Humidity-insensitive water evaporation from molecular complex fluids

We investigated theoretically water evaporation from concentrated supramolecular mixtures, such as solutions of polymers or amphiphilic molecules, using numerical resolutions of a one dimensional model based on mass transport equations. Solvent evaporation leads to the formation of a concentrated solute layer at the drying interface, which slows down evaporation in a long-time scale regime. In this regime, often referred to as the falling rate period, evaporation is dominated by diffusive mass transport within the solution, as already known. However, we demonstrate that, in this regime, the rate of evaporation does not also depend on the ambient humidity for many molecular complex fluids. Using analytical solutions in some limiting cases, we first demonstrate that a sharp decrease of the water chemical activity at high solute concentration, leads to evaporation rates which depend weakly on the humidity, as the solute concentration at the drying interface slightly depends on the humidity. However, we also show that a strong decrease of the mutual diffusion coefficient of the solution enhances considerably this effect, leading to nearly independent evaporation rates over a wide range of humidity. The decrease of the mutual diffusion coefficient indeed induces strong concentration gradients at the drying interface, which shield the concentration profiles from humidity variations, except in a very thin region close to the drying interface.Comment: 13 pages, 10 figure

Coefficients d'échange thermique superficiel : détermination par méthodes inverses en conduction bidimensionnelle transitoire

This work concerns the cooling by compressed air flow of the plungers used in the glass forming processes. The bad control of the plunger temperature leads to an increase of the fabrication default. Thus the objective of the study is to develop a thermal model which will allow the calculation of the plunger temperature field and which-will be used to optimize the cooling system configuration. In a first stage, the temperature field in the plunger/glass/mold system is determined with a on dimensional model. The radiation in the glass is treated with the discrete ordinate method and the model takes into account the different steps of forming cycle. This simplified model is used tio better understand the dehavior of the heat transfer in the plunger/glass/mold system and to determine the parameter that must be known accurately. The boundary conditions at the plunger/glass interface and at the inner surface cooled by air must be known accuraltely ti determine the plunger temperature. Two experimental set-up have been realized to determine by inverse heat conduction methods thes unknown boundary condition. The heat transfer coefficient between the plunger and the galss and between the plunger and the air are shown and examined for different experimental situations. The two dimensional thermal model solved with the finite elements technique is then described. Based on the boundary conditions determined experimentally, an example of the plunger temperature field during a glass forming cycle is given.Ce travail traite du refroidissement par air comprimé des poinçons utilisés pour le formage d'ébauches en verre. Une mauvaise maîtrise de la température de ces poinçons entraînant une augmentation des défauts de fabrication, le but est de développer un modèle thermique permettant de prédire leur température, afin d'améliorer la conception du système de refroidissement.Dans une première étape, le champ de température dans le système poinçon/verre/moule est calculé grâce à un modèle monodimensionnel, reproduisant les différentes étapes d'un cycle de formage. Ce modèle est utilisé dans le but de mieux comprendre la nature des transferts de chaleur dans le système poinçon/verre/moule, et pour préciser les hypothèses du modèle destiné à l'optimisation du système de refroidissement du poinçon.La modélisation fine du poinçon nécessitant la connaissance des conditions aux limites aux surfaces soumises au contact avec le verre d'une part, et à l'action de la ventilation d'autre part, ces dernières sont obtenues expérimentalement, en utilisant les méthodes inverses en conduction. Les valeurs du coefficient d'échange entre le poinçon et le verre, et entre le poinçon et l'air comprimé sont présentées et discutées, pour différentes conditions expérimentales.Pour finir, le modèle bidimensionnel de simulation du poinçon, basé sur la résolution de l'équation de la conduction en éléments finis, et utilisant les conditions aux limites déterminées expérimentalement est décrit, et un exemple de simulation est détaillé

Reactive Objects

In the reactive approach, system components are not supposed to execute at their own rate, but are instead driven by a logical common clock, defining global instants. The Reactive Object Model introduced in this paper, is an object based formalism matching the reactive paradigm. In this model, methods can be invoked using instantaneous non-blocking send orders, which are immediately processed (that is, processed during the current instant); moreover, a method cannot execute more than once at each instant. The Reactive Object Model is described and compared to the Actor Model; then a prototype language based on this model is introduced; finally its expressive power is shown on the example of a broadcast communication mechanism

Développement de cellules convectives lors du séchage de solutions polymères

Dans le cadre des travaux développés à l'heure actuelle sur les phénomènes d'instabilités couplés à l'évaporation, une étude expérimentale de séchage d'une solution polymère/solvant a été réalisée. Nous nous intéressons ici au premier régime qui apparaît en début de séchage, lorsque des mouvements convectifs se développent dans la solution dont la viscosité est encore relativement faible. Plusieurs types de visualisations ont été réalisées pour caractériser les structures convectives: visualisation en vue de dessus par caméra IR et par caméra vidéo, PIV dans une section verticale. Nous présentons une analyse des structures convectives obtenues, leur origine et leur domaine d'existence en fonction de l'épaisseur et de la viscosité initiale

Numerical simulation of an evaporative meniscus on a moving substrate

A hydrodynamic model based on lubrication theory has been developed to describe an evaporative meniscus in a complete wetting configuration, when evaporation takes place in ambient air. We focus on combined effects of evaporation and the substrate motion on the effective contact angle. Numerical simulations show two distinct regimes when varying the substrate velocity on several orders of magnitude. At a small velocity, the effective contact angle is governed by evaporation and is independent of the substrate velocity, while the substrate motion is dominant at a high velocity. In the latter case, a Landau-Levich regime is obtained for the receding contact line, and a Cox-Voinov regime for the advancing contact line. Finally, we use our numerical results to test the simplified model developed by Pham et al. [5,6]

Numerical simulation of complex fluid drying in a Hele-Shaw cell

A 2D model is currently under development to describe the flow and concentration fields inside a Hele-Shaw cell with evaporation for a receding meniscus (dip-coating-like configuration). An original approach is proposed to address the difficult problem of the boundary conditions close to the contact line. The model is used to study the deposit thickness as a function of some process parameters

Numerical simulation of dip-coating in the evaporative regime

International audienceA hydrodynamic model is used for numerical simulations of a polymer solution in a dip-coating-like experiment. We focus on the regime of small capillary numbers where the liquid flow is driven by evaporation, in contrast to the well-known Landau-Levich regime dominated by viscous forces. Lubrication approximation is used to describe the flow in the liquid phase. Evaporation in stagnant air is considered (diffusion-limited evaporation), which results in a coupling between liquid and gas phases. Self-patterning due to the solutal Marangoni effect is observed for some ranges of the control parameters. We first investigate the effect of evaporation rate on the deposit morphology. Then the role of the spatial variations in the evaporative flux on the wavelength and mean thickness of the dried deposit is ascertained, by comparing the 2D and 1D diffusion models for the gas phase. Finally, for the very low substrate velocities, we discuss the relative importance of diffusive and advective components of the polymer flux, and consequences on the choice of the boundary conditions

Role of solutal free convection on interdiffusion in a horizontal microfluidic channel

International audienceWe theoretically investigate the role of solutal free convection on the diffusion of a buoyant solute at themicrofluidic scales, 5–500 µm. We first consider a horizontal microfluidic slit, one half of which initiallyfilled with a binary solution (solute and solvent), and the other half with pure solvent. The buoyant forcesgenerate a gravity current that couples to the diffusion of the solute. We perform numerical resolutions ofthe 2D model describing the transport of the solute in the slit. This study allows us to highlight differentregimes as a function of a single parameter, the Rayleigh number Ra which compares gravity-inducedadvection to solute diffusion. We then derive asymptotic analytical solutions to quantify the width of themixing zone as a function of time in each regime and establish a diagram that makes it possible to identifythe range of Ra and times for which buoyancy does not impact diffusion. In a second step, we presentnumerical resolutions of the same model but for a 3D microfluidic channel with a square cross-section. Weobserve the same regimes as in the 2D case, and focus on the dispersion regime at long time scales. We thenderive the expression of the 1D dispersion coefficient for a channel with a rectangular section, and analysethe role of the transverse flow in the particular case of a square section. Finally, we show that the impactof this transverse flow on the solute transport can be neglected for most of the microfluidic experimentalconfigurations

Modeling flow coating of colloidal dispersions in the evaporative regime: prediction of deposit thickness

International audienceWe investigate flow coating processes, i.e., the formation of dry coatings starting from dilute complex fluids confined between a static blade and a moving substrate. In particular, we focus on the evaporative regime encountered at low substrate velocity, at which the coating flow is driven mainly by solvent evaporation in the liquid meniscus. In this regime, general arguments based on mass conservation show that the thickness of the dry film decreases as the substrate velocity increases, unlike the behavior in the well-known Landau–Levich regime. This work focuses on colloidal dispersions, which deserve special attention. Indeed, flow coating is expected to draw first a solvent-saturated film of densely packed colloids, which further dries fully when air invades the pores of the solid film. We first develop a model based on the transport equations for binary mixtures, which can describe this phenomenon continuously, using appropriate boundary conditions and a criterion to take into account pore-emptying in the colloidal film. Extensive numerical simulations of the model then demonstrate two regimes for the deposit thickness as a function of the process parameters (substrate velocity, evaporation rate, bulk concentration, and particle size). We finally derive an analytical model based on simplified transport equations that can reproduce the output of our numerical simulations very well. This model can predict analytically the two observed asymptotic regimes and therefore unifies the models recently reported in the literature