The Convective Instabilities in a Liquid-Vapor System with a Non-equilibrium Evaporation Interface

Rayleigh-Marangoni-B,nard instability in a system consisting of a horizontal liquid layer and its own vapor has been investigated. The two layers are separated by a deformable evaporation interface. A linear stability analysis is carried out to study the convective instability during evaporation. In previous works, the interface is assumed to be under equilibrium state. In contrast with previous works, we give up the equilibrium assumption and use Hertz-Knudsen's relation to describe the phase change under non-equilibrium state. The influence of Marangoni effect, gravitational effect, degree of non-equilibrium and the dynamics of the vapor on the instability are discussed.

热学微流控系统中瞬态相变传热研究

微尺度相变传热广泛存在于微反应器、微型燃料电池、微蒸发器、微冷凝器、微热管、微汽泡执行器等微流控器件中，研究微流控系统中的相变问题对于微流控器件的设计和运行具有重要的科学意义。本文针对三类典型的微尺度相变问题，即微尺度流动沸腾、微尺度流动凝结以及微加热器上的汽泡动力学进行了深入细致的研究，实验研究中所采用的实验件均为标准MEMS微加工工艺制作，克服了常规机械加工所造成的表面粗糙度的影响。 考虑到微流控系统中大量应用交叉型、弯曲型等复杂结构的微通道，在微尺度流动沸腾研究中，设计了一种具有交错微通道结构的微流控芯片，并以丙酮为工质，对该芯片内的流动沸腾进行了研究。发现了周期为毫秒量级微时间尺度的流型结构，整个周期包括单相液体充液、两相分层流以及部分蒸干的液膜流三个阶段；在单个微通道区域，由于蒸发动量力的作用，液膜沿流动方向呈非均匀分布，蒸干首先发生在上游；由于液相弗劳德数较小，导致微通道中依然存在分层流流型。由于毕渥数较小，芯片背面温度几乎与芯片内壁面温度保持同步变化。虽然红外热像仪的响应频率较低，但仍然可以鉴别出由于流型周期性转换导致的壁面温度脉动。 在微尺度流动凝结换热研究中，为便于获取凝结过程的动态流动特性，设计了一种低高宽比的单微通道，并以水为工质，对该微通道中的流动凝结换热进行了研究。实验中采取了空气自然对流冷却和 水强制对流冷却两种冷却强度。研究发现，该微通道中的凝结换热呈周期性，其周期在毫秒量级。在通道上游入口处，存在一个呈准静止状态的长汽弹，汽弹前端周期性脱离汽泡。增加冷却强度会使汽泡的脱离频率增大，脱离直径减小；长汽弹前端周期性脱离汽泡是由于汽液界面具有较大的韦伯数。汽泡在该微通道内的运动过程中直径基本不变是由于汽泡在通道内的滞留时间远小于汽泡完全冷凝所需的总时间。 为澄清并联通道的多通道效应对微尺度凝结换热的影响，作者设计了由三个矩形通道组成的并联微通道冷凝器。研究发现，通道中的流型结构与单通道凝结过程类似，均为上游呈准静止状态的长汽弹和下游周期性的汽泡脱离。在中间通道和侧通道中，总共发现了三种不同的汽泡脱离模式，即单汽丝断裂模式、双汽丝同步断裂模式以及双汽丝非同步断裂模式。多通道效应主要表现在由于硅基固体导热的影响，三个通道中具有不同的温度分布，中间通道的温度关于其中心线成对称分布，而两侧通道中的高温区域均靠向中间通道。虽然硅具有良好的导热性，整个硅基上的温差很小，但在微尺度下，小温差依然可以导致较大的温度梯度，造成中间通道的双汽丝关于其中心线成对称分布，并且总是发生同步断裂；侧通道中的双汽丝偏向中间通道，并且在靠近中间通道的一侧汽丝总是首先发生断裂。由于温度梯度引起的Maragnoni对流效应，侧通道中的汽泡脱离后便靠向高温侧。 在微汽泡动力学研究中，设计了一种尺寸为 的Pt薄膜微加热器，研究了脉冲控制参数对微加热器上汽泡动力学特性的影响。研究发现在该微加热器上发生汽泡核化时，核化温度均达到液体的过热极限，因此为均质核化过程。在不同的脉冲控制参数下，存在三类不同的汽泡动力学特性，即（1）汽泡爆炸性生长和冷凝以及汽泡二次生长；（2）汽泡爆炸性生长继而分裂、吸引并聚合；（3）汽泡振荡生长而后持续生长并最终达到稳定状态。在第（1）类中，汽泡二次生长是由于脉冲加热过程中在玻璃基片上储存了热量；在第（2）类中，汽泡冷凝过程中的Marangoni效应导致分裂后的汽泡互相吸引并最终聚合。在第（3）类中，汽泡尺寸最终达到稳定是由于汽泡内蒸汽的发生量与汽液界面上蒸汽的凝结量相等。 本文的研究将为微反应器、微型燃料电池、微换热器、微汽泡执行器等相变微流控系统的设计和运行提供科学指导。

Dynamic interfacial tension of aqueous solutions of PVAAs and its role in liquid-liquid dispersion stabilization

In liquid-liquid dispersion systems, the dynamic change of the interfacial properties between the two immiscible liquids plays an important role in both the emulsification process and emulsion stabilization. In this paper, experimentally measured dynamic interfacial tensions of 1-chlorobutane in the aqueous solutions of various random copolymers of polyvinyl acetate and polyvinyl alcohol (PVAA) are presented. Theoretical analyses on these results suggest that the adsorption of the polymer molecules is controlled neither by the bulk diffusion process nor the activation energy barrier for the adsorption but the conformation of polymer molecules. Based on the concept of critical concentration of condensation for polymer adsorption, as well as the observation that the rate at which the dynamic interfacial tension changes does not correlate to the PVAA's ability to stabilize a single drop, it is postulated that the main stabilization mechanism for the PVAAs is by steric hindrance, not the Gibbs-Marangoni effect offered by the small molecule surfactants.

Physicochemical complexity in complex chemical systems

Self-replication and compartmentalization are two central properties thought to be essential for minimal life, and understanding how such processes interact in the emergence of complex reaction networks is crucial to exploring the development of complexity in chemistry and biology. Autocatalysis can emerge from multiple different mechanisms such as formation of an initiator, template self-replication and physical autocatalysis (where micelles formed from the reaction product solubilize the reactants, leading to higher local concentrations and therefore higher rates). Amphiphiles are also used in artificial life studies to create protocell models such as micelles, vesicles and oil-in-water droplets, and can increase reaction rates by encapsulation of reactants. So far, no template self-replicator exists which is capable of compartmentalization, or transferring this molecular scale phenomenon to micro or macro-scale assemblies. Here a system is demonstrated where an amphiphilic imine catalyses its own formation by joining a non-polar alkyl tail group with a polar carboxylic acid head group to form a template, which was shown to form reverse micelles by Dynamic Light Scattering (DLS). The kinetics of this system were investigated by 1H NMR spectroscopy, showing clearly that a template self-replication mechanism operates, though there was no evidence that the reverse micelles participated in physical autocatalysis. Active oil droplets, composed from a mixture of insoluble organic compounds in an aqueous sub-phase, can undergo processes such as division, self-propulsion and chemotaxis, and are studied as models for minimal cells, or protocells. Although in most cases the Marangoni effect is responsible for the forces on the droplet, the behaviour of the droplet depends heavily on the exact composition. Though theoretical models are able to calculate the forces on a droplet, to model a mixture of oils on an aqueous surface where compounds from the oil phase are dissolving and diffusing through the aqueous phase is beyond current computational capability. The behaviour of a droplet in an aqueous phase can only be discovered through experiment, though it is determined by the droplet's composition. By using an evolutionary algorithm and a liquid handling robot to conduct droplet experiments and decide which compositions to test next, entirely autonomously, the composition of the droplet becomes a chemical genome capable of evolution. The selection is carried out according to a fitness function, which ranks the formulation based on how well it conforms to the chosen fitness criteria (e.g. movement or division). Over successive generations, significant increases in fitness are achieved, and this increase is higher with more components (i.e. greater complexity). Other chemical processes such as chemiluminescence and gelation were investigated in active oil droplets, demonstrating the possibility of controlling chemical reactions by selective droplet fusion. Potential future applications for this might include combinatorial chemistry, or additional fitness goals for the genetic algorithm. Combining the self-replication and the droplet protocells research, it was demonstrated that the presence of the amphiphilic replicator lowers the interfacial tension between droplets of a reaction mixture in organic solution and the alkaline aqueous phase, causing them to divide. Periodic sampling by a liquid handling robot revealed that the extent of droplet fission increased as the reaction progressed, producing more individual protocells with increased self-replication. This demonstrates coupling of the molecular scale phenomenon of template self-replication to a macroscale physicochemical effect.

Double diffusive Marangoni convection flow of electrically conducting fluid in a square cavity with chemical reaction

Double diffusive Marangoni convection flow of viscous incompressible electrically conducting fluid in a square cavity is studied in this paper by taking into consideration of the effect of applied magnetic field in arbitrary direction and the chemical reaction. The governing equations are solved numerically by using alternate direct implicit (ADI) method together with the successive over relaxation (SOR) technique. The flow pattern with the effect of governing parameters, namely the buoyancy ratio W, diffusocapillary ratio w, and the Hartmann number Ha, is investigated. It is revealed from the numerical simulations that the average Nusselt number decreases; whereas the average Sherwood number increases as the orientation of magnetic field is shifted from horizontal to vertical. Moreover, the effect of buoyancy due to species concentration on the flow is stronger than the one due to thermal buoyancy. The increase in diffusocapillary parameter, w caus

The onset of oscillatory Marangoni convection in a two-layer system of conducting fluid in the presence of a uniform magnetic field

A combination of numerical and analytical techniques is used to analyse the effect of magnetic field and encapsulated layer on the onset of oscillatory Marangoni instability in a two layer system. Oscillatory Marangoni instability is possible for a deformed free surface only when the system is heated from above. It is observed that the existence of a second layer has a positive effect on Marangoni overstability with magnetic field whereas it has an opposite effect without magnetic field.

Effect of liquid bridge volume on the instability in small-Prandtl-number half zones

A perturbation method is used to examine the linear instability of thermocapillary convection in a liquid bridge of floating half-zone filled with a small Prandtl number fluid. The influence of liquid bridge volume on critical Marangoni number and flow features is analyzed. The neutral modes show that the instability is mainly caused by the bulk flow that is driven by the nonuniform thermocapillary forces acting on the free surface. The hydrodynamic instability is dominant in the case of small Prandtl number fluid and the first instability mode is a stationary bifurcation. The azimuthal wave number for the most dangerous mode depends on the liquid bridge volume, and is not always two as in the case of a cylindrical liquid bridge with aspect ratio near 0.6. Its value may be equal to unity when the liquid bridge is relatively slender.

Influence Of Free Surface Curvature Of A Liquid Layer On The Critical Marangoni Convection

The Pearson instability was suggested to discuss the onset of Marangoni convection in a liquid layer of large Prandtl number under an applied temperature difference perpendicular to the free surface in the microgravity environment. In this case, the temperature distribution on the curved free surface is nonuniform, and the thermocapillary convection is induced and coupled with the Marangoni convection. In the present paper the effect of volume ratio of the liquid layer on the critical Marangoni convection and the corresponding spatial variation of the convection structure in zero-gravity condition were numerically investigated by two-dimensional model. (C) 2008 Elsevier Ltd. All rights reserved.

Effects of g-jitter on the critical marangoni number

A half floating zone is fixed on a vibrational deck, which supports a periodical applied acceleration to simulate the effect of g-jitter. This paper deals with the effects of g-jitter on the fluid fields and the critical Marangoni number, which describes the transition from a forced oscillation of thermocapillary convection into an instability oscillatory convection in a liquid bridge of half floating zone with top rod heated. The responses of g-jitter field on the temperature profiles and flow pattern in the liquid bridge were obtained experimentally. The results indicated that the critical Marangoni number decreases with the increasing of g-jitter effect and is slightly smaller for higher frequency of g-jitter with fixed strength of applied gravity.

Experimental Study on Bénard-Marangoni Convection in Double Immiscible Liquid Layer

An experimental investigation of Bénard-Marangoni convection has been performed in double immiscible liquid layers of rectangular configuration on the ground. The two kinds of liquid are 10cst silicon oil and FC-70 respectively. The size of rectangular chamber is 100mm×40mm in horizontal cross-section. The evolution processes of convection are observed in the differential thickness ratio of two liquid layers. The critical temperature difference was measured via the detections of fluid convection by a particle image velocimetry (PIV) in the vertical cross-section of the liquid layer. The critical temperature difference or the critical Marangoni number was given. And the influence of the thickness ratio of two liquid layers on the convection instability was discussed. The evolution processes of patterns and temperature distributions on the interface are displayed by using thermal liquid crystal. The velocity distributions on the interface were also obtained. In comparison with the thermocapillary effect, the effect of buoyancy convection will relatively increase when the depth of the liquid layer increases. Because of the coupling of buoyancy and thermocapillary effect, the convection instability is much more complex than that in the microgravity environment. And the critical convection depends on the change of the thickness of liquid layers and also the change of thickness ratio of two liquid layers.

Thermal effects on Rayleigh-Marangoni-Benard instability in a system of superposed fluid and porous layers

Thermal effects of the heat transfer at free surface (represented by Biot number) on the Rayleigh-Marangoni-Benard instability in a system of liquid-porous layers with top free surface are investigated numerically. The results indicate that this thermal effect can evidently lead to the mode transition of convection, which is overlooked in previous works. (C) 2010 Elsevier Ltd. All rights reserved.

Effect of volume ratio on thermocapillary flow in liquid bridges of high-Prandtl-number fluids

In present study, the transition of thermocapillary convection from the axisymmetric stationary flow to oscillatory flow in liquid bridges of 5cst silicon oil (aspect ratio 1.0 and 1.6) is investigated in microgravity conditions by the linear instability analysis. The corresponding marginal instability boundary is closely related to the gas/liquid configuration of the liquid bridge noted as volume ratio. With the increasing volume ratio, the marginal instability boundary consists of the increasing branch and the decreasing branch. A gap region exists between the branches where the critical Marangoni number of the corresponding axisymmetric stationary flow increases drastically. Particularly, a unique axisymmetric oscillatory flow (the critical azimuthal wave number is m=0) in the gap region is reported for the liquid bridge of aspect ratio 1.6. Moreover, the energy transfer between the basic state and the disturbance fields of the thermocapillary convection is analyzed at the corresponding critical Marangoni number, which reveals different major sources of the energy transfer for the development of the disturbances in regimes of the increasing branch, the gap region and the decreasing branch, respectively.

Numerical modelling and validation of Marangoni and surface tension phenomena using the finite volume method

Surface tension induced flow is implemented into a numerical modelling framework and validated for a number of test cases. Finite volume unstructured mesh techniques are used to discretize the mass, momentum and energy conservation equations in three dimensions. An explicit approach is used to include the effect of surface tension forces on the flow profile and final shape of a liquid domain. Validation of this approach is made against both analytical and experimental data. Finally, the method is used to model the wetting balance test for solder alloy material, where model predictions are used to gain a greater insight into this process. Copyright © 2000 John Wiley & Sons, Ltd.

Effect of umbelliferone (7-hydroxycoumarin, 7-HOC) on the enzymatic, edematogenic and necrotic activities of secretory phospholipase A2 (sPLA2) isolated from Crotalus durissus collilineatus venom

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AMINO-ACID-SEQUENCE OF TSTX-V, AN ALPHA-TOXIN FROM TITYUS-SERRULATUS SCORPION-VENOM, AND ITS EFFECT ON K+ PERMEABILITY OF BETA-CELLS FROM ISOLATED RAT ISLETS OF LANGERHANS

Highly purified Tityustoxin V (TsTX-V), an alpha-toxin isolated from the venom of the Brazilian scorpion Tityus serrulatus, was obtained by ion exchange chromatography on carboxymethylcellulose-52. It was shown to be homogeneous by reverse phase high performance liquid chromatography, N-terminal sequencing (first 39 residues) of the reduced and alkylated protein and by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate and tricine. Following enzymatic digestion, the complete amino acid sequence (64 residues) was determined. The sequence showed higher homology with the toxins from the venoms of the North African than with those of the North and South American scorpions. Using the rate of Rb-86(+) release from depolarized rat pancreatic beta-cells as a measure of K+ permeability changes, TsTX-V (5.6 mu g/ml) was found to increase by 2.0-2.4-fold the rate of marker outflow in the presence of 8.3 mM glucose. This effect was persistent and slowly reversible, showing similarity to that induced by 100 mu-M veratridine, an agent that increases the open period of Na+ channels, delaying their inactivation. It is suggested that, by extending the depolarized period, TsTX-V indirectly affects beta-cell voltage-dependent K+ channels, thus increasing K+ permeability.