The study presented in this dissertation concerns the evaporation, in normal ambient conditions, of sessile droplets (pinned and freely receding) of various HFE liquids (instead of the widely used water), which are considered so far as environmentally friendly and are often used as heat-transfer fluids in thermal management applications. They are pure perfectly-wetting and volatile liquids with low thermal conductivity and high vapor density. These properties affect in their own way many aspects concerning droplet evaporation such as the evaporation-induced contact angles, evaporation rate of a droplet, contact line pinning and Marangoni flow, all of which are treated in the present dissertation.<p>In general, the thesis starts with a general introduction including but not limited to sessile droplets (Chapter 1). In Chapter 2 we provide a general overview of capillarity-related concepts. Then, in Chapter 3 we present the interferometric setup, along with the liquids and the substrate that is used in the experiments, and also explain the reasons why this particular method is chosen. In Chapter 4 we address, among others, the issue of evaporation-induced contact angles under complete wetting conditions. The behavior of the global evaporation rate is also examined here, whereas in Chapter 5 we discuss the influence of thermocapillary stresses on the shape of strongly evaporating droplets. Finally, before concluding in Chapter 7, we address in Chapter 6 the still open question of the influence of non-equilibrium effects, such as evaporation, on the contact-line pinning at a sharp edge, a phenomenon usually described in the framework of equilibrium thermodynamics. The experimental results obtained are also compared with the predictions of existing theoretical models giving rise to interesting conclusions and promising perspectives for future research.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

A numerical code for solving the time-dependent incompressible 3D Navier-Stokes equations with finite volumes on overlapping staggered grids in cylindrical and rectangular geometry is developed. In the code, written in FORTRAN, the momentum equation for the velocity is solved by projection method and Poisson equation for the pressure is solved by ADI implicit method in two directions combined with discrete fast Fourier transform in the third direction. A special technique for overcoming the singularity on the cylinder's axis is developed. This code, taking into account dependence upon temperature of the viscosity, density and surface tension of the liquid, is used to study the fluid motion in a cylinder with free cylindrical surface (under normal and zero-gravity conditions); and in a rectangular closed cell with a source of thermocapillary convection (bubble inside attached to one of the cell's faces). They are significant problems in crystal growth and in general experiments in fluid dynamics respectively. Nevertheless, the main study is dedicated to the liquid bridge problem.<p><p>The development of thermocapillary convection inside a cylindrical liquid bridge is investigated by using a direct numerical simulation of the 3D, time-dependent problem for a wide range of Prandtl numbers, Pr = 0.01 - 108. For Pr > 0.08 (e.g. silicon oils), above the critical value of temperature difference between the supporting disks, two counter propagating hydrothermal waves bifurcate from the 2D steady state. The existence of standing and traveling waves is discussed. The dependence of viscosity upon temperature is taken into account. For Pr = 4, 0-g conditions, and for Pr = 18.8, 1-g case with unit aspect ratio an investigation of the onset of chaos was numerically carried out. <p><p>For a Pr = 108 liquid bridge under terrestrial conditions ,the appearance and the development of thermoconvective oscillatory flows were investigated for different ambient conditions around the free surface.<p><p>Transition from 2D thermoconvective steady flow to a 3D flow is considered for low-Prandtl fluids (Pr = 0.01) in a liquid bridge with a non-cylindrical free surface. For Pr < 0.08 (e.g. liquid metals), in supercritical region of parameters 3D but non-oscillatory convective flow is observed. The computer program developed for this simulation transforms the original non-rectangular physical domain into a rectangular computational domain.<p><p>A study of how presence of a bubble in experimental rectangular cell influences the convective flow when carrying out microgravity experiments. As a model, a real experiment called TRAMP is numerically simulated. The obtained results were very different from what was expected. First, because of residual gravity taking place on board any spacecraft; second, due to presence of a bubble having appeared on the experimental cell's wall. Real data obtained from experimental observations were taken for the calculations.<p> / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

Motivated by the existence of complex behaviors arising from interactions between chemistry and fluid dynamics in numerous research problems and every-day life situations, we theoretically investigate the dynamics resulting from the interplay between chemistry, diffusion, and fluid motions in a reactive aqueous solution. As a chemical reaction induces changes in the temperature and in the composition of the reactive medium, such a reaction can modify the properties of the solution (density, viscosity, surface tension,…) and thereby trigger convective motions, which in turn affect the reaction. Two classes of convective flows are commonly occurring in solutions open to air, namely Marangoni flows arising from surface tension gradients and buoyancy flows driven by density gradients. As both flows can be induced by compositional changes as well as thermal changes and in turn modify them, the resulting experimental dynamics are often complex. The purpose of our thesis is to gain insight into these intricate dynamics thanks to the theoretical analysis of model systems where only one type of convective flow is present. In particular, we numerically study the spatio-temporal evolution of model chemical fronts resulting from the coupling between reactions, diffusion, and convection. Such fronts correspond to self-organized interfaces between the products and the reactants, which typically have different density and surface tension. Fluid motions are therefore spontaneously induced due to these differences across the front.<p><p>In this context, we first address the propagation of a model autocatalytic front in a horizontal solution layer, in the presence of pure Marangoni convection on the one hand and of pure buoyancy convection on the other hand. We evidence that, in both cases, the system attains an asymptotic dynamics characterized by a steady fluid vortex traveling with the front at a constant speed. The presence of convection results in a deformation and acceleration of the chemical front compared to the reaction-diffusion situation. However we note important differences between the Marangoni and buoyancy cases that could help differentiate experimentally between the influence of each hydrodynamic effect arising in solutions open to the air. We also consider how the kinetics and the exothermicity of the reaction influence the dynamics of the system. The propagation of an isothermal front occurring when two diffusive reactants are initially separated and react according to a simple bimolecular reaction is next studied in the presence of chemically-induced buoyancy convection. We show that the reaction-diffusion predictions established for convection-free systems are modified in the presence of fluid motions and propose a new way to classify the various possible reaction-diffusion-convection dynamics./En induisant des changements de composition et de température, une réaction chimique peut modifier les propriétés physiques (densité, viscosité, tension superficielle,…) de la solution dans laquelle elle se déroule et ainsi générer des mouvements de convection qui, à leur tour, peuvent affecter la réaction. Les deux sources de convection les plus courantes en solution ouverte à l’air sont les gradients de tension superficielle, ou effets Marangoni, et les gradients de densité. Comme ces deux sources sont en compétition et peuvent toutes deux résulter de différences de concentration ou de température, les dynamiques observées expérimentalement sont souvent complexes. Le but de notre thèse est de contribuer à la compréhension de telles dynamiques par une étude théorique analysant des modèles réaction-diffusion-convection simples. En particulier, nous étudions numériquement l’évolution spatio-temporelle de fronts chimiques résultant du couplage entre chimie non-linéaire, diffusion et hydrodynamique. Ces fronts constituent l’interface auto-organisée entre les produits et les réactifs qui typiquement ont des densités et tensions superficielles différentes. Des mouvements du fluide peuvent dès lors être spontanément initiés dus à ces différences au travers du front.<p> <p>Dans ce contexte, nous étudions la propagation d’un front chimique autocatalytique se propageant dans une solution aqueuse horizontale, d’une part en la seule présence d’effets Marangoni, et d’autre part en présence uniquement d’effets de densité. Nous avons montré que dans les deux cas, le système atteint une dynamique asymptotique caractérisée par la présence d’un rouleau de convection stationnaire se propageant à vitesse constante avec le front. Ce front est à la fois déformé et accéléré par les mouvements convectifs par rapport à la situation réaction-diffusion. Nous avons mis en évidence d’importantes différences entre les deux régimes hydrodynamiques qui pourraient aider les expérimentateurs à différencier les effets de tension superficielle de ceux de densité générés par la propagation de fronts chimiques en solution. Nous avons également considéré l’influence de la cinétique de réaction ainsi que de l’exothermicité sur la dynamique de ces fronts. Enfin, nous avons étudié la propagation en présence de convection d’un front de réaction impliquant deux espèces de densités différentes, initialement séparées et réagissant selon une cinétique bimoléculaire. Nous avons montré que la convection modifie les propriétés réaction-diffusion du système et nous proposons de nouveaux critères pour classifier les dynamiques réaction-diffusion-convection.<p><p><p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished