Effect of Laser Heating on Nonlinear Surface Deformation of Acoustically Levitated Droplet

Under the microgravity environment, new and high quality materials with a homogeneous crystal structure are expected to be manufactured by undercooling solidification, since the material manufacturing under the microgravity environment has no effect of gravity. However, the temperature change on the interface of the material in space is expected to affect on the material processing due to the changing of physical property corresponding temperature. The purpose of the present study is to investigate effect of the laser heating on surface deformation of large levitated droplet. A water droplet levitated in the acoustic standing wave is heated by YAG laser. In order to increase the water droplet temperature, rhodamine 6G is solved in it to achieve high absorbance of the laser. Droplet from 2.5 to 5.5 mm in diameter were levitated and heated. The deformation of the droplet interface has been observed by high speed video camera. We used the radiation thermometer for the measurement of the temperature of droplet surface. It is noticed that the heated droplet deforms with its resonance frequencies. The experimental result of shape oscillation makes it possible to simulate the oscillation.     

Internal Flow of Acoustically Levitated Droplet

Under the microgravity environment, products of new and high quality materials solidified into homogeneous crystal by under cooling solidification have been the subject of much interest. Manufacture of material under the microgravity environment can be performed more static than that under the normal gravity. Handling technology of molten metal is important for such processes to hold in the limit space. However, when a large levitated droplet exists in the limit space, internal flow can be appeared remarkably. Elucidation of the effect of the internal flow of the levitated droplet is required in order to establish the containerless processing for new material under the microgravity environment. In current research, the internal flow of a levitated droplet was investigated by Zhao et al. (J Acoust Soc Am 106:589–595, 1999a and 106:3289–3295, 1999b) and Trinh et al. (Phys Fluids 12(2):249–251, 2000). These studies were analyzed numerically and theoretically. However, experimental study about the internal flow of the levitated droplet is not enough. According to our study Abe et al. (Microgravity Sci Technol 19(3–4):33–34, 2007), the authors observed internal flow of the water and glycerol droplet in normal gravity environment. In the water droplet, which is a low viscosity fluid, internal flow of both left and right hand rotation was observed. On the other hand, in the glycerol droplet, which is a high viscosity fluid, only rigid body rotation was observed. This research measured only two dimensional flows. It is thought that internal flow in the water is not two-dimensional but three-dimensional flow. Then, in order to investigate a three-dimensional flow structure in levitated water droplet in detail, we try to measure the three-dimensional flow in the levitated droplet. In the present study, test fluid with different viscosity is levitated. And, multidimensional PIV measurement is conducted to investigate the internal flow structure in a levitated droplet. Stereo images at equatorial plane of a levitated droplet are observed for measuring the three-dimensional component of velocity in the levitated droplet. As a result, the velocity of z direction is observed in the water droplet. On the other hand, the v _z is hardly observed in the glycerol droplet. The three dimensional structures of water and glycerol are differed. The difference of such flow structure is supposed to be due to the influence of the viscosity.     

Melt Flow Instability and Turbulence in Electro-Magnetically Levitated Droplets

This article addresses fluid flow instabilities and flow transition to turbulent chaotic motions through numerical analysis and turbulence in electro-magnetically levitated droplets through direct numerical simulations. Numerical implementation and computed results are presented for flow instability and turbulence flows in magnetically levitated droplets under terrestrial and microgravity conditions. The linear melt flow stability is based on the solution of the Orr-Sommerfeld linearized equations with the base flows obtained numerically using high order numerical schemes. The resulting eigenvalue problems are solved using the linear transformation or Arnold's method. Melt flow instability in a free droplet is different from that bounded by solid walls and flow transits to an unstable motion at a smaller Reynolds number and at a higher wave number in a free droplet. Also, flow instability depends strongly on the base flow structure. Numerical experiments suggest that the transition to the unstable region becomes easier or occurs at a smaller Reynolds number when the flow structures change from two loops to four loops, both of which are found in typical levitation systems used for micro-gravity applications. Direct numerical simulations (DNS) are carried out for an electro-magnetically levitated droplet in a low to mild turbulence regime. The DNS results indicate that both turbulent kinetic energy and dissipations attain finite values along the free surface, which can be used to derive necessary boundary conditions for calculations employing engineering k--ε models.     

Effect of Temperature Change on Interfacial Behavior of an Acoustically Levitated Droplet

Under the microgravity environment, new and high quality materials with a homogeneous crystal structure are expected to be manufactured by undercooling solidification, since the material manufacturing under the microgravity environment is more static than that under the normal gravity. However, the temperature change on the interface of the material in space can affect on the material processing. The purpose of the present study is to investigate effect of the temperature change of interface on the large levitated droplet interface. A water droplet levitated by the acoustic standing wave is heated by YAG laser. In order to heat the water droplet by the laser heating, rhodamine 6G is solved in it to achieve high absorbance of the laser. The droplet diameter is from 4 to 5.5 mm. The deformation of the droplet interface is observed by high speed video camera. The temperature of droplet is measured by the radiation thermometer. It is noticed that the larger droplet under the higher sound pressure tends to oscillate remarkably by the laser heating.     

Study on Heat Transfer and Flow Characteristic Under Phase-Change Process of an Acoustically Levitated Droplet

Acoustic levitation is one of the levitation technique which is expected to be used for analytical chemistry and manufacturing new materials. Thus, it is important to gather the knowledge about acoustically levitated droplet. The purpose of this study is to investigate the heat transfer and flow behavior under phase change process of an acoustically levitated droplet. The following results were obtained from experiments. Evaporation process and external flow structure of the levitated droplet is visualized by a high speed camera and it is found that they differ by the type of fluid. Toroidal vortices are observed near the surface of the ethanol solution droplet. Heat transfer coefficient is estimated from the volume change and temperature gradient. It is substantially higher than that estimated by the existing experimental correlation.     

Droplet Deformation and 2-D/3-D Marangoni Flow Phenomena in Droplets Levitated by Electric Fields

An integrated numerical model is presented for free surface phenomena and Marangoni fluid flows in electrically levitated droplets under both terrestrial and microgravity conditions. The model development is based on the boundary element solution of the Maxwell equations simplified for electrostatic levitation applications and the free surface deformation that is primarily caused from the surface Maxwell stresses resulting from the applied electric fields. The electric and free surface model is further integrated with a finite element model for the surface-tension-induced fluid flows in the levitated droplets. Both 2-D and 3-D fluid flow structures may be developed in the electrically levitated droplets depending on the applied laser heating sources. The integrated model is applied to study the electric field distribution, free surface deformation, and 2-D and 3-D internal fluid flow structures in normal and microgravity for single, symmetric two-beam, four-beam, and six-beam laser heating arrangements. Among these arrangements, the six-beam arrangement with equal heating intensity gives the smallest temperature difference and the smallest maximum velocity.     

External Flow of an Acoustically Levitated Droplet

One of the major recent advances for experiments in containerless processing is acoustic levitation. Although there are a lot of previous studies for acoustic levitation, characteristic of external flow of an acoustically levitated droplet is not experimentally examined enough. In this study, external flow field has been observed by using high speed camera and Particle Image Velocimetry. In the case of any levitated droplet at a velocity antinode of standing wave, toroidal vortex are generated around levitated droplet. It is found that toroidal vortex around a levitated droplet is strongly affected by viscosity of levitated samples and input voltage. In terms of water droplet, as input voltage is decreased, location of toroidal vortex is moved from bottom to top of levitated samples.     

Precise Density Measurements for Electromagnetically Levitated Liquid Combined with Surface Oscillation Analysis

A new method is proposed for accurately measuring the densities of high-temperature liquids which involves analyzing the surface oscillations of levitated droplets. This method makes it easy to improve on the accuracy of density measurements obtained by using conventional electromagnetic levitation systems. In addition, the errors in density measurements made on the ground are further reduced by applying a static magnetic field to suppress surface oscillations in levitated liquid droplets. The magnetic field interacts with electrical currents in the levitated droplet, thereby generating a Lorentz force; this force suppresses flow within the liquid droplet. By combining both these methods, the scatter in density measurements for molten Si at temperatures in the range from 1,500 K to 1,900 K is reduced by an order of magnitude compared with previously reported data. Using this new method, the density of molten SiGe has been measured at temperatures from 1,350 K to 1,650 K.     

Evaluation of Simulated Microgravity Environments Induced by Diamagnetic Levitation of Plant Cell Suspension Cultures

Ground-Based Facilities (GBF) are essetial tools to understand the physical and biological effects of the absence of gravity and they are necessary to prepare and complement space experiments. It has been shown previously that a real microgravity environment induces the dissociation of cell proliferation from cell growth in seedling root meristems, which are limited populations of proliferating cells. Plant cell cultures are large and homogeneous populations of proliferating cells, so that they are a convenient model to study the effects of altered gravity on cellular mechanisms regulating cell proliferation and associated cell growth. Cell suspension cultures of the Arabidopsis thaliana cell line MM2d were exposed to four altered gravity and magnetic field environments in a magnetic levitation facility for 3 hours, including two simulated microgravity and Mars-like gravity levels obtained with different magnetic field intensities. Samples were processed either by quick freezing, to be used in flow cytometry for cell cycle studies, or by chemical fixation for microscopy techniques to measure parameters of the nucleolus. Although the trend of the results was the same as those obtained in real microgravity on meristems (increased cell proliferation and decreased cell growth), we provide a technical discussion in the context of validation of proper conditions to achieve true cell levitation inside a levitating droplet. We conclude that the use of magnetic levitation as a simulated microgravity GBF for cell suspension cultures is not recommended.     

Mechanism of stabilization of location of a droplet cluster above the liquid-gas interface

We studied the influence of sizes of droplets, forming the ??droplet cluster?? dissipative structure, on their levitation height in the vapor-air flow, which appears when free surface of horizontal water layer is locally heated. A sharp decrease in the velocity of the vapor-air flow takes place at a distance from the surface comparable with the droplet diameter. Allowing for the aerodynamic nature of the droplet levitation, this peculiarity of the flow determines the high stability of location of the droplet cluster above the interface. Existence of droplets that are anomalously heavy in the slope of the Stokes levitation mechanism is described.     

Surface Oscillations of an Electromagnetically Levitated Droplet

The natural oscillation frequency of freely suspended liquid droplets can be related to the surface tension of the material, and the decay of oscillations to the liquid viscosity. However, the fluid flow inside the droplet must be laminar to measure viscosity with existing correlations; otherwise the damping of the oscillations is dominated by turbulent dissipation. Because no experimental method has yet been developed to visualize flow in electromagnetically levitated oscillating metal droplets, mathematical modeling can assist in predicting whether or not turbulence occurs, and under what processing conditions. In this paper, three mathematical models of the flow: (1) assuming laminar conditions, (2) using the k−ɛ turbulence model, and (3) using the RNG turbulence model, respectively, are compared and contrasted to determine the physical characteristics of the flow. It is concluded that the RNG model is the best suited for describing this problem when the interior flow is turbulent. The goal of the presented work was to characterize internal flow in an oscillating droplet of liquid metal, and to verify the accuracy of the characterization by comparing calculated surface tension and viscosity values to available experimental results.     

Influencing Factors of Levitation Drift Caused by Magnet Vibration

In a levitated high-temperature superconducting system, the drift of levitated body due to vibration of the permanent magnet or an alternating magnetic field is a key and fatal subject in its applications. In this paper, the levitation drift caused by a PM vibration is investigated by a vibration measurement system. The influences of critical current density, levitation gap, weight of the levitated body, voltage across the shaker (amplitude of vibration), cooling height, etc. on the levitation drift have been measured, respectively. In the end, several methods which can reduce the levitation drift are presented and compared; the most reliable method is enhancing the critical current density of the superconductors.     

Role of vapor flow in the mechanism of levitation of a droplet-cluster dissipative structure

The rate of water evaporation has been experimentally determined under conditions that ensure the formation of a dissipative structure of the droplet cluster type [1, 2]. It is shown that, in the region of localization of a droplet cluster, the velocity of the vapor-air flow is sufficient to maintain the levitation of droplets over the liquid surface according to the Stokes mechanism.     

Effects of relaxation in levitating superconductors

The effect of the magnetic flux creep on the levitation stability of high-temperature superconductors was studied. It was shown experimentally that under a unipolar magnetization the levitation force decreased at a logarithmic velocity characteristic of the creep process. If the current structure was bi- or multi-polar one, which was formed in a sample exposed to a reversing external magnetic field, the force remained unchanged during a certain period of time. The theory of relaxation of magnetization and force for a partial and full penetration of the critical state was considered. It was shown that relaxation decelerated sharply if the region with a current producing the main magnetization was far from the superconductor surface. A concept of an open and internal magnetic relaxation was introduced. The time of the internal relaxation for different reverse depths was estimated. The calculated values approached the experimental values of the levitation stabilization time.     

Droplet Oscillations in High Gradient Static Magnetic Field

In high intensity and high gradient magnetic fields the volumetric force on diamagnetic material, such as water, leads to conditions very similar to microgravity in a terrestrial laboratory. In principle, this opens the possibility to determine material properties of liquid samples without wall contact, even for electrically non-conducting materials. In contrast, AC field levitation is used for conductors, but then terrestrial conditions lead to turbulent flow driven by Lorentz forces. DC field damping of the flow is feasible and indeed practiced to allow property measurements. However, the AC/DC field combination acts preferentially on certain oscillation modes and leads to a shift in the droplet oscillation spectrum.What is the cause? A nonlinear spectral numerical model is presented, to address these problems.     

Silicon purity controlled under electromagnetic levitation (SPYCE): influences on undercooling

The rapid evolution of photovoltaic Si production induced a shortage of high purity silicon raw material. The use of lowest purity silicon has a strong effect on the casting conditions and ingot structure and properties. During solidification, solute rejection at the growth interface leads to an increase of the impurities concentration in the liquid phase and then to the precipitation of silicon nitride and silicon carbide. As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation setup which has been developed in order to measure the undercooling versus impurity concentration is presented. The impurity concentration in the levitated Si drop is controlled by the partial pressure of nitrogen or hydrocarbon gas. As nucleation is a random phenomenon, statistical measurements are presented, from samples which showed numerous heating/melting and cooling/solidification phases. The effect of carbon impurities on the undercooling of silicon droplet is discussed.     

Spin-up instability of a levitated molten drop in magnetohydrodynamic-flow transition to turbulence

We describe the spinning behavior of a suspended molten droplet subjected to electromagnetic heating. Our observations are derived from video images of droplets of palladium-silicon alloy in experiments on the MSL-1 (First Microgravity Science Laboratory) mission of the Space Shuttle (STS-83 and STS-94, April and July 1997). We inferred the resultant magnetohydrodynamic (MHD) flow inside the drop from motion of impurities on the surface. Digital particle tracking of the impurities is used to quantify the axial rotation of the levitated droplet. The analysis suggests that the levitated drop attains a constant rotational speed during the melting phase and formation of the co-rotating axisymmetric laminar toroidal structures. With continued electromagnetic heating, the sample's viscosity drops and the MHD flow accelerates, giving rise to instabilities of the internal flow. The rate of axial rotation increases significantly during this flow transition. The new data suggests a surprising interaction between the flow inside the levitated molten drop and the driving coils in the experiments. We explore the mechanisms that may be responsible for this spinning behavior.     

Real‐Time Monitoring of Colloidal Crystallization in Electrostatically‐Levitated Drops

Colloidal crystallization is analogous to the crystallization in bulk atomic systems in various aspects, which has been explored as a model system. However, a real‐time probing of the phenomenon still remains challenging. Here, a levitation system for a study of colloidal crystallization is demonstrated. Colloidal particles in a levitated droplet are gradually concentrated by isotropic evaporation of water from the surface of the droplet, resulting in crystallization. The structural change of the colloidal array during crystallization is investigated by simultaneously measuring the volume and reflectance spectra of the droplet. The crystal nucleates from the surface of the droplet at which the volume fraction exceeds the threshold and then the growth proceeds. The crystal growth behavior depends on the initial concentrations of colloidal particles and salts which determine the overall direction of crystal growth and interparticle spacing, respectively. The results show that a levitating bulk droplet has a great potential as a tool for in situ investigation of colloidal crystallization.     

Mass spectrometry of acoustically levitated droplets

Containerless sample handling techniques such as acoustic levitation offer potential advantages for mass spectrometry, by eliminating surfaces where undesired adsorption/desorption processes can occur. In addition, they provide a unique opportunity to study fundamental aspects of the ionization process as well as phenomena occurring at the air-droplet interface. Realizing these advantages is contingent, however, upon being able to effectively interface levitated droplets with a mass spectrometer, a challenging task that is addressed in this report. We have employed a newly developed charge and matrix-assisted laser desorption/ionization (CALDI) technique to obtain mass spectra from a 5-microL acoustically levitated droplet containing peptides and an ionic matrix. A four-ring electrostatic lens is used in conjunction with a corona needle to produce bursts of corona ions and to direct those ions toward the droplet, resulting in droplet charging. Analyte ions are produced from the droplet by a 337-nm laser pulse and detected by an atmospheric sampling mass spectrometer. The ion generation and extraction cycle is repeated at 20 Hz, the maximum operating frequency of the laser employed. It is shown in delayed ion extraction experiments that both positive and negative ions are produced, behavior similar to that observed for atmospheric pressure matrix-assisted laser absorption/ionization. No ion signal is observed in the absence of droplet charging. It is likely, although not yet proven, that the role of the droplet charging is to increase the strength of the electric field at the surface of the droplet, reducing charge recombination after ion desorption.