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

A spatio-temporal study of rheo-oscillations in a sheared lamellar phase using ultrasound

We present an experimental study of the flow dynamics of a lamellar phase sheared in the Couette geometry. High-frequency ultrasonic pulses at 36 MHz are used to measure time-resolved velocity profiles. Oscillations of the viscosity occur in the vicinity of a shear-induced transition between a high-viscosity disordered fluid and a low-viscosity ordered fluid. The phase coexistence shows up as shear bands on the velocity profiles. We show that the dynamics of the rheological data result from two different processes: (i) fluctuations of slip velocities at the two walls and (ii) flow dynamics in the bulk of the lamellar phase. The bulk dynamics are shown to be related to the displacement of the interface between the two differently sheared regions in the gap of the Couette cell. Two different dynamical regimes are investigated under applied shear stress: one of small amplitude oscillations of the viscosity ($\delta\eta/\eta\simeq 3$%) and one of large oscillations ($\delta\eta/\eta\simeq 25$%). A phenomenological model is proposed that may account for the observed spatio-temporal dynamicsComment: 16 pages, 17 figures, submitted to Eur. Phys. J.

Collective diffusion coefficient of a charged colloidal dispersion: interferometric measurements in a drying drop

In the present work, we use Mach-Zehnder interferometry to thoroughly investigate the drying dynamics of a 2D confined drop of a charged colloidal dispersion. This technique makes it possible to measure the colloid concentration field during the drying of the drop at a high accuracy (about 0.5%) and with a high temporal and spatial resolution (about 1 frame/s and 5 $\mu$m/pixel). These features allow us to probe mass transport of the charged dispersion in this out-of-equilibrium situation. In particular, our experiments provide the evidence that mass transport within the drop can be described by a purely diffusive process for some range of parameters for which the buoyancy-driven convection is negligible. We are then able to extract from these experiments the collective diffusion coefficient of the dispersion $D(\varphi)$ over a wide concentration range $\varphi=0.24$-$0.5$, i.e. from the liquid dispersed state to the solid glass regime, with a high accuracy. The measured values of $D(\varphi)\simeq 5$-$12 D_0$ are significantly larger than the simple estimate $D_0$ given by the Stokes-Einstein relation, thus highlighting the important role played by the colloidal interactions in such dispersions

Towards local rheology of emulsions under Couette flow using Dynamic Light Scattering

We present local velocity measurements in emulsions under shear using heterodyne Dynamic Light Scattering. Two emulsions are studied: a dilute system of volume fraction $\phi=20$ % and a concentrated system with $\phi=75$ %. Velocity profiles in both systems clearly show the presence of wall slip. We investigate the evolution of slip velocities as a function of shear stress and discuss the validity of the corrections for wall slip classically used in rheology. Focussing on the bulk flow, we show that the dilute system is Newtonian and that the concentrated emulsion is shear-thinning. In the latter case, the curvature of the velocity profiles is compatible with a shear-thinning exponent of 0.4 consistent with global rheological data. However, even if individual profiles can be accounted for by a power-law fluid (with or without a yield stress), we could not find a fixed set of parameters that would fit the whole range of applied shear rates. Our data thus raise the question of the definition of a global flow curve for such a concentrated system. These results show that local measurements are a crucial complement to standard rheological tools. They are discussed in light of recent works on soft glassy materials.Comment: 13 pages, 21 figures, submitted to Eur. Phys. J.

Observation of Droplet Size Oscillations in a Two-Phase Fluid under Shear Flow

Experimental observations of droplet size sustained oscillations are reported in a two-phase flow between a lamellar and a sponge phase. Under shear flow, this system presents two different steady states made of monodisperse multilamellar droplets, separated by a shear-thinning transition. At low and high shear rates, the droplet size results from a balance between surface tension and viscous stress whereas for intermediate shear rates, it becomes a periodic function of time. A possible mechanism for such kind of oscillations is discussed

Séchage de fluides complexes en géométrie confinée

Dans ce travail de thèse, nous avons développé deux méthodologies permettant d'acquérir rapidement et facilement des propriétés physico-chimiques, cinétiques et thermodynamiques de fluides complexes. Nous nous sommes focalisés sur le rôle de la concentration sur ces propriétés. Les deux méthodes développées sont basées sur la concentration en continu d'une solution aqueuse par évaporation contrôlée du solvant. Le premier outil est une goutte de quelques microlitres confinée entre deux plaques dont la hauteur est de 100 m. Dans cette géométrie à deux dimensions, l'évaporation est entièrement décrite par un modèle que nous avons développé. L'observation du séchage de la goutte couplée à des mesures locales de concentration par spectroscopie Raman, permet d'accéder quantitativement au diagramme de phase d'une solution de copolymères, et de mesurer l'activité ainsi que d'estimer le coefficient d'interdiffusion de la solution. Le second outil est une puce microfluidique permettant de concentrer des solutions aqueuses grâce à la pervaporation de l'eau à travers une membrane. Cet outil permet avec quelques microgrammes de soluté, de bâtir un gradient de concentration stationnaire le long d'un microcanal. Les techniques de spectroscopie Raman et de diffusion des rayons X aux petits angles permettent à nouveau de mesurer des propriétés physico-chimiques de la solution mais également de mettre en évidence le caractère discontinu du coefficient d'interdiffusion en fonction de la concentration, dépendant des mésophases présentes.In this work, we developed two methods in order to access rapidly and easily physico-chemical, thermodynamic and kinetic properties of complex fluids. We focused on the role of the concentration on these properties. The two methods that we developed are based on the continuous concentration of an aqueous solution thanks to the evaporation of the solvent. The first tool is a microliter droplet confined between two circular plates with a cell height of about 100 m. Within this two dimensional cylindrical geometry, the evaporation of the droplet is totally described by a model that we developed. The observation of the droplet evaporation combined to local Raman spectroscopy measurements permits us to build a quantitative phase diagram, to measure the activity of the solution and to estimate its mutual diffusion coefficient. The second tool is a microfluidic chip in which water is removed through a thin membrane. This device permits us to build with a few micrograms of solutes a stationary concentration gradient along a microchannel. Raman confocal spectroscopy and small angle X-ray scattering give access to the quantitative phase diagram and also permit to evidence that the mutual diffusion coefficient is discontinuous at some of the phase boundaries.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Velocity profiles in shear-banding wormlike micelles

Using Dynamic Light Scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize on the complex, non-Newtonian nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let