Rheology of sheared suspensions of rough frictional particles

International audienceThis paper presents three-dimensional numerical simulations of non-Brownian concen- trated suspensions in a Couette flow at zero Reynolds number using a fictitious domain method. Contacts between particles are modelled using a DEM-like approach (Discrete Element Method), which allows for a more physical description, including roughness and friction. This work emphasizes the effect of friction between particles and its role on rheological properties, especially on normal stress differences. Friction is shown to no- tably increase viscosity and second normal stress difference |N2| and decrease |N1|, in better agreement with experiments. The hydrodynamic and contact contributions to the overall particle stress are particularly investigated. This shows that the effect of friction is mostly due to the additional contact stress since the hydrodynamic stress remains unaffected by friction. Simulation results are also compared with experiments, such as normal stresses or effective friction coefficient μ(Iv), and the agreement is improved when friction is accounted for. This suggests that friction is operative in actual suspensions

Rheology of non-Brownian suspensions of rough frictional particles under shear reversal: A numerical study

International audienceWe perform particle scale simulations of suspensions submitted to shear reversal. The simulations are based on the Force Coupling Method, adapted to account for short range lubrication interactions together with direct contact forces between particles, including surface roughness, contact elasticity and solid friction. After shear reversal, three consecutive steps are identified in the viscosity transient: an instantaneous variation, followed by a rapid contact force relaxation, and finally a long time evolution. The separated contributions of hydrodynamics and contact forces to the viscosity are investigated during the transient, allowing a qualitative understanding of each step. In addition, the influence of the contact law parameters (surface roughness height and friction coefficient) on the transient are evaluated. Concerning the long time transient, the difference between the steady viscosity and minimum viscosity is shown to be proportional to the contact contribution to the steady viscosity, allowing in principle easy determination of the latter in experiments. The short time evolution is studied as well. After the shear reversal, the contact forces vanish over a strain that is very short compared to the typical strain of the long time transient, allowing to define an apparent step between the viscosity before shear reversal and after contact force relaxation. This step is shown to be an increasing function of the friction coefficient between particles. Two regimes are identified as a function of the volume fraction. At low volume fraction, the step is small compared to the steady contact viscosity, in agreement with a particle pair model. As the volume fraction increases, the value of the viscosity step increases faster than the steady contact viscosity, and, depending on the friction coefficient, may approach it

Différences de contraintes normales dans les suspensions non-browniennes concentrées

Nous nous intéressons aux deux différences de contraintes normales qui apparaissent lorsqu'on cisaille une suspension de sphères non-browniennes suffisamment concentrée. Nous mesurons simultanément les contraintes normales totales dans la suspension et la pression dans le liquide suspendant afin d'évaluer les contraintes normales d'origine particulaire. Les deux différence de contraintes normales sont mesurées en utilisant deux géométries d'écoulement distinctes : deux plans parallèles rotatifs et une géométrie de poiseuille annulaire

Brain Vitamin E Deficiency During Development Is Associated With Increased Glutamate Levels and Anxiety in Adult Mice

Vitamin E, the most important lipophilic radical scavenging antioxidant in vivo, has a pivotal role in brain. In an earlier study, we observed that adult mice with a defect in the gene encoding plasma phospholipid transfer protein (PLTP) display a moderate reduction in cerebral vitamin E levels, and exacerbated anxiety despite normal locomotion and memory functions. Here we sought to determine whether dietary vitamin E supplementation can modulate neurotransmitter levels and alleviate the increased anxiety phenotype of PLTP-deficient (PLTP−/−) mice. To address this question, a vitamin E-enriched diet was used, and two complementary approches were implemented: (i) “early supplementation”: neurotransmitter levels and anxiety were assessed in 6 months old PLTP−/− mice born from vitamin E-supplemented parents; and (ii) “late supplementation”: neurotransmitter levels and anxiety were assessed in 6 months old PLTP−/− mice fed a vitamin E-enriched diet from weaning. Our results show for the first time that an inadequate supply of vitamin E during development, due to moderate maternal vitamin E deficiency, is associated with reduced brain vitamin E levels at birth and irreversible alterations in brain glutamate levels. They also suggest this deficiency is associated with increased anxiety at adulthood. Thus, the present study leads to conclude on the importance of the micronutrient vitamin E during pregnancy

Contraintes normales particulaires dans les suspensions non-browniennes concentrées

Nous étudions l'écoulement d'une suspension concentrée de particules non-browniennes entre deux disques parallèles en rotation. Le disque inférieur est immobile et il est percé de trous recouverts de grilles qui retiennent les particules. La pression du liquide derrière les grilles, appelée pression de grille, est mesurée. Par ailleurs le disque immobile est équipé de capteurs de pression qui affleurent sa surface. Ces derniers permettent de mesurer le profil radial de contrainte normale dans la direction du gradient de vitesse. Nous montrerons que l'on peut déduire de ce profil, les deux différences de contraintes normales. Par ailleurs, en adoptant une description diphasique de la suspension, la contrainte particulaire dans la direction du gradient de vitesse pourra être obtenue en soustrayant la pression de grille (identifiée à la pression dans le liquide) à la contrainte normale totale mesurée grâce aux capteurs directs. Connaissant la contrainte normale particulaire dans la direction du gradient de vitesse et les deux différences de contraintes normales, nous serons en mesure de présenter les trois composantes de contrainte normale. La connaissance des contraintes normales particulaires présente un intérêt certain car ce sont elles qui sont censées contrôler la migration de particules sous écoulement. Nous verrons cependant que l'interprétation des mesures présentées ici est extrêmement délicate et qu'il n'est pas certain que les grandeurs que nous mesurons soient celles qui interviennent dans la migration

In vitro expansion of human glioblastoma cells at non-physiological oxygen tension irreversibly alters subsequent in vivo aggresiveness and AC 133 expression

Among markers of glioblastoma initiating cells, AC133 has been shown to be associated with glioblastoma resistance and malignancy. Recently, it was demonstrated that increasing oxygen tension (pO(2)) down-regulated AC133 expression in glioblastoma cells in vitro. In order to better understand extrinsic factor regulation of AC133, this work aimed to investigate the relationship between cell culture pO(2), AC133 expression, and tumor development and phenotype. Using treatments with CoCl(2) and HIF-1α shRNA knockdowns on non-sorted human primary glioblastoma cells cultured at low (3%) versus high (21%) oxygen tension, we established a responsibility for low pO(2) in the maintenance of high levels of AC133 expression, with a major but non-exclusive role for HIF-1α. We also demonstrated that human glioblastoma cells previously cultured under high oxygen tension can lose part of their aggressiveness when orthotopically engrafted in SCID mice or lead to tumors with distinct phenotypes and no re-expression of AC133. These observations showed that the specific pO(2) microenvironment irreversibly impacts glioblastoma cell phenotypes, highlighting the pertinence of culture conditions when extrapolating data from xenogenic models to human cells in their source environment. They also raised AC133 as a marker of non-exposure to oxygenated areas rather than a marker of aggressiveness or low pO(2) niches

Advanced Neuroimaging with Computed Tomography Scanning

International audienceThe x-ray computed tomography (CT) is well known as a useful imaging method and thus CT images have continuingly been used for many applications, especially in medical fields. This book discloses recent advances and new ideas in theories and applications for CT imaging and its analysis. The 16 chapters selected in this book cover not only the major topics of CT imaging and analysis in medical fields, but also some advanced applications for forensic and industrial purposes. These chapters propose state-of-the-art approaches and cutting-edge research results