Search

Aggregation and disaggregation of clusters of attractive particles under flow are studied from numerical and theoretical points of view. Two-dimensional molecular dynamics simulations of both Couette and Poiseuille flows highlight the growth of the average steady-state cluster size as a power law of the adhesion number, a dimensionless number that quantifies the ratio of attractive forces to shear stress. Such a power-law scaling results from the competition between aggregation and disaggregation processes, as already reported in the literature. Here, we rationalize this behavior through a model based on an energy function, which minimization yields the power-law exponent in terms of the cluster fractal dimension, in good agreement with the present simulations and with previous works., 13 pages, 6 figures

International audience; Nuclear MOx -(U,Pu)O2- fuels, are shaped into pellets by dry-pressing a mixture of UO2/PuO2 . This process relies on several steps, among which the materials are powdered (crushing, mixing, sieving, pressing). It presents proven benefits, such as ease of implementation and high production rates. Alternative shaping processes based on materials in a wet or liquid state, are interesting alternatives for the mixing steps. Indeed, they reduce the dust accumulation due to dry-powder processes. Moreover, such processes should enable a better control over the fired fuel microstructures, through a more homogeneous mixing in the first steps. Extrusion processes could prove useful to engineer fuel rods. This work is focused on understanding the rheology of ceramic pastes, to be able to control its extrusion.Yttrium oxide and titanium dioxide, acting as plutonium dioxide and uranium dioxide, were selected for their respective proximity regarding morphology, granulometry and electric neutral point. Criteria believed to be the main parameters physically dictating the agglomeration and flow of the ceramic pastes. The pastes were obtained with a 30cm3 roller blade mixer (Brabender). Organic additives were added to enable the extrusion of non-model, highly charged systems. A capillary rheometer (RH2000, Malvern Panalytical) and less-conventional squeeze test flows were used to study the rheology of such pastes. A ram-extruder was built and mounted on an electromechanical testing machine (Instron). Ceramic pastes, shaped into 20cm rods, without surface defect and withstanding the specifications for nuclear fuels (fired density, mechanical strength, roughness, dimensions, purity and composition and microstructure) were further characterized. Six parameters fitted Benbow-Bridgwater’s equation to predict the extrusion pressure when changing geometry and speed. Constitutive modelling has been carried out on the forces obtained from squeezing test set-ups. The compressive forces were found to be dependent on sample deformation, compressive speed and volume fraction of ceramics. Friction seems to play an important role in flows of the system studied. Traditional capillary experiments have also been found to be poorly reliable to successfully characterize the rheological behaviour of these complex systems.Extruding highly charged ceramics rods, respecting nuclear specifications, is challenging. Modelling such flows requires reliable rheological flow-configurations. Squeezing tests and modified models successfully predict these pastes behaviour under controlled flow conditions. Further work will focus on applying these revisited models on extrusion flows. The final goal will be to transpose the study on ceramic pastes with nuclear oxides.

International audience; In a series of previous papers, models describing the breakage of lignocellulosefibers during melt mixing process were established and applied to predict fibersize changes when compounding composites in a twin-screw extruder. Differ-ent types of fibers were studied (flax, hemp, sisal, etc), but always with thesame matrix, namely polypropylene (PP) compatibilized with PP grafted withmaleic anhydride (PP-g-MA). In the present study, the influence of the matrixwas characterized, by comparing the results obtained with nonpolar PP andpolar poly(butylene succinate) (PBS) of similar viscosity. For flax and hempfibers, morphology changes (in length and diameter) were characterized alongthe screws under various processing conditions (screw speed and feed rate).Whatever the conditions, breakage was more important with the PBS matrix.Moreover, the rheological properties of the composites were also different,indicating specific interactions between fibers and matrices. Atomic forcemicroscopy was used to measure the adhesion between the different matricesand AFM levers functionalized with nanocrystalline cellulose. The results con-firmed a better interaction between the nanocrystalline cellulose and the PBSmatrix compared to the PP one

International audience

International audience

International audience

International audience

International audience

International audience

International audience

International audience

Colloque avec actes et comité de lecture. Internationale.; International audience; Une des phénomènes les plus élégants et néanmoins simples de la mécanique des fluides se produit lorsqu'un fluide visqueux tombe sur un plan, et vient ensuite s'enrouler de façon à former une spirale. Rencontrés aussi bien dans la vie de tous les jours (miel sur une biscotte) que dans des cas industriels plus pointus, ces instabilités de flambage cachent une physique riche et complexe malgré l'apparente simplicité des expériences qui les mettent en lumière. La complexité s'accroit encore davantage lorsque le matériau qui s'écoule est non-newtonien, par exemple dans le cas de suspensions, fréquemment rencontrées dans différents procédés (remplissage de récipients alimentaires, dépôts de couches...). La chute d'un jet et son enroulement impliquant des surfaces libres et une structuration du liquide après impact, il existe alors une forte demande pour décrire et prédire ces phénomènes, difficile à modéliser. C'est le sujet du présent travail dans lequel sont analysés numériquement des instabilités de flambage dans un fluide non-newtonien dont la viscosité varie selon une loi de puissance, afin de mettre en évidence des effets de rhéoépaississement sur ces instabilités. Quelques résultats numériques sont ensuite confrontés à des résultats expérimentaux en régime newtonien dont les paramètres ont été observés et mesurés à l'aide d'une caméra rapide.

International audience; We introduce an accurate numerical method for the computation of 2D and 3D granular collapse under gravity flows using the μ(I) rheology. We show that this rheology can capture the two experimentally observed types of spreading and the corresponding scaling laws, and we provide an accurate sensitivity analysis to rheological constants. Finally, we underline the role of an additional length scale on the spreading dynamics.; Ce travail concerne la résolution numérique du problème d’effondrement 2D et 3D de colonne granulaire à l’aide du modèle μ(I). Nous montrons que ce modèle est capable de reproduire les deux régimes d’étalement observés expérimentalement, et nous présentons une étude de sensibilité aux paramètres du modèle. Finalement, nous mettons en évidence le rôle d’une échelle de longueur supplémentaire sur l’étalement, confirmé par analyse dimensionnelle.

International audience

International audience

International audience

International audience

International audience

During the exploitation of a nuclear plant, or all other installation which uses radioactive materials, maintenance tasks or decommissioning operations are mandatory in order to preserve people health and environment. Among existing processes, decontaminating gels have been development by CEA in order to overcome the drawbacks of the traditionally used methods. These colloidal gels were originally formulated in an empirical way; however, the knowledge of their structures is important as it rules all the rheological behaviors of the material. The way these gels flow is an important parameter to the process and our laboratory measurements can be transposed to the industrial world. Moreover, other composition refinements have been developed in order to extend their field of use and efficiency; Au cours de la vie d'une installation dans laquelle est utilisée de la matière radioactive, des étapes de maintenance, ou bien à terme de démantèlement, sont nécessaires afin d'assurer une protection convenable des travailleurs et de l'environnement. Parmi les procédés existants, des gels de décontamination ont été développés au CEA afin de pallier aux inconvénients des méthodes classiquement utilisées. Ces gels colloïdaux, formulés à l'origine de façon plutôt empirique, ont une structure qu'il convient de connaitre, celle-ci étant la clef de voute du comportement rhéologique du matériau. La façon dont ils s'écoulent est en effet un paramètre important du procédé, et les mesures en laboratoire qui sont effectuées sur ces systèmes permettent d'en améliorer l'implémentation. En outre, de nombreux raffinements du procédé, par le biais notamment d'additifs, en étendent le champ d'utilisation, et l'efficacité

International audience; Aqueous slurries made of silica or alumina particles may behave like sols or like gels depending on the particle concentration, but also the pH or the ion force of the medium. The present study aimed at investigating the different sol-gel transitions observed in systems between a diluted state and concentrated formulations. For this purpose zeta potential and G', G '' measurements, reflecting elastic and viscous behaviors, were selected as the tools of choice to get insights into the mechanisms accounting for the sol-gel transition. We discuss about the relevancy of these simple measurements and affirm that they are sufficient to explain the evolutions in an alumina based system. Indeed, particles can adopt a "repulsive" organization at low salt concentration values (i.e. high surface potential values) or form an attractive percolating network, when the ion strength reaches a critical level. Moreover, the addition of NaOH can lead to very complex moduli variations as both the electrostatic shielding and the pH change; yet, these variations are easily predictable using our reasoning. (C) 2013 Elsevier B.V. All rights reserved.

During the exploitation of a nuclear plant, or all other installation which uses radioactive materials, maintenance tasks or decommissioning operations are mandatory in order to preserve people health and environment. Among existing processes, decontaminating gels have been development by CEA in order to overcome the drawbacks of the traditionally used methods. These colloidal gels were originally formulated in an empirical way; however, the knowledge of their structures is important as it rules all the rheological behaviors of the material. The way these gels flow is an important parameter to the process and our laboratory measurements can be transposed to the industrial world. Moreover, other composition refinements have been developed in order to extend their field of use and efficiency, Au cours de la vie d'une installation dans laquelle est utilisée de la matière radioactive, des étapes de maintenance, ou bien à terme de démantèlement, sont nécessaires afin d'assurer une protection convenable des travailleurs et de l'environnement. Parmi les procédés existants, des gels de décontamination ont été développés au CEA afin de pallier aux inconvénients des méthodes classiquement utilisées. Ces gels colloïdaux, formulés à l'origine de façon plutôt empirique, ont une structure qu'il convient de connaitre, celle-ci étant la clef de voute du comportement rhéologique du matériau. La façon dont ils s'écoulent est en effet un paramètre important du procédé, et les mesures en laboratoire qui sont effectuées sur ces systèmes permettent d'en améliorer l'implémentation. En outre, de nombreux raffinements du procédé, par le biais notamment d'additifs, en étendent le champ d'utilisation, et l'efficacité