The yielding dynamics of a colloidal gel

Attractive colloidal gels display a solid-to-fluid transition as shear stresses above the yield stress are applied. This shear-induced transition is involved in virtually any application of colloidal gels. It is also crucial for controlling material properties. Still, in spite of its ubiquity, the yielding transition is far from understood, mainly because rheological measurements are spatially averaged over the whole sample. Here, the instrumentation of creep and oscillatory shear experiments with high-frequency ultrasound opens new routes to observing the local dynamics of opaque attractive colloidal gels. The transition proceeds from the cell walls and heterogeneously fluidizes the whole sample with a characteristic time whose variations with applied stress suggest the existence of an energy barrier linked to the gelation process. The present results provide new test grounds for computer simulations and theoretical calculations in the attempt to better understand the yielding transition. The versatility of the technique should also allow extensive mesoscopic studies of rupture mechanisms in soft solids ranging from crystals to glassy materials.Comment: 8 pages, 5 figure

Propagation des fronts de fissure plane dans les matériaux fragiles hétérogènes de dimensions finies

Notre approche vise à tester les modèles classiques de piègage de fissure pour des motifs de ténacité contrôlée et des géométries d'éprouvette réalistes, c'est-à-dire de tailles finies. Nous modélisons l'effet de l'épaisseur du matériau. Les prédictions du modèle théorique sont validées par des calculs de type éléments finis qui convergent vers les deux régimes asymptotiques: milieu semi-infini et géométrie plaque. Dans le cas d'une fissure interagissant avec un défaut unique, nous comparons les résultats expérimentaux avec les calculs par éléments finis sans paramètre ajustable

Knife-bladed vortices in non-Newtonian fluids

A tank is filled with a non-Newtonian fluid. We report on the deformation of the free surface resulting from the presence of an underlying vortex. In a tiny range of the experimental parameters, the flow spontaneously loses its initial axi-symmetry, leading to the formation of a stationary knife-bladed vortex. We report on the series of patterns observed experimentally and summarize the conditions of existence of the latter by establishing a state diagram. Received: 1 July 2016, Accepted: 24 October 2016; Edited by: J. P. Hulin; Reviewed by: C. Nouar, LEMTA, Université de Lorraine, Nancy, France; DOI: http://dx.doi.org/10.4279/PIP.080007 Cite as: E Freyssingeas, D Frelat, Y Dossmann, J-C Géminard, Papers in Physics 8, 080007 (2016

Coplanar perturbation of a crack lying on the mid-plane of a plate

International audienceSeveral groups have studied experimentally the deformation of the front of mode I cracks propagating quasistatically along the interface between bonded plates. The theoretical interpretation of such experiments has always been based up to now on a formula of Rice (1985); this formula provides the first-order variation of the local mode I stress intensity factor resulting from some small, but otherwise arbitrary coplanar perturbation of the front of a semi-infinite crack in an infinite body. To be applicable to bonded plates, this formula requires that the characteristic distance of variation of this perturbation in the direction of the crack front be small compared to all other characteristic dimensions of the problem, and first of all the thickness of the plates. This condition is unfortunately frequently violated in practice. The purpose of this paper is therefore to provide a more exact formula for the variation of the local stress intensity factor, for the specific cracked geometry and boundary conditions used in experiments; this should allow for more accurate theoretical interpretations. This is done in two steps. The first one consists in adapting Rice (1985)'s treatment, applicable to the extreme case of plates of infinite thickness, to the other extreme one of plates of infinitesimal thickness, using the standard Love-Kirchhoff plate theory. An interesting outcome of the analysis is that the distance from the crack front to the boundary of the plate acts as a "cutoff length", in the sense that when the distance between two points on the crack front gets larger than it, the influence of the crack advance at the first point upon the stress intensity factor at the second diminishes quickly; the plate thickness , however, plays no similar role. The second step consists in supplementing the theoretical expressions applicable to extreme values of the plate thickness with finite element computations providing results for intermediate values. These computations lead to the definition of a simple, approximate but accurate "interpolation formula" for the variation of the local stress intensity factor, applicable to plates of arbitrary thickness

Increased risk of severe COVID-19 in hospitalized patients with SARS-CoV-2 Alpha variant infection: a multicentre matched cohort study

International audienceBackground: The impact of the variant of concern (VOC) Alpha on the severity of COVID-19 has been debated. We report our analysis in France.Methods: We conducted an exposed/unexposed cohort study with retrospective data collection, comparing patients infected by VOC Alpha to contemporaneous patients infected by historical lineages. Participants were matched on age (± 2.5 years), sex and region of hospitalization. The primary endpoint was the proportion of hospitalized participants with severe COVID-19, defined as a WHO-scale > 5 or by the need of a non-rebreather mask, occurring up to day 29 after admission. We used a logistic regression model stratified on each matched pair and accounting for factors known to be associated with the severity of the disease.Results: We included 650 pairs of patients hospitalized between Jan 1, 2021, and Feb 28, 2021, in 47 hospitals. Median age was 70 years and 61.3% of participants were male. The proportion of participants with comorbidities was high in both groups (85.0% vs 90%, p = 0.004). Infection by VOC Alpha was associated with a higher odds of severe COVID-19 (41.7% vs 38.5%-aOR = 1.33 95% CI [1.03-1.72]).Conclusion: Infection by the VOC Alpha was associated with a higher odds of severe COVID-19