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.     

Self-arraying of charged levitating droplets

Diamagnetic levitation of water droplets in air is a promising phenomenon to achieve contactless manipulation of chemical or biochemical samples. This noncontact handling technique prevents contaminations of samples as well as provides measurements of interaction forces between levitating reactors. Under a nonuniform magnetic field, diamagnetic bodies such as water droplets experience a repulsive force which may lead to diamagnetic levitation of a single or few micro-objects. The levitation of several repulsively charged picoliter droplets was successfully performed in a ~1 mm(2) adjustable flat magnetic well provided by a centimeter-sized cylindrical permanent magnet structure. Each droplet position results from the balance between the centripetal diamagnetic force and the repulsive Coulombian forces. Levitating water droplets self-organize into satellite patterns or thin clouds, according to their charge and size. Small triangular lattices of identical droplets reproduce magneto-Wigner crystals. Repulsive forces and inner charges can be measured in the piconewton and the femtocoulomb ranges, respectively. Evolution of interaction forces is accurately followed up over time during droplet evaporation.     

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.     

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.     

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.     

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.     

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.     

The field-induced ion evaporation effect in the bipartition of charged droplets

A physical mechanism explaining the formation of a cluster phase during the electrostatic bipartition of droplets unstable with respect to their own charge is suggested. The mechanism assumes the possibility of the field-induced evaporation of ions from the surface of droplets of a micron size formed in the final stage of a droplet decay cascade.     

Numerical and experimental investigations on the use of mist flow process in refrigerated display cabinets

This study concerns the use of mist flow whereby fine water droplets are injected into the air curtain to improve the performance of Refrigerated Display Cabinets (RDCs). The deposition and evaporation of droplets on the surface of products partially compensate the radiative heat gained by the products by removing from it the amount of latent heat of the evaporated droplets.The experiments were carried out on an actual display cabinet. Numerical modelling was performed using Fluent Computational Fluid Dynamics (CFD) software. In two-phase flow, an Euler–Lagrange approach was adopted to predict the transport of droplets by the air curtain and their spatial distribution on the product surface of the RDC. An original numerical procedure was built in the CFD model in order to compute the deposited droplets while taking into account the evaporative flux of droplets on the product surface.The two-phase flow model was used to analyse the performance of the mist cooling process in terms of surface temperature decrease and the homogeneity of droplet deposition on the product surface of the RDC as a function of inlet droplet injection configurations.     

Study of chemistry in droplets with net charge before and after Coulomb explosion: ion-induced nucleation in solution and implications for ion production in an electrospray

Droplets with net charge are essential intermediaries in the production of gaseous ions in the electrospray (ES) ion source. There could be a wealth of knowledge regarding the chemistry that occurs in such droplets as a result of their violation of electroneutrality. Such information could lead to improved understanding of the ion generation process in an ES along with factors that affect it. The experiments performed involved the levitation of individually charged droplets that were, and were not, allowed to undergo Coulomb explosion while they remained levitated in an electrodynamic balance (EDB). Through examination of precipitates formed within the levitated droplets, it was observed that onset of NaCl precipitation was promoted in droplets (glycerol:water 9:1 v/v) that had mass-to-net-charge (m/z) ratio <-4.8 x 10(9) amu/e. This threshold m/z value is exceeded in essentially all droplets generated in an ES because it is above the calculated threshold for Coulomb explosion. This finding suggests that cluster formation in droplets having net charge could occur at solute concentrations lower than would be anticipated on the basis of homogeneous nucleation. The effect of large entities such as precipitates existing in the droplet on the dynamics of droplet Coulomb explosion was also examined. Using droplets whose initial size and magnitude of net charge were equivalent within experimental error but having different concentration of solutes, we showed that the m/z of their main residues following Coulomb explosion were different. Micrometer-size droplets containing 20 nm fluorescent beads that underwent Coulomb explosion resulted in main residues that had higher m/z for higher initial bead concentration in the starting solution (320 nM) when compared to main residues resultant from starting solutions having lower initial bead concentration (21 nM).     

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.     

High-speed imaging and CFD simulations of a deforming liquid metal droplet in an electromagnetic levitation experiment

Electromagnetic levitation of a liquid metal droplet is of great interest to study gas–liquid metal reactions. An important prerequisite for the evaluation of the overall mass transfer between the gas and metal is to characterize the geometry of the deforming molten droplet, which determines the interfacial reaction area. In this article, the free surface shape and dynamics of a molten 80%Ni–20%Cr droplet is investigated both experimentally and numerically. The frequencies associated to the oscillatory translational motions of the drop and to the vibrations of its free surface are measured using high-speed video image analysis. A 2D transient model is then presented, in which three interacting phenomena are considered: electromagnetic phenomena, the turbulent flow of liquid metal in the drop and the change in the drop shape. The numerical results presented demonstrate the capabilities of the model.     

Microstructure Evolution of Cu-Pb Monotectic Alloys Processed in Drop Tube

Rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3m drop tube.Both macrosegregated and uniformly dispersed structures are observed in Cu-40wt pct Pb alloy droplets,whereas droplets of composition Cu-64wt pct Pb exhibit only macrosegregation morphologies.The microstructures are strongly dependent on droplet size.The higher undercooling tends to facilitate liquid phase separation and results in more extensive macrosegregation in smaller droplets.There exists a pronounced tendency for the Pb-rich liquid to occupy the surface of the droplets of both compositions,resulting from the quite lower surface tension of the Pb-rich phase and cauing a Pb-rich layer at the surface of the solidified droplet.The nucleation of monotectic cells in the Cu-40 wt pct Pb droplets with dispersed structures preferentially occurs at the droplet surface.A single nucleation event takes place more frequently as droplet size is reduced.     

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.     

Formation of droplets of alternating composition in microfluidic channels and applications to indexing of concentrations in droplet-based assays

For screening the conditions for a reaction by using droplets (or plugs) as microreactors, the composition of the droplets must be indexed. Indexing here refers to measuring the concentration of a solute by addition of a marker, either internal or external. Indexing may be performed by forming droplet pairs, where in each pair the first droplet is used to conduct the reaction, and the second droplet is used to index the composition of the first droplet. This paper characterizes a method for creating droplet pairs by generating alternating droplets, of two sets of aqueous solutions in a flow of immiscible carrier fluid within PDMS and glass microfluidic channels. The paper also demonstrates that the technique can be used to index the composition of the droplets, and this application is illustrated by screening conditions of protein crystallization. The fluid properties required to form the steady flow of the alternating droplets in a microchannel were characterized as a function of the capillary number Ca and water fraction. Four regimes were observed. At the lowest values of Ca, the droplets of the two streams coalesced; at intermediate values of Ca the alternating droplets formed reliably. At even higher values of Ca, shear forces dominated and caused formation of droplets that were smaller than the cross-sectional dimension of the channel; at the highest values of Ca, coflowing laminar streams of the two immiscible fluids formed. In addition to screening of protein crystallization conditions, understanding of the fluid flow in this system may extend this indexing approach to other chemical and biological assays performed on a microfluidic chip.     

基于SPH方法的低韦伯数下三维液滴碰撞聚合与反弹数值模拟研究

光滑粒子流体动力学方法（SPH方法）作为纯拉格朗日粒子方法,可以有效避免网格法在模拟大变形过程中带来的网格扭曲等缺陷,适合模拟含大变形的液滴碰撞聚合与反弹过程。该文基于Ott和Schnetter提出的修正SPH方法,利用有限差分与SPH一阶导数相结合的方法处理粘性项中的二阶导数问题,进行Couette流算例验证,数值解...     

Nonisothermal flow in a conic liquid trap under conditions of atmosphere-free space

A nonisothermal film flow is considered, which is formed on the inside surface of a conic liquid trap, to the inlet of which a homogeneous flow of monodisperse droplet medium is delivered under conditions of atmosphere-free space. Asymptotic models of steady film flow are constructed and investigated for the conditions of inertial deposition of droplets under the assumption of small relative thickness of film and of the effect of secondary droplets formed upon deposition on the film surface. For a slow flow, the shape of the surface and the parameters of the film are found analytically. A parametric numerical investigation of distributions of velocity, temperature, and film thickness is performed for the general case. The parameters of flow in the inlet section of the offtake channel are found, and the conditions are determined which are required for maintaining a steady-state mode of operation of the drip pan.     

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.     

蒸发液滴中的流动与传质行为:理论与应用

液滴蒸发过程伴随着热量和物质的传输,是现代技术领域普遍存在而又未充分认识的一个复杂过程。综述了蒸发液滴中的流动与传质规律及其应用研究进展,主要包括典型的蒸发液滴过程及其中流动方式、聚合物溶液液滴蒸发成膜花样、复合材料溶液液滴蒸发成膜花样等液滴蒸发理论的实验研究。同时简要分析了蒸发液滴理论在喷墨打印、纳米材料制备等领域的应用研究。最后对聚合物溶液液滴蒸发理论的现状及未来应用需求进行了总结与展望。