quasi-static heating and cooling process. The first resonant spacing H1 between the reflector and emitter is larger than that predicted for plane standing waves, and its temperature dependence comes mainly from the variation of wavelength, which is proportional to T1/2. The maximum levitation force FM has a drastic decreasing tendency with temperature rise due to its sensitivity to the ratios of the geometric parameters to wavelength. For the containerless processing of water and the Pb–Sn eutectic alloy, pm decreases whereas pM increases with the enhancement of temperature, which narrows the allowed pressure range for the safe and stable levitation of the processed drops at higher temperatures. As an experimental application of these analyses, the acoustically levitated water and the Pb–Sn eutectic alloy melt are highly undercooled by up to 24 and 38 K, respectively.

磁気浮上状態で溶融凝固したパラフィンの配向効果

Processing of materials under the magnetic levitation condition is an important new technique for synthesis of novel materials, because containerless melting can be performed in a quasi-microgravity environment. For materials with anisotropic magnetic susceptibility, the magnetic orientation effect can be expected in addition to the microgravity effect. We performed containerless melting of paraffin under the magnetic levitation condition, using a newly developed YAG laser furnace, in order to examine the possibility of synthesizing oriented spherical materials. By magnetization measurement and X-ray diffraction measurement, we confirmed the orientation of paraffin solidified under the magnetic levitation condition. This containerless melting method is expected to provide opportunities for obtaining materials with new functions.

反発浮上小型磁気軸受の回転特性と振動回避法の提案

This paper reports on the performance characteristics of the permanent magnet repulsion-type magnetic bearing, which has a vertical-shaft-type inner rotor with polygon scanner mirror facets. In general, though this type of magnetic bearing system has the feature that the radial stiffness is constant in the permanent magnet sections at a steady position, if the radial stiffness can be changed by an axial active control, various applications of this system can be expected. We propose the generation of a radial restoring force, using an iron board with a conical surface set up in the rotor shaft. Stable rotation at a maximum rotation speed of about 50000 rpm is achieved, and a change in the radial stiffness in the noncontact levitation condition is confirmed.

Eutectic growth under acoustic levitation conditions.

Samples of Pb-Sn eutectic alloy with a high density of 8.5 x 10(3) kg/m(3) are levitated with a single-axis acoustic levitator, and containerlessly melted and then solidified in argon atmosphere. High undercoolings up to 38 K are obtained, which results in a microstructural transition of "lamellas-broken lamellas-dendrites." This transition is further investigated in the light of the coupled zone for eutectic growth and the effects of ultrasound. The breaking of regular eutectic lamellas and suppression of gravity-induced macrosegregation of (Pb) and (Sn) dendrites are explained by the complicated internal flow inside the levitated drop, which is jointly induced by the shape oscillation, bulk vibration and rotation of the levitated drop. The ultrasonic field is also found to drive forced surface vibration, which subsequently excites capillary ripples and catalyzes nucleation on the sample surface.

Crystal growth and materials processing in the magnetic levitation condition

Magnetic levitation effects in high magnetic fields and its application to crystal growth and materials processing have been investigated at High Field Laboratory for Superconducting Materials of IMR, Tohoku University. We have succeeded in (1) growing of some ionic crystals in a levitating solution, and (2) melting of levitating glass that makes a perfect sphere and fine particles.

Crystal growth of ammonium chloride in magnetic levitation conditions

A gradient strong magnetic field was applied to levitate an ammonium chloride solution droplet, and the behavior of crystal nucleation and growth in the droplet was investigated. Moreover, the gradient strong magnetic field was applied to control the crystal position in the solution, and the “levitating” crystal growth in the solution was succeeded without any contacts with the container walls and the solution surface.

A method for magnetic field determination inside magnetic fluids

The method is based on the phenomenon of levitation of a nonmagnetic spherical body immersed in a magnetic fluid situated in a magnetic field characterized by a vertical gradient oriented from top to bottom. This structure of the field has been obtained in a wedge-shaped air gap. Taking into account the levitation condition and the hyperbolic structure of the field, a theoretical formula for the z coordinate was used. The comparison between experimental and theoretical data allow to obtain the magnetic field intensity inside the magnetic fluids.

Gyroscopic Stabilization of Passive Magnetic Levitation

A nonlinear mathematical model able to describe the motion of a passive magnetic levitation device, known as Levitron, is presented in this paper. Using the standard approach usually applied in rotordynamics and without introducing any preliminary assumptions, the equations of motion for all six degrees of freedom of the magnetic spinning top are obtained. By computing the four natural frequencies characterizing the horizontal translational vibrations of the rotor and the whirling motion of its axis, the conditions for stable levitation in terms of the spin speed are obtained. Some results coming from the numerical integration of the equations of motion are also presented and compared with those obtained using the simplified model based upon the fast top assumption.

Cubic electrodynamic levitation trap with transparent electrodes

An unconventional electrodynamic suspension system with transparent planar electrodes is described which stably levitates charged solid particles or liquid droplets without the need for feedback control. The system has been used with particles ranging from about 1 to 100 μm diam, under vacuum and within stationary and flowing gases. Operation within low conductivity liquids is possible in principle. The suspension system consists of six transparent conducting electrodes arranged as faces of a hollow cube. Four of these electrodes are driven by a variable-frequency two-phase ac source operating in the low audio frequency range. Advantages of this type of trap for aerosol studies include relatively wide-angle optical access and a geometry naturally suited to the use of three-axis dc crossfields for particle manipulation. Conditions for stable levitation are reviewed as well as methods for determining the radius, mass, charge, and density of a spherical levitated object.

Electrical levitation for micromotors, microgyroscopes and microaccelerometers

Electrical levitation is an excellent candidate method for solving the problem of surface-contact friction resulting from the instability of rotors in micromotors. One of the most important aspects of electrical levitation is the achievement of stable equilibrium suspension. The theories and experiments in previous work are not sufficient for practical application of electrical levitation in microelectromechanical systems (MEMS). In particular, there are unrestrained rotatable disc structures in these devices. In this paper, the developed stable conditions of electrical levitation using three independent resonant circuits are presented, and the experimental procedure related to it is described. Finally, the application of electrical levitation in micromotors, electrostatic microgyroscopes and microaccelerometers is examined.

Numerical simulation of the dynamics of an electrostatically levitated drop

In this paper we use the boundary integral method to simulate the dynamics of a liquid drop levitated by electrostatic force. Our numerical code reproduces the Rayleigh limit, the Taylor limit and the stable equilibrium shape of a leviated drop obtained by previous authors. It also reveals the mechanism by which a levitated drop might breakup. Using this code, we have obtained the natural frequencies of the drop under various levitation conditions. We have also examined the effect of the electrostatic field on the resonant interactions between the modes. We found that the presence of the electric field enhances the modal interactions; modes which otherwise would not interact become coupled to each other. Calculations of the dynamics of a levitated drop under a time-periodic electric field indicate that the main resonance occurs when the frequency of the electric field is near the free oscillation frequency of the fundamental mode.

Jeans instability of a dusty plasma.

We study the linear and the nonlinear stages of the Jeans instability of charged grains with mass ${\mathit{m}}_{\mathit{d}}$ and charge ${\mathit{q}}_{\mathit{d}}$ in the regime ${\mathit{Gm}}_{\mathit{d}}^{2}$/${\mathit{q}}_{\mathit{d}}^{2}$\ensuremath{\approxeq}O(1). Various conditions for stable electrostatic levitation, condensation, and dispersion of grains in the plasma background are obtained. The nonlinear solutions show that there is a condensation of grains even when the effect of self-gravity is annulled by self-electrostatic repulsion. Astrophysical situations relevant to the results are briefly discussed.

General conditions for dielectrophoretic and magnetohydrostatic levitation

Contactless separation of particles, droplets, or bubbles in solid-liquid, solid-solid, liquid-liquid, and gas-liquid systems can be achieved using electric and magnetic fields. Dielectric levitation and magnetohydrostatic separation techniques exemplify such separation processes. In this paper, the analogy between dielectric and magnetic levitation in non-uniform fields is exploited to provide general conditions for stable, contactless suspension of particles, droplets, or bubbles. Certain general properties of axisymmetric (cusped) electric and magnetic fields are used to obtain general equations of motion for levitated objects.

An Application of Electromagnetic Levitation to Welding

The results of a study of levitation by induction applied to metal welding are presented. To achieve the levitation of liquid metal, a special flat inductor, based on the "magnetic potential hole," is used. This inductor consists of a double oscillating circuit supplied by an induction generator and is analyzed from an electrical point of view. It is shown that the optimal levitation conditions can be obtained by two working modes: the first one with a phase shift between the two currents of the two half-circuits having the same oscillating frequency and the second one with each of these currents having different frequencies. Some practical details are also given for the optimization of the inductor. Finally, a laboratory levitation experiment is described.

Rate of decarburization of iron-carbon melts: Part II. a mixed-control model

The observed retarding effect of sulfur on the decarburization of Fe-C melts has been interpreted by means of a mixed-control mechanism involving gas-phase mass transfer and dissociative adsorption of CO2. A mathematical model formulated on the basis of the proposed mechanism gave an excellent fit to the experimental data. The application of the model to the data provided a value of 4.42 x 10−3 mole · cm−2 · s−1 · atm−1 for the dissociative adsorption rate constant for CO2 on liquid iron at 1973 K; the fraction of surface sites that cannot be occupied by sulfur, even at apparent surface-saturation, was found to be 0.085. The model predicts a residual rate of decarburization at high sulfur concentrations; this prediction is borne out by the experiment. The effect of convective motion within the levitated melt on the rate of decarburization below a characteristic carbon concentration was quantified. The liquid-phase mass transfer was greatly enhanced by the stirring effect of the electromagnetic field. The effective diffusivity of carbon in Fe-C melts under levitation conditions has been found to be 3.24 x 10−3 cm2 · s−1, a value ten times as large as that under stationary conditions.