Surface Fluctuations of an Aging Colloidal Suspension: Evidence for Intermittent Quakes

We present measurements of the thermal fluctuations of the free surface of an aging colloidal suspension, Laponite. The technique consists in measuring the fluctuations of the position of a laser beam that reflects from the free surface. Analysing the data statistics, we show that, as the fluid ages, the dynamics becomes intermittent. The intermittent events correspond to large changes in the local slope of the free surface over a few milliseconds. We show that those quakes are uncorrelated, although they are kept in memory by the surface over short time scales

Etude de la sédimentation de particules non-browniennes dans un fluide thixotrope à seuil (la laponite)

La sédimentation de particules non-browniennes dans une suspension de laponite a été étudiée expérimentalement. Un protocole de préparation spécifique a été mis en place, la caractérisation rhéologique du fluide a été réalisée : la laponite constitue un fluide thixotrope, à seuil, rhéofluidifiant et viscoélastique. La caractéristique principale est l'augmentation exponentielle de la viscosité du fluide avec le temps de vieillissement. La sédimentation de particules isolées a été étudiée sous différents aspects : vitesse de la particule, champs de vitesse autour de la particule, biréfringence induite par la sédimentation. La vitesse de la particule évolue en exponentielle décroissante avec le temps de vieillissement. Deux régimes d'écoulement suivant la contrainte exercée par la particule ont été repérés : un écoulement avec une traînée négative (remontée du fluide après le passage de la particule) ou avec une évolution spatiale quasi-similaire entre l'amont et l'aval. La biréfringence induite par la sédimentation de la particule relaxe très lentement après le passage de la particule. Le temps de relaxation correspondant augmente avec le temps de vieillissement du fluide. La sédimentation de deux particules a été observée dans deux cas : en présence et en absence d'une traînée négative. L'agrégation des particules peut être observée dans chaque cas. De plus, la sédimentation de suspensions a été entreprise : on observe une sédimentation sur une longueur finie conduisant à l'arrêt de la sédimentation contrairement au ca newtonien. La dynamique de cet arrêt est en exponentielle décroissante.The settling of non-Bownian spheres in a laponite suspension has been studied experimentally. A specific preparation has been set up. The rheology of the fluid has been characterized~: a suspension of laponite constitutes a thixotropic, yield stress, shear thinning, viscoelastic fluid. The main characteristic is that the viscosity increases exponentially with the aging time. The settling of single spheres has been studied under different aspects : the velocity of the sphere, the flow field around the sphere and the birefringence induced by the settling sphere. The velocity of the sphere slows down exponentially with aging time. Two flow field patterns have been observed : with a negative wake (an upward motion in the wake of the sphere) or with a near fore-and-aft symmetry. The settling-induced birefringence relaxes slowly in the wake of the sphere. The characteristic relaxation time increases with the aging time exponentially. The settling of a couple of spheres has been observed in two cases : with and without a negative wake. Spheres may cluster in each case. A first experiment of a settling suspension has been made (a single size of small spheres). A settling on finite length has been observed with a kinetic described with an exponential.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Les fluctuations de surface pour mesurer les propriétés de systèmes complexes en l'absence de sollicitation

La dynamique des fluctuations thermiques d un milieu révèle les propriétés de ce milieu, sans qu il soit nécessaire de le solliciter. Afin d exploiter ce principe, nous avons mis au point une technique optique destinée à mesurer des fluctuations spontanées de hauteur de surfaces libres, basée sur la mesure de la déflexion d un laser réfléchi à la surface. Nous montrons que l on peut ainsi mesurer les fluctuations de surface de milieux très variés, allant des liquides peu visqueux aux solides viscoélastiques. Les propriétés viscoélastiques du milieu sondé peuvent être déterminées à partir du spectre expérimental des fluctuations. On compare les valeurs obtenues avec des mesures rhéométriques, la technique s avère être un moyen fiable pour caractériser les propriétés rhéologiques dans une gamme de fréquences allant de 0,1 Hz à quelques dizaines de kHz. Par ailleurs, on s intéresse à l influence du confinement sur les fluctuations de surface d un liquide newtonien. On étudie l effet du confinement en utilisant des substrats de différentes natures : solide plan, solide en relief et liquide. On montre que les fluctuations de surface dépendent fortement du substrat utilisé. On étudie en particulier le cas où le film liquide est déposé sur une surface solide plane. En créant un effet Marangoni induit par un gradient de température, on parvient à contrôler l épaisseur du film sondé. On mesure ainsi les fluctuations de surface de films dont l épaisseur varie entre 30 nanomètres et quelques micromètres. Les spectres mesurés sont sensibles à la condition hydrodynamique à l interface liquide-solide et permettent d évaluer une éventuelle longueur de glissement.The dynamics of thermal fluctuations of a medium reveals the properties of this medium, without the need to apply any stress. In order to exploit this principle, we have developed an optical technique for measuring spontaneous free surface height fluctuations, based on the measurement of the deflection of a laser reflected at the surface. We show that we can measure surface fluctuations of various materials, ranging from low viscosity liquids to viscoelastic solids. The viscoelastic properties of the medium probed can be determined from the experimental fluctuations spectrum. Comparing the data obtained with rheometric measurements, the technique turns out to be a reliable way to characterize the rheological properties of frequencies ranging from 0.1 Hz to tens of kHz. In addition, we are interested in the influence of confinement on the surface fluctuations of a newtonian liquid. We study the effect of confinement using substrates of different types : solid plane, solid with a relief and liquid. We show that the surface fluctuations strongly depend on the substrate used. We study in particular the case where a liquid film is deposited onto a solid surface. By creating a Marangoni effect induced by a temperature gradient, it is possible to control the thickness of the probed film. We can therefore measure surface fluctuations of films whose thickness varies between 30 nanometers and a few micrometers. The measured spectra are sensitive to the hydrodynamic condition at the liquid-solid interface and makes it possible to estimate a possible slip length.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Rapid settling of a colloidal gel

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Control of particle morphology in the spray drying of colloidal suspensions

International audiencePowders of nanoparticles are volatile, i.e. easily disperse in air, which makes their handling difficult. Granulation of nanoparticle powders provides a solution to that issue, and it is generally performed by spray drying the nanoparticles that have been suspended in a liquid. Spray drying of a colloidal suspension consists of atomising the suspension into droplets by a fast flowing and hot gas. Once the droplets dried, the resulting dry grains/microparticles can be used in a wide range of applications – food, pharmaceutics, fillers, ceramics, etc. It is well known that the grains resulting from spray-drying may be spherical but may also exhibit other diverse morphologies. Although different influencing parameters have been identified, no clear overview can be found in the literature for the driving mechanisms of grain shaping. In the present work, we review the assumptions made in the literature to explain the different morphologies. We analyse the orders of magnitude of the different effects at stake and show that the grain shape does not result from a hydrodynamic instability but is determined by the drying stage. However, we emphasize that neither the drying time nor the associated Péclet number are critical parameters for the determination of shape morphology. In light of those results, we also review and discuss the single droplet experiments developed to mimic spray drying. Generalising our previous works, we further analyse how the control of morphology can be achieved by tuning the colloidal interactions in the suspension. We detail the model we have developed that relates the colloidal interaction potential to a critical pressure exerted by the solvent as it flows, and we provide a quantitative prediction of the grain shape. Finally, we offer perspectives with regard to spray drying of systems such as molecular solutions, widely performed in e.g. the pharmaceutical industry

Surface fluctuations of liquids confined on flat and patterned solid substrates

International audienceWe report experimental measurements of the surface fluctuations of micron-thick oil films spread onto a solid substrate. We use a recently developed optical technique based on the measurement of the deflection of a laser beam triggered by changes in the local surface slope. When the liquid is spread on a flat substrate, fluctuation dynamics slow down as the thickness is decreased, in quantitative agreement with previous predictions. In addition, we investigate the consequences on surface fluctuations of the patterning of the substrate with a rectangular grating. For liquid film thicknesses smaller than the typical wavelength probed, we demonstrate that surface fluctuations are modified by the underlying pattern: The shape of the fluctuation spectra varies periodically with the spatial position over the pattern and, in addition, the fluctuations become locally anisotropic. However, the spatially averaged spectrum is isotropic

Wetting of polymers by their solvents

International audienceWe review the studies on the wetting of soluble polymeric substrates by their solvents, both in the literature and conducted in our group in the past decade. When a droplet of solvent spreads on a soluble polymer layer, its wetting angle can strongly vary with the contact line velocity even at capillary numbers smaller than unity, in contrast to non-soluble substrates. The solvent content in the polymer is a key parameter for the spreading dynamics; that content is set by the initial conditions, but also by the transfers occurring from the droplet to the polymer layer during spreading. We focus on hydrophilic amorphous polymers that are glassy at room temperature, and we discuss the consequences on wetting of the very large changes in the polymer physical properties induced by solvent sorption. We finally present new results on polymers of varying molar masses, and show how they open new perspectives for a better understanding of powder dissolution

Slow dynamics and intermittent quakes in soft glassy systems

We present measurements of the thermal fluctuations of the free surface of oil-in-water emulsions which exhibit a glassy behavior. The Surface Fluctuation Specular Reflection (SFSR) technique was applied to measure the free surface fluctuations. When the volume fraction of the oil droplets is close to or larger than the disordered packing volume fraction, the emulsion ages and its free surface exhibits abnormal fluctuations, consisting of rare but large amplitude quakes. From a statistical analysis of the signal, we observe that the relative importance of such intermittent quakes grows as the system ages. More precisely, we observe for the first time that the kurtosis of the fluctuations increases with aging time with an exponent of about 1.3. These quakes correspond to large changes in the local slope of the free surface over a few tenths of a second. We conjecture that such quakes reflect the dynamics peculiar to glassy systems driven by the relaxations of internal stress

Exploring the mechanisms of emulsion separation via millimetric droplet coalescence

International audienceDuring petroleum production, emulsification phenomena occur and the metastable phase of W/O emulsion (a dense-packed zone) forms due to the presence of the surfactant species in the oil. Recent studies [1] have described the separation kinetics of such an emulsion with the micrometric droplet size distribution (below 50 μm) at the bottle-test conditions. However, the real separator unit is a dynamic system with inlet and outlet flows where the droplet size distribution varies from micrometres (at the separator inlet) to millimetres (at the water-DPZ interface).In this research, we use a simple millifluidics setup to generate monodisperse droplets from 0.2 mm to 2 mm one-by-one and stock them in the recipient. The experiment has two stages: the emulsion accumulation while the droplet generation and coalescence are concurrent,the emulsion separation with no more droplets arriving into the DPZ. Emulsion interfaces are traced in time with spatiotemporal images to obtain the data on the separation speed. The first significant difference with [1] is the linear separation rate instead of the formation of the metastable phase for the same system (“water in dodecane + 800 ppm Span 80”). This can be explained with a complete droplet deformation depending on the emulsion height and the droplet size. Another major difference is the coalescence only at the water-emulsion interface (contrary to the model [1]). Visually the droplets from below have a polyhedral “foam-like” shape, while ones on top are spherical with a thick oil layer around them. The presented results and perspective of millifluidics application in the emulsion generation will be useful in the study of the different factors of coalescence

Dynamic Wetting on a Thin Film of Soluble Polymer: Effects of Nonlinearities in the Sorption Isotherm

International audienceThe wetting dynamics of a solvent on a soluble substrate interestingly results from the rates of the solvent transfers into the substrate. When a supported film of a hydrosoluble polymer with thickness e is wet by a spreading droplet of water with instantaneous velocity U, the contact angle is measured to be inversely proportionate to the product of thickness and velocity, eU, over two decades. As for many hydrosoluble polymers, the polymer we used (a polysaccharide) has a strongly nonlinear sorption isotherm phi(a(w)), where phi is the volume fraction of water in the polymer and a(w) is the activity of water. For the first time, this nonlinearity is accounted for in the dynamics of water uptake by the substrate. Indeed, by measuring the water content in the polymer around the droplet phi at distances as small as 5 mu m, we find that the hydration profile exhibits (i) a strongly distorted shape that results directly from the nonlinearities of the sorption isotherm and (ii) a cutoff length xi below which the water content in the substrate varies very slowly. The nonlinearities in the sorption isotherm and the hydration at small distances from the line were not accounted for by Tay et al., Soft Matter 2011, 7, 6953. Here, we develop a comprehensive description of the hydration of the substrate ahead of the contact line that encompasses the two water transfers at stake: (i) the evaporation condensation process by which water transfers into the substrate through the atmosphere by the condensation of the vapor phase, which is fed by the evaporation from the droplet itself, and (ii) the diffusion of liquid water along the polymer film. We find that the eU rescaling of the contact angle arises from the evaporation condensation process at small distances. We demonstrate why it is not modified by the second proces