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.     

Interfacial stability and internal flow of a levitated droplet

Under the microgravity environment, production of new and high quality material is expected. Large droplet is preferable for such a containerless processing in microgravity environment. There are a lot of previous studies for droplet levitation [1]. However, effect of surface instability and internal flow appear remarkable when the droplet becomes large. Elucidation of effect of surface instability and internal flow of the levitated droplet is required for the quality improvement of new material. The objective of present study is to clarify critical conditions of the occurrence of the internal flow and the surface instability. At first, the condition between the stable region and the unstable region of the droplet levitation was evaluated by using the existing critical Weber number theory. The experimental result agreed well with the theory. It was suggested that the stability of droplet can be evaluated by using the theory for the interfacial instability. Finally, two-dimensional visual measurement was conducted to investigate the internal flow structure in a levitated droplet. The effect of physical properties on the internal flow structure of the droplet is investigated by Particle Image Velocimetry (PIV) technique. As the result, it is indicated that the internal flow structure is affected by the physical property such as viscosity.     

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.     

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.     

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.     

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.     

Transcritical phenomena of autoignited fuel droplet at high pressures under microgravity

An experimental study has been performed under microgravity to obtain the detailed information needed for the deep understanding of the combustion phenomena of single fuel droplets which autoignite in supercritical gaseous environment. The microgravity environments both in a capsule of a drop shaft and during the parabolic flight of an aircraft were utilized for the experiments. An octadecanol droplet suspended at the tip of a fine quartz fiber in the cold section of the high-pressure combustion chamber was transferred quickly to be subjected to a hot gaseous medium in an electric furnace, this followed by autoignition and combustion of the fuel droplet in supercritical gaseous environment. High-pressure gaseous mixture of oxygen and nitrogen was used as the ambient gas. Temporal variation of temperature of the fuel droplet in supercritical gaseous environment was examined using an embedded fine thermocouple. Sequential backlighted images of the autoignited fuel droplet or the lump of fuel were acquired in supercritical gaseous environment with reduced oxygen concentration. The observed pressure dependence of the ignition delay and that of the burning time of the droplet with the embedded thermocouple were consistent with the previous results. Simultaneous imaging with thermometry showed that the appearance of the fuel changed remarkably at measured fuel temperatures around the critical temperature of the pure fuel. The interface temperature of the fuel rose well beyond the critical temperature of the pure fuel in supercritical gaseous environment. The fuel was gasified long before the end of combustion in supercritical gaseous environment. The proportion of the gasification time to the burning time decreased monotonically with increasing the ambient pressure.     

UV‐Resistant and Thermally Stable Superhydrophobic CeO2 Nanotubes with High Water Adhesion

A novel type of sticky superhydrophobic cerium dioxide (CeO₂) nanotube material is prepared by hydrothermal treatment without any chemical modification. A water droplet on the material surface shows a static water contact angle of about 157° but the water droplet is pinned on the material surface even when the material surface is turned upside down. Interestingly, the as‐prepared CeO₂ nanotube material displays durable superhydrophobicity and enhanced adhesion to water under ultraviolet (UV) light irradiation. Importantly, this change in water adhesion can be reversed by heat treatment to restore the original adhesive value of 20 µL. Further, the maximum volume of the water droplet adhered on the material surface of CeO₂ nanotubes can be regulated without loss of superhydrophobicity during the heating treatment/UV‐irradiation cycling. Meanwhile, the superhydrophobic CeO₂ nanotube material shows remarkable thermal stability even at temperatures as high as 450 °C, long‐term durability in chemical environment, and air‐storage and good resistance to oily contaminant. Finally, the potential application in no‐loss water transportation of this sticky superhydrophobic CeO₂ material is demonstrated.     

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.     

Dynamics for Droplets in Normal Gravity and Microgravity

A numerical study has been carried out to investigate the deformation dynamics of droplets under normal gravity and the thermocapillary migration of droplets under microgravity. The Navier–Stokes equations coupled with the energy conservation equation are solved on a staggered grid by the method of lines, and the mass conserving level set method is used to predict the surface deformation of the droplet. The simulation for the falling droplet in the air under normal gravity shows that the value of Weber number affects mainly the deformation of the droplet, while the value of Reynolds number has direct impact on the falling velocity of the droplet. From the simulation for the droplet thermocapillary migration and lateral oscillation under microgravity, it is found that the value of Marangoni number has obvious effects on the moving velocity and temperature distribution of the droplet.     

Colloidal Material Box: In-situ Observations of Colloidal Self-Assembly and Liquid Crystal Phase Transitions in Microgravity

To study the self-assembly behavior of colloidal spheres in the solid/liquid interface and elucidate the mechanism of liquid crystal phase transition under microgravity, a Colloidal Material Box (CMB) was designed which consists of three modules: (i) colloidal evaporation experimental module, made up of a sample management unit, an injection management unit and an optical observation unit; (ii) liquid crystal phase transition experimental module, including a sample management unit and an optical observation unit; (iii) electronic control module. The following two experimental plans will be performed inside the CMB aboard the SJ-10 satellite in space. (i) Self-assembly of colloidal spheres (with and without Au shell) induced by droplet evaporation, allowing observation of the dynamic process of the colloidal spheres within the droplet and the change of the droplet outer profile during evaporation; (ii) Phase behavior of Mg₂Al LDHs suspensions in microgravity. The experimental results will be the first experimental observations of depositing ordered colloidal crystals and their self-assembly behavior under microgravity, and will illustrate the influence of gravity on liquid crystal phase transition.     

Experimental study on convergence of droplet streams under microgravity

Experiments on the convergence of two droplet streams have been carried out under microgravity in order to develop a technique for converging droplet streams under microgravity and to examine the behavior of droplets in a vacuum and under microgravity after the binary droplets collide with each other. The working fluid is silicone oil with a low vapor pressure. In this study, a method of orienting the droplet generators toward a convergence point has been tested. In all of the 68 experiments conducted under microgravity, it is confirmed that droplet streams are converged. It has been concluded that the method of orienting multiple droplet generators to a convergeing point is effective for converging droplet streams under microgravity. The behaviors of the colliding droplets under microgravity and in a vacuum have been classified into five types. The five types of behavior are mapped on a We (Weber number) — B (impact parameter) diagram. The range of Weber numbers in the experiments is from 200 to more than 3000.     

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.     

Practical shape optimization of a levitation device for single droplets

The rigorous optimization of the geometry of a glass cell with computational fluid dynamics (CFD) is performed. The cell will be used for non-invasive nuclear magnetic resonance (NMR) measurements on a single droplet levitated in a counter current of liquid in a conical tube. The objective function of the optimization describes the stability of the droplet position required for long-period NMR measurements. The direct problem and even more the optimization problem require an efficient method to handle the high numerical complexity implied. Here, the flow equations are solved two-dimensionally and in steady state with the finite-element code SEPRAN for a spherical droplet with ideally mobile interface. The optimization is performed by embedding the CFD solver SEPRAN in the optimization environment EFCOSS. The underlying derivatives are computed using the automatic differentiation software ADIFOR. An overall concept for the optimization process is developed, requiring a robust scheme for the discretization of the geometries as well as a model for horizontal stability in the axially symmetric case. The numerical results show that the previously employed measuring cell described by Schröter is less suitable to maintain a stable droplet position than the new cell.     

A coupled boundary/finite element method for the computation of magnetically and electrostatically levitated droplet shapes

A coupled finite element and boundary element method is developed to predict the magnetic vector and scalar potential distributions in the droplets levitated in an alternating magnetic or electrostatic field. The computational algorithm entails the application of boundary elements in the region of free space and finite elements in the droplet region, the two being coupled along the droplet–air interface. The coupled boundary and finite element scheme is further integrated with a WRM‐based algorithm to predict the free surface deformation of magnetically and electrostatically levitated droplets. Several corner treatments for the boundary and finite element coupling and their implications to free surface calculations are discussed. Detailed formulation and numerical implementation are given. Numerical results are compared with available analytical solutions whenever available. A selection of computed results is presented for mag‐ netically or electrostatically levitated droplets under both terrestrial and microgravity conditions. Copyright © 1999 John Wiley & Sons, Ltd.     

Investigation of laser-induced destruction of droplets by acoustic methods

Experimental investigations of acoustic signals generated by individual laser-irradiated water droplets are reported. The dependence of droplet destruction thresholds on droplet radius and radiative heating rate is determined. A theoretical explanation of our experimental results is provided in terms of a model that includes the processes of droplet evaporation and fragmentation in response to intense laser heating.     

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.     

A new method for simultaneous measurement of surface tension and viscosity

A new method for the simultaneous mesurement of the surface tension and viscosity of a liquid was developed by combining the principle of the oscillating drop method with a microgravity environment. This new method can be used in an ordinary laboratory. A droplet falls for 1.5 m in approximately 0.55 s. During this short period, the surface oscillation of the droplet is recorded by two high speed line sensors equipped with a laser backlight and cylindrical lenses. The recording speed and resolution of the line sensors are 84000 line/s and 2048 pixels, respectively. The laser backlight forms a shadow of the droplet, and each of the cylindrical lenses makes the shadow into be a line, allowing the maximum diameter to be precisely measured by a line sensor. Before focusing the laser column to a line, it was split into two columns and each of them is forcused into a different line in order to determine the changes in the diameters in two right-angled directions. The measured oscillations show only a single peak for the n = 2 mode in the Fourier spectrum. This fact guarantees that the surface oscillation is almost ideal, and the simple equations for a spherical droplet can be used without any corrections.     

Sessile Drop in Microgravity: Creation, Contact Angle and Interface

We present in this paper the results obtained from a parabolic flight campaign regarding the contact angle and the drop interface behavior of sessile drops created under terrestrial gravity (1g) or in microgravity (μg). This is a preliminary study before further investigations on sessile drops evaporation under microgravity. In this study, drops are created by the mean of a syringe pump by injection through the substrate. The created drops are recorded using a video camera to extract the drops contact angles. Three fluids have been used in this study : de-ionized water, HFE-7100 and FC-72 and two heating surfaces: aluminum and PTFE. The results obtained evidence the feasibility of sessile drop creation in microgravity even for low surface tension liquids (below 15 mN m^− 1) such as FC-72 and HFE-7100. We also evidence the contact angle behavior depending of the drop diameter and the gravity level. A second objective of this study is to analyze the drop interface shape in microgravity. The goal of the these experiments is to obtain reference data on the sessile drop behavior in microgravity for future experiments to be performed in an French-Chinese scientific instrument (IMPACHT).