Konstantin Kostarev

POLYMERIZATION PROCESSES OF EPOXY PLASTIC IN SIMULATED FREE SPACE CONDITIONS

Abstract This paper presents the results of laboratory modeling of polymerization under the action of free-space factors. The relationships between temperature and stoichiometric ratios were obtained for different sublimation parameters of the reaction mixture components and rates of matrix polymerization. It was shown that there exists an optimal relation between the rates of polymerization and sublimation of low-molecular mixture components. The possibility of using low-molecular components with high sublimation rates under certain conditions of reaction development was experimentally verified. Two options for triggering the reaction have been tested—raising the temperature to the value at which polymerization begins and by means of sunlight photoinitiation (ultraviolet spectrum). It was shown that both variants can be used in the conditions of free space.

Thermal and Concentrational Maragoni Convection at Liquid/Air Bubble Interface

The paper presents the experimental study of thermo- and solutocapillary Marangoni convection around a gas bubble in an inhomogeneous fluid with a vertical thermal or surfactant concentration gradient. The stationary bubble in the form of a short horizontal cylinder with a free lateral surface was placed into a vertically oriented thin liquid layer (Hele-Shaw cell). The evolution of thermal and concentration fields and fluid flows was studied applying the interferometric method. In contrast to a thermocapillary convection representing a stationary flow and stable temperature distribution, the periodic concentration disturbances around the bubble were observed in the solutocapillary case. The regularities of the discovered effect were revealed, and its interpretation was proposed.

The development of Marangoni convection induced by local addition of a surfactant

The development of concentration convection induced by local addition of a surfactant solution onto a horizontal free surface of water is studied experimentally and theoretically. The experiment revealed that the capillary motion develops in a threshold manner, with the threshold value depending on the degree of purification of the fluid, the initial concentration of the surfactant, and the area of the free surface. To describe the threshold mechanism of the concentration convection, a number of theoretical models is considered. Different rheological properties of the surface phase, including the nonlinear dependence of the surface shear stress on the surface velocity, are examined. In the numerical experiment, the convective-flow patterns are calculated for different free-surface boundary conditions, and the time dependence of the flow intensity is investigated.

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.

Flow development at the surfaces of bubbles and droplets in gradient solutions of a surfase-active liquid

The development of solutocapillary flows at the surfaces of air bubbles and chlorobenzene droplets was experimentally studied in nonuniform aqueous solutions of ethanol and isopropanol, which have a low surface tension and, hence, exhibit surface-active properties with respect to water. The experiments demonstrated the retardation of the onset of the development of the Marangoni concentration-induced convection relative to the moment of the contact between an inflowing surfactant (alcohol) and the surface. The critical concentration gradients (the Marangoni diffusion numbers) necessary for the initiation of mass transfer of a liquid along the interface were determined as dependent on the rate of inflow of a tongue of a more concentrated solution and the initial alcohol concentration around the bubble.

Development of concentration-capillary convection on an interfacial surface

The onset of Marangoni convection initiated by introducing a surfactant solution drop onto a horizontal liquid interface is studied experimentally and theoretically. It is revealed that the film of admixtures adsorbed by this surface from the bulk of the contacting liquids begins to move only as a certain shear stress is reached, that is, the capillary flow development is of threshold nature. If compared with Marangoni convection development on the free liquid surface, the threshold shear stress may turn out to be either lower or higher, depending on the surfactant admixture concentration in both liquids and the pattern of changes in the surface tension with approaching the interfacial surface. In order to describe the threshold development of concentration convection, a nonlinear dependence of the shear stress at the liquid surface on the liquid velocity is proposed. In a numerical experiment, the convective flow pattern on the interface is determined and the time dependence of the flow intensity is obtained.

Shock-wave-like structures induced by an exothermic neutralization reaction in miscible fluids

We report shock-wave-like structures that are strikingly different from previously observed fingering instabilities, which occur in a two-layer system of miscible fluids reacting by a second-order reaction A+B→S in a vertical Hele-Shaw cell. While the traditional analysis expects the occurrence of a diffusion-controlled convection, we show both experimentally and theoretically that the exothermic neutralization reaction can also trigger a wave with a perfectly planar front and nearly discontinuous change in density across the front. This wave propagates fast compared with the characteristic diffusion times and separates the motionless fluid and the area with anomalously intense convective mixing. We explain its mechanism and introduce a new dimensionless parameter, which allows to predict the appearance of such a pattern in other systems. Moreover, we show that our governing equations, taken in the inviscid limit, are formally analogous to well-known shallow-water equations and adiabatic gas flow equations. Based on this analogy, we define the critical velocity for the onset of the shock wave which is found to be in the perfect agreement with the experiments.

Convective instability in a two-layer system of reacting fluids with concentration-dependent diffusion

The development of convective instability in a two-layer system of miscible fluids placed in a narrow vertical gap has been studied theoretically and experimentally. The upper and lower layers are formed with aqueous solutions of acid and base, respectively. When the layers are brought into contact, the frontal neutralization reaction begins. We have found experimentally a new type of convective instability, which is characterized by the spatial localization and the periodicity of the structure observed for the first time in the miscible systems. We have tested a number of different acid–base systems and have found a similar patterning there. In our opinion, it may indicate that the discovered effect is of a general nature and should be taken into account in reaction–diffusion–convection problems as another tool with which the reaction can govern the movement of the reacting fluids. We have shown that, at least in one case (aqueous solutions of nitric acid and sodium hydroxide), a new type of instability called as the concentration-dependent diffusion convection is responsible for the onset of the fluid flow. It arises when the diffusion coefficients of species are different and depend on their concentrations. This type of instability can be attributed to a variety of double-diffusion convection. A mathematical model of the new phenomenon has been developed using the system of reaction–diffusion–convection equations written in the Hele–Shaw approximation. It is shown that the instability can be reproduced in the numerical experiment if only one takes into account the concentration dependence of the diffusion coefficients of the reagents. The dynamics of the base state, its linear stability and nonlinear development of the instability are presented. It is also shown that by varying the concentration of acid in the upper layer one can achieve the occurrence of chemo-convective solitary cell in the bulk of an almost immobile fluid. Good agreement between the experimental data and the results of numerical simulations is observed.

Surfactant Transfer in Multiphase Liquid Systems under Conditions of Weak Gravitational Convection

To day, investigation of mass transfer between a drop and a surrounding medium is one of the most promising and at the same time the most complicated fields of research. The relevance of these investigations should be attributed to the fact that drops participate in many technological processes including extraction, mixing, dissolution, etc. The complexity of studies is caused by small dimensions and spherical shape of the drops which severely restricts the possibilities of experimental investigation (Henschke & Pfennig , 1999; Agble & Mendes-Tatsis, 2000; Kosvintsev & Reshetnikov, 2001; Amar et al., 2005; Waheed et al, 2002). As a result the majority of studies into the process of mass transfer between a drop and an ambient fluid cover theoretical aspects of the problem and are based on different simplifications and assumptions on the character and the level of interaction between mass transfer mechanisms (Myshkis. et al, 1987; Subramanian et al., 2001; Bratukhin et al., 2001). Therefore the results of these studies require experimental verification especially in the case of surfactant diffusion, which can locally change the surface tension at the interface and initiate the solutocapillary motion even in initially homogeneous solutions (Wegener et al., 2007, 2009a, 2009b; Burghoff & Kenig, 2006). The character of interaction between buoyancy and capillary convections generally strongly depends on the gravity level. At most times it is suggested that the buoyancy plays the main role under normal gravity and that the capillary forces dominate in microgravity conditions. However at normal gravity conditions the free convection motion provides conditions for recovering the percentage composition of the liquid mixtures near the interface. With reduction in the gravity level the outflow of both the diffusing component and depleted mixture decreases. Lowering of the concentration gradient normal to the interface reduces the probability of formation of the concentration inhomogeneities along this boundary. Therefore the possibility for the occurrence of solutocapillary motion during mass transfer processes under microgravity conditions is still an open question. In the autumn of 2007 the space experiment Diffusion of a surfactant from a drop was carried out during the orbital flight of the spacecraft Foton-M3 (Kostarev et al., 2010). The performed experiment was aimed at studying the development of the capillary convection

Oscillatory Regimes of Solutocapillary Marangoni Convection

It is common knowledge that a fluid motion can be initiated both by the volume (buoyancy) forces, due to density variations in presence of a gravity field, and the surface (capillary) forces, due to variations of surface tension along a fluid/fluid interface. Involving in motion the surface and the near-surface layers the capillary forces gives rise to a volumetric flow generally known as the Marangoni convection. In turn, inhomogeneity of the surface tension may result from its dependence on temperature or concentration of the dissolved surfacetension active (surfactant) component. Such convection, thermocapillary or solutocapillary one, respectively, plays an important role in hydrodynamics and heat/mass transfer of inhomogeneous multi-phase media with the liquid/liquid interface or the free surface between liquid and gas. The Marangoni convection has a marked effect on the intensity of many technological processes encountered in food, chemical, oil, metallurgical and other industries, including those proceeding in microgravity conditions where the gravitational mechanisms of convective motion are weakened or absent. Particular interest on research in this field has quickened in recent years due to new achievements in the development of space technologies and life-support and survival systems for orbital stations. Marangoni phenomena is a problem of great application value for such areas as ecology (purification of water surface from petroleum products), meteorology, biology (motion of bacteria and microbodies), medicine (spreading of pulmonic surfactants during aerosol inhalation in treatment of lung diseases) and so on. Despite the global abundance of the Marangoni convection the direct experimental investigation of the Marangoni flows in the so-called pure form is a challenging task. The reason is that under normal terrestrial conditions the existence of temperature and concentration gradients in a fluid also leads to development of the gravitational Rayleigh convection with considerably higher intensity than that of the Marangoni convection. Hence the latter proves to be almost suppressed by a more severe convective driving mechanism or veiled by its action. At the same time, under certain conditions, the surface forces may dominate over the buoyancy ones. Such conditions are realized, for example, in shallow layers and fluid films, liquid bridges and zones confined between solid surfaces, and also for small insoluble drops or gas bubbles suspended in liquids. In the past decades, the Marangoni convection in such systems has been the subject of intensive investigation. However, most of the studies are concerned with thermocapillary variant of the Marangoni