Development of an interferometer for measurement of the diffusion coefficient of miscible liquids

A common-path interferometer (CPI) system was developed to measure the diffusivity of transparent liquid pairs by real-time visualization of the concentration gradient profile. The CPI is an optical technique that can be used to measure changes in the gradient of the refractive index of transparent materials. The CPI is a shearing interferometer that shares the same optical path from a laser light source to the final imaging plane. Molecular diffusivity of liquids can be determined by use of physical relations between changes in the optical path length and the liquid phase properties. The data obtained by this interferometer are compared with similar results from other techniques. This demonstrates that the instrument is reliable for measurement of the diffusivity of miscible liquids and allows the system to be compact and robust. It can also be useful for studies in interface dynamics as well as other applications in a low-gravity environment.     

Measurement of the complex refractive-index spectrum for birefringent and absorptive liquids

The optical constants of birefringent and/or opaque liquids, e.g., liquid crystals and magnetic fluids, are difficult to measure at wavelengths at which a strong light source such as a laser or an arc lamp is not accessible. The refractive index n and the extinction coefficient kappa of these liquids can be simultaneously evaluated from the reflectance curves that are measured in the large incident angle range. A semicylindrical sample cell allows the spectral reflectance measurement with a weak light source even at large incident angles. By using this method, we evaluated the ordinary and the extraordinary indices of a nematic liquid crystal in the continuous wavelength range of 0.55-1.60 mum. The complex refractive indices of magnetic fluids were also evaluated, and the affect of the magnetic field was demonstrated.     

二元系纳米多层薄膜非线性互扩散研究

采用非均匀体系非线性动力学离散模型计算AB二元系纳米多层薄膜的非线性互扩散,研究了A、B原子间作用力之差VBB-VAA和有序能V对纳米多层薄膜浓度分布和界面结构的影响.对于V≤0的纳米多层薄膜,随着VSS-VAA由0减小到-0.05eV,扩散过程中多层薄膜浓度偏离对称分布,界面由宽化向平直转变,有序能V由0减小到-0.025eV,导致多层薄膜的扩散速度加快.当V>0时,扩散过程中多层薄膜出现相分离趋势导致的上坡扩散,初始宽化互混的界面变得平直,且随着VBB-VAA由0减小到-0.05V,多层薄膜扩散过程中形成的浓度起伏长大成为新的亚层.AB二元系纳米多层薄膜扩散过程中的浓度分布和界面结构演变与扩散非对称性系数m'、有序能V和初始时刻的浓度梯度有关.     

Geometrical size effect on the interface diffusion of micro solder joint in electro-thermal coupling aging

This paper investigates the influence of diffusion layer (solder layer) thickness (δ) on interface diffusion in both thermal aging and electro-thermal coupling aging. The different δ (δ = 60, 120 and 240 μm) of Cu/Sn–3.0Ag–0.5Cu (SAC305)/Cu butt solder joints are used. The results indicate that the geometrical size (solder layer thickness) of solder joint has significant effect on element diffusion behavior. The diffusion coefficient, time exponent, element concentrations and diffusion flux are greatly dependent upon δ. The effects of δ on the interface diffusion is different between thermal aging and electro-thermal coupling aging, due to driving force for diffusion is different. During thermal aging, concentration gradient is the main driving force of diffusion, and diffusion coefficient, time exponent and diffusion flux are relatively low for a thin solder layer. However, under electro-thermal coupling condition, the electron wind force provides the dominating driving force for diffusion, and diffusion coefficient and diffusion flux of thin δ are significantly larger than the thick ones. The Cu concentration of the area near interface is relatively high for a thin solder layer in both tests. Under the same experimental temperature, the effects of δ on the electro-thermal coupling aging are more obvious than thermal aging.     

Thermal management with self-rewetting fluids

The present paper describes the results of a series of microgravity experiments on thermal management device, actually wickless heat pipes, with using the so-called “self t-rewetting fluids” (dilute aqueous solutions of high carbon alcohols) as a working fluid. Although most of liquids show a decrease in the surface tension with increasing temperature, self-rewetting fluids show exceptionally an increase in the surface tension with increasing temperature. This particular characteristic allows for a spontaneous liquid supply to hotter interface by the thermocapillary flow. When liquid/vapor phase change takes place, furthermore, additional Marangoni effect due to concentration gradient by the preferential evaporation of alcohol-rich composition in the aqueous solutions is induced. A considerably strong liquid inflow to dry patch or thin film is therefore expected at three-phase interline or liquid/vapor interface. One of the most promising applications of the self-rewetting fluids in space is wickless heat pipes in which condensate spontaneously returns to evaporation region by enhanced Marangoni effect. Demonstrational experiments on the fluid behavior in a transparent glass tube wickless heat pipe were conducted in JAMIC, and spontaneous liquid return velocities were measured. The present authors then performed parabolic flight experiments on heat transfer characteristics of prototype wickless copper heat pipes, and the performance was compared with ordinary heat pipe having wick structure and with other working fluid.     

Optimizing the low-pressure carburizing process of 16Cr3NiWMoVNbE gear steel

Compared with the traditional atmospheric carburization, low-pressure carburization has the benefits of producing no surface oxidation and leaving fine, uniformly dispersed carbides in the carburized layer. However, the process parameters for low-pressure carburization of 16Cr3NiWMoVNbE steel have yet to be optimized. Thus, we use the saturation-value method to optimize these parameters for aviation-gear materials. Toward this end, the microstructure and properties of 16Cr3NiWMoVNbE steel after different carburization processes are studied by optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron probe microanalysis. Considering the saturated austenite carbon concentration, we propose a model of carbon flux and an alloy coefficient for low-pressure carburization to reduce the carbon concentration in austenite and avoid the surface carbide network. At the early stage of carburization (˜30 s), the gas-solid interface has a higher concentration gradient. The averaging method is not ideal in practical applications, but the carbon flux measured by using the segmented average method is 2.5 times that measured by the overall average method, which is ideal in practical applications. The corresponding carburization time is reduced by 60%. By using the integral average method, the actual carburization time increases, which leads to the rapid formation of carbide on the surface and affects the entire carburization process. Nb and W combine with C to form carbides, which hinders carbon diffusion and consumes carbon, resulting in a sharp decrease in the rate of C diffusion in austenite (the diffusion rate is reduced by ˜52% for 16Cr3NiWMoVNbE steel). By changing the diffusion coefficient model and comparing the hardness gradient of different processes, the depth of the actual layer is found to be very similar to the design depth.     

Nonlinear interdiffusion in binary nanometer-scale multilayers submitted to thermal annealing

The concentration profile in binary A-B nanometer-scale multilayers submitted to thermal annealing was calculated based on the Martin's kinetic discrete model for one-dimension nonlinear interdiffusion by a diffusion asymmetry coefficient m′ and an ordering energy V between A and B atoms. With decreasing the diffusion asymmetry coefficient m′ from 0 to − 6, the concentration profile of the multilayers deviated from symmetrical distribution, and their interfaces became sharp and shifted towards the side of the sublayer with lower pair interaction energy. The difference of diffusion coefficient of A and B atoms caused by the diffusion asymmetry coefficient m′ led to the difference of net fluxes of A and B atoms in the multilayers. When the ordering energy V changed from − 0.001 eV to − 0.05 eV, change in the concentration profile and interface structure was same for the multilayers with a given diffusion asymmetry coefficient m′, but the calculated diffusion time decreased correspondently. The lower ordering energy V makes the A and B atoms aggregating more easily during the interdiffusion. It is found that the nonlinear interdiffusion of a series of binary nanometer-scale multilayers can be characterized by the diffusion asymmetry coefficient m′ and the ordering energy V, to explore the solid state reaction between the sublayers of nanometer-scale multilayers.     

Interferometric measurement of light scattering behavior in continuous SiO2 fiber in PMMA matrix composite

Possibility of an interferometric method was examined for evaluating local light scattering behaviors in light transmitting materials. Phase profiles of wavefronts through a continuous SiO₂ glass fiber–PMMA composite was measured by detecting transmitted light though the composite using the interferometric system with a pair of interferometers. Wavefront though the composite shows an anisotropic phase profile, and the phase delay originates from local light scattering due to refractive index difference at an interface between SiO₂ fiber and PMMA matrix. Measured wavefronts demonstrated that the interferometric method is effective tool for an evaluation of local light scattering.     

Rayleigh–Taylor-Like Instability in Near-Critical Pure Fluids

The hydrodynamic stability of a thermodiffusive interface in a near-supercritical fluid is studied. The Navier-Stokes equations written for a van der Waals gas above its critical point are solved by means of a finite volume numerical method. The growth rate of the fluctuations shows that there exists a cutoff wave number beyond which the short wavelengths are stabilized by diffusion. The good agreement between the obtained values and recent theories for miscible fluids confirms that a near-critical fluid subjected to a thermal gradient may develop a gravitational instability for which the density gradient is driven by thermal diffusion and large compressibility.     

Lipopolymer gradient diffusion in supported bilayer membranes

We measure the gradient diffusion coefficient of a model lipopolymer in supported lipid bilayer membranes from Fourier-transform post-electrophoresis relaxation. The experiments and accompanying quantitative interpretation furnish the concentration dependence of the gradient diffusion coefficient. In striking contrast to the recent measurements of the self-diffusion coefficient from fluorescence recovery after photobleaching, the lipopolymer gradient diffusion coefficient increases with concentration. We interpret the enhancement at small but finite concentrations using the Scalettar–Abney–Owicki (SAO) statistical mechanical theory (1988) and the Bussell–Koch–Hammer (BKH) hydrodynamic theory (1995), which are customarily adopted to model membrane protein dynamics. The SAO theory furnishes an effective disc radius and soft repulsive interaction radius that are comparable to the Flory radius of the unperturbed polyethylene glycol chains. On the other hand, the BKH theory predicts a gradient diffusion coefficient that decreases with disc/membrane protein concentration. Thus, in contrast to membrane proteins, we conclude that lipopolymer hydrodynamic interactions are weak because the principal disturbances are in the low-viscosity aqueous phase. Accordingly, lipopolymer interactions are dominated by thermodynamic interactions among polymer chains. Interestingly, our experiments suggest that increasing (decreasing) the polymer molecular weight should increase (decrease) the relaxation rate of lipopolymer concentration fluctuations.     

Liquid-phase mass transfer in high-pressure systems with a supercritical and a liquid phase

Liquid-phase diffusion coefficients and mass-transfer coefficients were measured in binary two-phase systems at high pressures. Both were determined from the rates of absorption of the gaseous component into the liquid. Diffusion coefficients were measured by observing unsteady-state diffusion into a sell) i infinite liquid phase and fitting transient counterdiffusion mass-flow rates to the visually determined change of the position of the interface between the liquid and the supercritical phase. Mass-transfer coefficients in the liquid phase were determined from the absorption rate of the gaseous component into a falling liquid film of known flow and physical properties. Experiments were performed with the binary systems carbon dioxide-oleic acid, carbon dioxide -methyl myristate, and carbon dioxide-methyl palmitate. Liquid-phase diffusion coefficients rise significantly with concentration as viscosity decreases. Temperature also has a strong effect on diffusivities. Experimental mass-transfer coefficients in nearly saturated liquids agree well with calculations for falling films with known properties, whereas far from equilibrium, Marangoni convection greatly enhances mass-transfer rates. Close to the critical point of the binary system at a given temperature, a sharp decline of the mass-transfer coefficient is observed.     

Determination and verification of the gap dependent heat transfer coefficient during permanent mold casting of A356 aluminum alloy

In complex castings, the heat transfer across the casting / mold interface depends on the local gap size and contact pressure. Thus, an experimental setup is constructed to measure and evaluate the air‐gap dependent heat transfer coefficient during solidification of an A356 permanent mold casting. In order to evaluate the heat transfer coefficient, the temperature gradient and air gap development is measured at the casting / mold interface. This allows the interface temperatures and the time‐dependent heat flux across the gap to be calculated as a function of the measured gap size. Furthermore, the heat transfer coefficient and gap size are correlated to the interface temperature of the casting. The experimental setup and the evaluation procedure provide consistent and reproducible results. The heat transfer coefficient thus evaluated is employed to simulate the experimental setup. The temperatures measured are well reproduced. The results of the present work are compared to simulations using two heat transfer coefficient functions found in literature. This comparison shows a substantial improvement over the state of the art. This improvement is due to the exact knowledge of gap formation and the corresponding values of the heat transfer coefficient.     

Modelling of Diffuse Interfaces with Temperature Gradients

The work is devoted to capillary phenomena in miscible liquids under the assumption that they have a constant and the same density. The model consists of the heat equation, diffusion equation, and the Navier-Stokes equations with the Korteweg stress. We study several configurations corresponding to the microgravity experiments planned for the International Space Station. The basic conclusion of the numerical simulations is that transient capillary phenomena in miscible liquids exist and can produce convective flows sufficiently strong to be observed experimentally. In particular, there exists a miscible analogue to the Marangoni convection where the temperature gradient is applied along the transition zone between two fluids. Convection also appears if, instead of the temperature gradient, the case where the width of the transition zone varies in space is considered. Finally, similar to the immiscible case, miscible drops move in a temperature gradient.     

Ion-exchange waveguides formed in glasses using silver-containing melts

The activation energy has been determined and the concentration dependence of the diffusion coefficient have been studied for ion-exchange diffusion from silver-containing salt melts into K8 optical glass. The dependence of the refractive index at the surface of a gradient waveguide, formed by diffusion, on the melt composition is experimentally measured. Using these data, it is possible to model the optical parameters of the waveguides and to optimize the diffusion process parameters.     

The role of interface concentration gradient in the formation of silver dendritic particles

Dendritic structures are widely present in nature, from river networks to snowflakes. There is a long-term interest in discovering their common formation mechanism, especially the driving force leading to the branching and the reason to keep symmetry. The inhibition of lithium dendrites in secondary batteries also calls for the deep understanding on the formation of dendritic structures. Here in this article, we report an investigation on the driving force of the formation of dendritic structures. Silver particles are synthesized by Galvanic replacement reaction (GRR) in which metal rods are immersed into the silver nitrate solution to reduce silver ions followed by silver particles formation on the surface of rods. The silver ions concentration profile near the rods is measured by Mach-Zehnder interferometer during the reaction. It is found that the formation of silver dendritic particles is accompanied by the interface concentration gradient. A regulation on the gradient leads to the change of silver morphology, experimentally confirming the dominant role of the interface concentration gradient in the formation of diverse structures.     

Mutual Diffusion Coefficient and Dynamic Viscosity Near the Critical Consolute Point Probed by Dynamic Light Scattering

The possibility of applying dynamic light scattering to simultaneous determination of the mutual diffusion coefficient and the viscosity of binary liquid systems was studied near the critical consolute point. When seed particles are added to the system, the particle diffusion coefficient is measured, and the viscosity is obtained using the Stokes–Einstein relation. Since the amplitude of light scattered from concentration fluctuations is low in a mixture with a small difference between the refractive indices of the pure components, this approach allowed the determination of the viscosity of a critical mixture of nitroethane and isooctane, without a signal component from mutual diffusivity superimposed. In contrast, particle aggregation prevented the determination of the viscosity of a critical mixture of triethylamine and water. Despite this difficulty, and an unidentified contribution in the signals obtained, the mutual diffusion coefficient and the critical exponent v could be determined in this system without a noticeable influence from the addition of seed particles.     

Nanostructure of vortex during explosion welding

The microstructure of a bimetallic joint made by explosion welding of orthorhombic titanium aluminide (Ti-30Al-16Nb-1Zr-1Mo) with commercially pure titanium is studied. It is found that the welded joint has a multilayered structure including a severely deformed zone observed in both materials, a recrystallized zone of titanium, and a transition zone near the interface. Typical elements of the transition zone-a wavy interface, macrorotations of the lattice, vortices and tracks of fragments of the initial materials-are determined. It is shown that the observed vortices are formed most probably due to local melting of the material near the contact surface. Evidence for this assumption is deduced from the presence of dipoles, which consist of two vortices of different helicity and an ultrafine duplex structure of the vortex. Also, high mixing of the material near the vortex is only possible by the turbulent transport whose coefficient is several orders of magnitude larger than the coefficient of atomic diffusion in liquids. The role played by fragmentation in both the formation of lattice macrorotations and the passage of coarse particles of one material through the bulk of the other is determined.     

Characterization and resolution of evaporation-mediated osmolality shifts that constrain microfluidic cell culture in poly(dimethylsiloxane) devices

Evaporation is a critical problem when handling submicroliter volumes of fluids. This paper characterizes this problem as it applies to microfluidic cell culture in poly(dimethylsiloxane) (PDMS) devices and provides a practical solution. Evaporation-mediated osmolality shifts through PDMS membranes with varying thicknesses (10, 1, 0.2, or 0.1 mm) were measured over 96 h. Even in humidified cell culture incubators, evaporation through PDMS and associated shifts in the osmolality of culture media was significant and prevented mouse embryo and human endothelial cell growth and development. A simple diffusion model, where the measured diffusion coefficient for PDMS matches reported values of approximately 10-9 m2/s, accounts for these evaporation and osmolality shifts. To overcome this problem, a PDMS-parylene-PDMS hybrid membrane was developed that greatly suppresses evaporation and osmolality shifts, yet possesses thinness and the flexibility necessary to interface with deformation-based microfluidic actuation systems, maintains the clarity for optical microscopy, and enables the successful development of single-cell mouse embryos into blastocysts under static conditions and culture of human endothelial cells under dynamic recirculation of submicroliter volumes of media. These insights and methods demonstrated specifically for embryo and endothelial cell studies will be generally useful for understanding and overcoming evaporation-associated effects in microfluidic cell cultures.