Shear induced drainage in foamy yield-stress fluids

Shear induced drainage of a foamy yield stress fluid is investigated using MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes the system at rest, a fast drainage is observed when a horizontal shear is imposed. It is shown that the sheared interstitial material behaves as a viscous fluid in the direction of gravity, the effective viscosity of which is controlled by shear in transient foam films between bubbles. Results provided for several bubble sizes are not captured by the R^2 scaling classically observed for liquid flow in particulate systems, such as foams and thus constitute a remarkable demonstration of the strong coupling of drainage flow and shear induced interstitial flow. Furthermore, foam films are found to be responsible for the unexpected arrest of drainage, thus trapping irreversibly a significant amount of interstitial liquid.Comment: Published in Physical Review Letters. http://prl.aps.org/abstract/PRL/v104/i12/e12830

Rheological behaviour of suspensions of bubbles in yield stress fluids

The rheological properties of suspensions of bubbles in yield stress fluids are investigated through experiments on model systems made of monodisperse bubbles dispersed in concentrated emulsions. Thanks to this highly tunable system, the bubble size and the rheological properties of the suspending yield stress fluid are varied over a wide range. We show that the macroscopic response under shear of the suspensions depends on the gas volume fraction and the bubble stiffness in the suspending fluid. This relative stiffness can be quantified through capillary numbers comparing the capillary pressure to stress scales associated with the rheological properties of the suspending fluid. We demonstrate that those capillary numbers govern the decrease of the elastic and loss moduli, the absence of variation of the yield stress and the increase of the consistency with the gas volume fraction, for the investigated range of capillary numbers. Micro-mechanical estimates are consistent with the experimental data and provide insight on the experimental results.Comment: submitted to Journal of non Newtonian Fluid Mechanic

Coupling of elasticity to capillarity in soft aerated materials

We study the elastic properties of soft solids containing air bubbles. Contrary to standard porous materials, the softness of the matrix allows for a coupling of the matrix elasticity to surface tension forces brought in by the bubbles. Thanks to appropriate experiments on model systems, we show how the elastic response of the dispersions is governed by two dimensionless parameters: the gas volume fraction and a capillary number comparing the elasticity of the matrix to the stiffness of the bubbles. We also show that our experimental results are in good agreement with computations of the shear modulus through a micro-mechanical approach.Comment: submitted to Soft Matte

L'autre élection présidentielle: les groupes d'intérêt en campagne

National audienceUne tradition d'analyse américaine permet d'étudier les élections comme des conjonctures où les groupes d'intérêt agissent. A priori cette tradition est étrangère au cas français. Pourtant elle offre un cadre original et stimulant pour comprendre les campagnes électorales. C'est là le pari que nous avons fait pour les élections présidentielles et législatives françaises de 2012. A partir d'une question centrale (comment les groupes d'intérêt français agissent-ils pendant une campagne électorale ?), le protocole d'enquête élaboré ouvre une nouvelle perspective sur les élections françaises. Ce chapitre présente le protocole et les premiers résultats obtenus

On the existence of a simple yield stress fluid behavior

Materials such as foams, concentrated emulsions, dense suspensions or colloidal gels, are yield stress fluids. Their steady flow behavior, characterized by standard rheometric techniques, is usually modeled by a Herschel-Bulkley law. The emergence of techniques that allow the measurement of their local flow properties (velocity and volume fraction fields) has led to observe new complex behaviors. It was shown that many of these materials exhibit shear banding in a homogeneous shear stress field, which cannot be accounted for by the standard steady-state constitutive laws of simple yield stress fluids. In some cases, it was also observed that the velocity fields under various conditions cannot be modeled with a single constitutive law and that nonlocal models are needed to describe the flows. Doubt may then be cast on any macroscopic characterization of such systems, and one may wonder if any material behaves in some conditions as a Herschel-Bulkley material. In this paper, we address the question of the existence of a simple yield stress fluid behavior. We first review experimental results from the literature and we point out the main factors (physical properties, experimental procedure) at the origin of flow inhomogeneities and nonlocal effects. It leads us to propose a well-defined procedure to ensure that steady-state bulk properties of the materials are studied. We use this procedure to investigate yield stress fluid flows with MRI techniques. We focus on nonthixotropic dense suspensions of soft particles (foams, concentrated emulsions, Carbopol gels). We show that, as long as they are studied in a wide (as compared to the size of the material mesoscopic elements) gap geometry, these materials behave as 'simple yield stress fluids': they are homogeneous, they do not exhibit steady-state shear banding, and their steady flow behavior in simple shear can be modeled by a local continuous monotonic constitutive equation which accounts for flows in various conditions and matches the macroscopic response.Comment: Journal of Non-Newtonian Fluid Mechanics (2012) http://dx.doi.org/10.1016/j.jnnfm.2012.06.00

Competition between alkenes in intramolecular ketene-alkene [2 + 2] cycloaddition: What does it take to win?

Abstract: In the course of developing a new synthetic methodology using ketenes in sequential cycloaddition steps, we were faced with a competition problem with molecules containing a ketene tethered to more than one reacting partner. To pinpoint the electronic and tethering requirements for a chemoselective reaction, we undertook a series of ketene−alkene [2 + 2] cycloaddition competition experiments. Those experiments were conducted on molecules containing either two identical alkenes having different tether lengths or two alkenes having the same tether length but being electronically different. We demonstrated that the reaction is much faster for forming five-membered rings than six-membered rings and calculated the Hammett constant ρ for intramolecular ketene−alkene [2 + 2] cycloadditions to be −1.39

LGWP & HC Refrigerants Solubility Tests Performed in Running Scroll Compressor

This paper presents the results of investigations on Low Global Warming Potential refrigerants and R290 solubility with different lubricants. Solubility impacts a lot the final lubricant-refrigerant mixture reaching the bearing lubricant inlet and in the same way can impact compressor reliability on wet vapor flowing back to compressor inlet Experimental data, measured in operating compressors, are compared to static steady state values coming from PVT plots. R407C and R410A, two well-known HFC refrigerants, are added as a reference. This work highlights the differences between dynamic solubility (tests performed in a compressor) and static solubility (from PVT plots). A comparison of the impact on the dynamic solubility has been done when a steering feature is added. This work allows us to define the best compromise between different lubricants in these new applications

Français écrit au collégial et marché du travail : développe-t-on adéquatement la compétence à produire des écrits professionnels en réponse aux attentes des employeurs?

Affiche présentée dans le cadre du Colloque de l'ARC, «La relève scientifique et la recherche collégiale : pratiques inspirantes au regard des chercheuses et chercheurs, et enjeux spécifiques à la formation des étudiantes et étudiants», dans le cadre du 84e Congrès de l'Acfas, Université du Québec à Montréal, Montréal, le 10 mai 2016.L’école devrait mener, entre autres, à devenir compétent en matière d’écriture dans l’exercice d’un métier ou d’une profession, c’est-à-dire dans le cadre de la production d’écrits professionnels, des écrits qu’on dits « utilitaires » ou « fonctionnels ». Or, pour près de 40 % des employeurs que nous avons interrogés, la situation est critique quant à la capacité à communiquer par écrit en français des recrues diplômées de la formation technique du collégial, qui ne répondent pas à leurs attentes en la matière. Notre recherche avait pour but de décrire, de manière comparative, en quoi la formation spécifique de cinq programmes techniques développe la compétence à produire des écrits professionnels de genres variés satisfaisant aux attentes des employeurs des domaines d’emplois afférents. Notre design méthodologique mixte nous a conduits à recueillir et à analyser de manière parallèle et convergente des données provenant de documents constitutifs des programmes ainsi que d’enquêtes auprès de deux groupes cibles. Dix employeurs ont été interrogés individuellement quant à leurs attentes en matière de production d’écrits, puis 230 enseignants ont été interrogés par sondage quant à leurs pratiques. Ces résultats, ensuite éclairés et approfondis au cours de deux entretiens de groupe avec 15 enseignants, nous ont suggéré des pistes d’action visant à favoriser le développement de la compétence à produire des écrits professionnels au cœur d’enjeux socio-économiques de différents ordres

PC-MSDM: A quality metric for 3D point clouds

International audienceIn this paper, we present PC-MSDM, an objective metric for visual quality assessment of 3D point clouds. This full-reference metric is based on local curvature statistics and can be viewed as an extension for point clouds of the MSDM metric suited for 3D meshes. We evaluate its performance on an open subjective dataset of point clouds compressed by octree pruning; results show that the proposed metric outperforms its counterparts in terms of correlation with mean opinion scores