Oscillatory marangoni convection around bubbles and drops in heterogenous solutions of surfactants

Experimental investigation was performed to study the concentration convection around stationary gas bubbles and insoluble drops in a thin liquid layer placed in a vertical Hele-Shaw cell. The bubbles or drops, squeezed between the two parallel cell walls, took the shape of short cylinders with free lateral surfaces. The cell was filled in with an aqueous solution of a surface-tension active fluid (surfactant) with vertically stratified concentration. A special wire frame prevented bubbles from rising up under the buoyancy force, thus modelling the microgravity conditions. A convective motion in the mixture develops at the bubble or drop interface, due to the solutocapillary Marangoni forces. Owing to a small thickness of the liquid layer (∼1mm), the arising flows and surfactant concentration distributions are nearly two-dimensional so that it is possible to investigate their structure and evolution by interferometric technique. The experiments revealed the development of oscillatory convection around the drop interface, which was similar to that observed in bubble tests. The period and duration of oscillations were determined in relation to time, surfactant concentration gradient and concentration Marangoni number. The analysis of bubble and drop behavior showed that the existence of self-oscillatory modes is related to the specific interaction between the solutocapillary and soluto-gravitational mechanisms of motion.     

Effect of Viscosity on Vibration-Induced Motion of a Spherical Particle Suspended in a Fluid Cell

Production of protein and semi-conductor crystals with advanced quality and properties is possible under microgravity conditions due to the suppression of convection effects. However, aboard space platforms, g-jitter induced motions of solid particles can cause unsteady convection that may result in degradation of the properties of crystals produced. There are different effects of g-jitter on small particles suspended in a fluid cell which are not fully understood. To investigate these small vibration effects, ground experiments were conducted by suspending a spherical particle with a thin wire in a rectangular fluid cell and subjecting the cell and particle to horizontal vibrations with different frequencies and amplitudes. The fluid viscosity was varied to investigate the attraction or repulsion force induced in the direction normal to the direction of the vibration. The force was found to change from attraction to repulsion with an increase in the fluid viscosity and increase with the increasing vibration frequency and amplitude.     

Protein crystallization in space

The microgravity environment of space is an ideal place to study the complicated protein crystallization process and to grow good-quality protein crystals. A series of crystal growth experiments of 10 different proteins was carried out in space on a Chinese re-entry satellite FSW-2 in August, 1992. The experiments were performed for about two weeks at a temperature of 18.5 +/- 0.5 degrees C using a tube-like crystallization apparatus made in the Shanghai Institute of Technical Physics, Academia Sinica. More than half of 48 samples from 6 proteins produced crystals, and the effects of microgravity on protein crystal growth were observed, especially for hen-egg white lysozyme and an acidic phospholipase A2 from the venom of Agkistrodon halys Pallas. Analyses of the crystallization of these two enzymes in this mission showed that the microgravity environment in space may be beneficial to improve size, external perfection, morphology, internal order, and nucleation of protein crystals. Some of these positive microgravity effects were also demonstrated by the growth of protein crystals in gelled solution with the above two enzymes. A structural analysis of the tetragonal lysozyme crystal grown in space is in progress.     

Heat and mass transfer during crystal growth

Quality of semiconductor and oxide crystals which are grown from the melts plays an important role for electronic and/or optical devices. The crystal quality is significantly affected by the heat and mass transfer in the melts during crystal growth in a growth furnace such as Czochralski or horizontal Bridgman methods. This paper reviews the present understanding of phenomena of the heat and mass transfer of the melts, especially instability of melt convection from the detailed numerical calculation, which helps to understand the melt convection visualized using X-ray radiography. Large scale simulation of melt convection during crystal growth is also reviewed.

Characteristics of flow instabilities of melt convection with a low Prandtl number (ratio between momentum and thermal diffusivities) are also reviewed by focusing on the instabilities of baroclinic, the Rayleigh-Benard and the Marangoni-Benard, from the points of view of temperature, rotating and/or magnetic field effects during crystal growth. Oxygen concentration in grown crystals is also discussed how melt convection affects.     

Simulation studies of the hydrodynamics in high-temperature solutions for crystal growth. VI. Temperature fluctuations at the crystal/liquid interface

The temperature fluctuations at the crystal/liquid interface have been studied using model liquids. The character of the fluctuations varied depending on whether in the liquid there was only free convection or both free and forced convections were simultaneously present. It has been established that in the second case the amplitude of fluctuations is determined mainly by the rate of the resultant convection flow and the liquid viscosity. A maximum amplitude of fluctuations is observed at a resultant flow rate w_fl = O when the free and forced convections proceed in opposite directions, and at w_fl = min when both convection flows have the same direction. Larger amplitudes of temperature fluctuations are registered in low-viscosity liquids. The conditions under which a flat crystal/liquid interface showing no temperature fluctuations should be formed have been determined for crystal diameters of 10, 15 and 20 mm, respectively.     

Feedback Control of the Marangoni-Bénard Convection in a Horizontal Fluid Layer with Internal Heat Generation

Feedback control is applied to the steady Marangoni-Bénard convection in a horizontal layer of fluid with internal heat generation heated from below and cooled from above. The analytical technique is used to obtain the close form analytical expression for the onset of Marangoni-Bénard convection when the lower boundary is conducting. The effects of feedback control are studied by examining the critical Marangoni numbers and wave numbers. It is shown that the critical Marangoni number decreases as the value of internal heat generation but the critical Marangoni number can be increased through the use of feedback control.     

Effect of gravity on InGaSb crystal growth

The effect of gravity on dissolution of GaSb in InSb melt and growth of InGaSb was experimentally investigated. Experiments were carried out in a GaSb(seed)/InSb/GaSb(feed) sandwich system under an imposed temperature gradient. In the experiments, the GaSb feed crystal dissolved into the InSb melt to supply the required GaSb component for the growth of In_0.1Ga_0.9Sb crystal. Two parameters were considered: (1) the inclination angle (θ) of the sample for gravity as 0° and 53°, and (2) the sample diameter (D) as 9 mm and 5mm. When θ was 0°, the interface was almost flat, indicating that convection was axisymmetric and stable. Whereas the interface was distorted towards gravitational direction when θ was 53°, indicating that solutal convection was dominant. The decrease of growth temperature and sample diameter reduced the distortion of interface and the dissolution amount of GaSb feed. The homogeneous crystals were grown at the initial growth stage by supplying the GaSb component during growth.     

Convective Auto-Oscillations Near a Drop–Liquid Interface in a Horizontal Rectangular Channel

The concentration convection in an isothermal liquid near a drop (or an air bubble) clamped between the vertical walls of a horizontal channel is studied numerically within the framework of two simple mathematical models: with and without the surface phase at the drop–liquid interface formed by adsorption/desorption process. The interaction between the buoyancy and the Marangoni convective flows is responsible for the onset of auto-oscillation regime. Such oscillations have been experimentally investigated in other works. In our numeric experiments, more than 20 outbursts of the Marangoni convection were observed. The surfactant distributions obtained numerically at different oscillation phases agree well the experimen tal data.     

Nonlinear Benard-Marangoni convection in a rotating layer

Weakly nonlinear Bérnard-Marangoni convection in a rotating layer of fluid is investi gated. Conditions on the conductivity ratio between the boundary and the fluid, rotation rate, Marangoni parameter and the amplitude of convection are determined under which either two-dimensional rolls, square cells or hexagonal cells can be the preferred form of convection.     

Effect of throughflow on Marangoni convection in micropolar fluids

Summary The effect of throughflow on the onset of Marangoni convection in a horizontal layer of micropolar fluid flow bounded below by a rigid isothermal surface and above by a nondeformable free adiabatic surface, for marginal state, is studied. The determination of the critical Marangoni number entails solving the eigenvalue problem numerically for which the Single-term Galerkin method is employed.     

Suction-injection effects on the onset of Rayleigh-Bénard-Marangoni convection in a fluid with suspended particles

Summary The effect of Suction-Injection-Combination (SIC) on the linear stability of Rayleigh-Bénard Marangoni convection in a horizontal layer of an Boussinesq fluid with suspended particles confined between an upper free adiabatic boundary and a lower rigid isothermal/adiabatic boundary is considered. The Rayleigh-Ritz technique is used to obtain the eigenvalues. The influence of various parameters on the onset of convection has been analysed. It is found that the effect of Prandtl number on the stability of the system is dependent on the SIC being pro-gravity or anti-gravity. A similar Pe-sensitivity is found in respect of the critical wave number. It is observed that the fluid layer with suspended particles heated from below is more stable compared to the classical fluid layer without suspended particles. The problem has possible applications in microgravity situations.     

微重力条件下熔体中对流过程的数值模拟

微重力条件下生长优质晶体遇到的最大问题是要控制晶体生长的条件，抑制由于重大的减弱而引起的熔体中的热毛细时流。但是，用实验来解决这些问题费用高，周期也长，而且有时完全用实验来模拟也是很困难的。用数值计算方法来模拟微重力条件下熔体中的对流过程是空间晶体生长研究的一个重要的方向，计算结果对控制空间生长晶体和抑制熔体中的对流有指导意义。对微重力条件下熔体中对流发生、发展的过程进行了数值研究。以有限差分法研究了沿上表面为自由表面的水平区域不同边界条件下的熔体中的对流过程。     

φ300mm的大直径直拉单晶硅勾形磁场下生长的数值模拟

直径300mm硅片的生产技术是当今硅材料生产研究的重要方向之一,而晶体生长 界面的形状、温度分布、晶体中氧的浓度和均匀性等对熔体流动状态十分敏感,采用实验的方 法来测量熔体的流动、温度场分布是很困难的,因此很难通过实验的方法获得熔体的流动是如 何影响晶体生长的质量的,而数值模拟能提供熔体流动、温度分布等详细内容,为单晶硅的生 长提供有利的指导.本文采用低雷诺数的K-ε紊流模型,对直径300mm的大直径单晶硅生 长进行了数值模拟,通过熔体在有、无勾形磁场作用时的流场、温度场的分析,阐明了勾形磁 场影响熔体流动的机理.     

An experimental study of bubble behavior in a temperature gradient near a heated wall under low-gravity and microgravity conditions aboard NASA DC9 airplane

The behavior of a single bubble and a pair of bubbles under microgravity conditions has been investigated using the NASA-DC9 aircraft in order to understand the effects of various parameters and to control the bubble behavior in space. Silicone oil was used as the test liquid, and a nitrogen gas bubble was injected from the top wall under different experimental conditions. In an isothermal case, two different microgravity conditions were achieved by either fixing the experimental apparatus to the aircraft floor or freely floating the apparatus in the aircraft cabin. The bubble behavior was found to be clearly influenced by the quality of the microgravity environment, and variations of the bubble aspect ratio with the Bond number were presented. The results indicate that there is a critical Bond number of the order of 10⁻¹ which determines the bubble shape deformation. In the free-floating experiments, a temperature gradient was imposed on the liquid around the bubble near the heated top wall. Marangoni convection was expected to occur around the bubble and the bubble behavior was studied under various temperature gradients. The bubble aspect ratio was found to decrease with an increase in the Marangoni number. A theoretical model for the relation between the Marangoni flow around the bubble and the aspect ratio is proposed based on simple assumptions. Visualization of Marangoni convection around the bubbles using the photochromic dye activation method was successfully performed. The aspect ratios predicted by the model agreed with the experimental results reasonably well. Direct measurements of surface velocity are, however, necessary to further evaluate the validity of the model.     

The flow behavior of granular material due to horizontal shaking

This paper studied the convection phenomena that occurred in a granular bed under horizontal vibration. High-speed photography was employed to measure the movements of particles. The fields of the instantaneous velocities and the long-term velocities were plotted. Two symmetric convection rolls were observed. The granular temperature distributions were also measured and analyzed. The controlling parameters included the acceleration amplitude and the wall condition. The convection strength was found to increase linearly with the total granular temperature in the bed.     

Effects of convective fluid motion upon oxide crystal growth in high temperature solution

For understanding of the influence of convective flow on crystal growth, space high temperature in situ observation instrument (SHITISOI) is dedicated to visualize and record the whole growth process of oxide crystals in high temperature up to 1,000°C. Model experiments using transparent liquids such as KNbO₃ and a mixture of Li₂B₄O₇+KNbO₃ were chosen to investigate effects on ground and in space. On the earth, an investigation of growth kinetics of KNbO₃ crystal related to two different states of convection: diffusive-advective flow and diffusive-convective flow, has been performed. The per unit length of a step e is calculated from the experimental data for two different states of convection. Analyses of these data show the effect of buoyancy convection is to enhance the sharpness of the interface. The growth of KNbO₃ crystals from solution of KNbO₃+Li₂B₄O₇ was investigated in space. The streamlines of the steady thermocapillary convection in Li₂B₄O₇ solvent was observed. Due to thermocapillary convection, KNbO₃ crystal grains grew and filled the whole solution homogeneously. Earth-based quenching experiments are designed in order to study polyhedral instability of KNbO₃ crystal, which is controlled by diffusion mechanism limitation. In all cases, when the crystal was nucleated near air/solution surface, it lost its polyhedral stability and varied from polyhedrons to dendrites. The thickness of diffusion mechanism limitation layer is about 60 μm.     

Influence of crystal—melt interface shape on self-seeding and single crystalline quality

The growth of Sb-based crystals (InSb, GaSb etc) was undertaken using resistive heater furnace by vertical directional solidification (VDS) technique. Crystal—melt interface shape during the growth was shown to convert from concave to convex along the crystal axis of the ingots. Many antimonide (Sb) crystals of 8 mm to 18 mm diameter were grown by optimized growth parameters. The forced convection and absence of conducting support to ampoule showed improvement in crystal quality of as grown ingots. Crystals showed preferred orientation and self-seeding. Results on interface shape and crystallinity of ingots were found to be in good agreement with the experiments.     

Counterdiffusion protein crystallisation in microgravity and its observation with PromISS (protein microscope for the international space station)

The crystallisation by counterdiffusion is a very efficient technique for obtaining high-quality protein crystals. A prerequisite for the use of counterdiffusion techniques is that mass transport must be controlled by diffusion alone. Sedimentation and convection can be avoided by either working in gelled systems, working in systems of small dimensions, or in the absence of gravity. We present the results from experiments performed on the ISS using the Protein Microscope for the International Space Station (PromISS), using digital holography to visualise crystal growth processes. We extensively characterised three model proteins for these experiments (cablys3*lysozyme, triose phosphate isomerase, and parvalbumin) and used these to assess the ISS as an environment for crystallisation by counterdiffusion. The possibility to visualise growth and movement of crystals in different types of experiments (capillary counterdiffusion and batch-type) is important, as movement of crystals is clearly not negligible.     

Effects of convective fluid motion upon oxide crystal growth in high temperature solution

For understanding of the influence of convective flow on crystal growth, space high temperature in situ observation instrument (SHITISOI) is dedicated to visualize and record the whole growth process of oxide crystals in high temperature up to 1,000°C. Model experiments using transparent liquids such as KNbO₃ and a mixture of Li₂B₄O₇+KNbO₃ were chosen to investigate effects on ground and in space. On the earth, an investigation of growth kinetics of KNbO₃ crystal related to two different states of convection: diffusive-advective flow and diffusive-convective flow, has been performed. The per unit length of a step e is calculated from the experimental data for two different states of convection. Analyses of these data show the effect of buoyancy convection is to enhance the sharpness of the interface. The growth of KNbO₃ crystals from solution of KNbO₃+Li₂B₄O₇ was investigated in space. The streamlines of the steady thermocapillary convection in Li₂B₄O₇ solvent was observed. Due to thermocapillary convection, KNbO₃ crystal grains grew and filled the whole solution homogeneously. Earth-based quenching experiments are designed in order to study polyhedral instability of KNbO₃ crystal, which is controlled by diffusion mechanism limitation. In all cases, when the crystal was nucleated near air/solution surface, it lost its polyhedral stability and varied from polyhedrons to dendrites. The thickness of diffusion mechanism limitation layer is about 60 μm.     

高温氧化物晶体界面非稳定性研究

设计了一套模拟实验,以获得关于晶体形态和界面非稳定性的差异的可靠数据,如高温溶液生长的骸晶和枝蔓晶．这些实验是在高温实时观察装置（ＨＩＴＩＳＯＴ）内进行的．高温溶液晶体生长实验是在环形铂金丝炉圈内进行的．炉圈直径为２ｍｍ．铂金丝既起加热又起支撑熔体的作用．选用ＫＮｂＯ_３和Ｌｉ_２Ｂ_４Ｏ_７的混合物进行晶体生长实验．在只存在扩散机制的快速生长过程中,会形成不同的晶体不完整性,如晶面凹坑、骸晶和枝蔓晶．采用淬火实验以分辨不同的ＫＮｂＯ_３晶体形态,并用扫描电镜研究Ｌｉ_２Ｂ_４Ｏ_７溶体中ＫＮｂＯ_３晶体生长的形貌．在一般情况下,当晶体在气液界面附近液相区成核时,会产生晶体界面非稳定性．导致晶体形状不稳定的溶液层的厚度为６０μｍ．通过扫描电镜观察,发现晶体在这一溶液层中由多面体晶变为枝蔓晶．骸晶和枝蔓晶的各向异性反映了ＫＮｂＯ_３的立方特性,也反映了界面非稳定性是沿［１１０］晶棱扩大的,［１１０］晶棱方向的分支证实了晶体生长形状的各向异性·形成界面非稳定性的临界尺寸为１０μｍ．与此相反,中持稳定的晶面形状是通过６０μｍ厚度以下的溶液内的晶体生长来实现的．晶体生长过程是由高温实时观察装置进行实时观察和记录的,并能观察到晶体固液