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

Frequency jumps in the planar vibrations of an elastic beam

The small amplitude transverse vibrations of an elastic beam clamped at both extremities are studied. The beam is modeled as an extensible, shearable planar Kirchhoff elastic rod under large displacements and rotations, and the vibration frequencies are computed both analytically and numerically as a function of the loading. Of particular interest is the variation of mode frequencies as the load is increased through the buckling threshold. While for some modes there is no qualitative changes in the mode frequencies, other modes experience rapid variations after the buckling threshold. For slender beams, these variations become stiffer, eventually resulting in a discontinuous jump of frequency at buckling, in the limit of inextensible, unshearable beams

A novel method for pair-matching using three-dimensional digital models of bone:mesh-to-mesh value comparison

The commingling of human remains often hinders forensic/physical anthropologists during the identification process, as there are limited methods to accurately sort these remains. This study investigates a new method for pair-matching, a common individualization technique, which uses digital three-dimensional models of bone: mesh-to-mesh value comparison (MVC). The MVC method digitally compares the entire three-dimensional geometry of two bones at once to produce a single value to indicate their similarity. Two different versions of this method, one manual and the other automated, were created and then tested for how well they accurately pair-matched humeri. Each version was assessed using sensitivity and specificity. The manual mesh-to-mesh value comparison method was 100 % sensitive and 100 % specific. The automated mesh-to-mesh value comparison method was 95 % sensitive and 60 % specific. Our results indicate that the mesh-to-mesh value comparison method overall is a powerful new tool for accurately pair-matching commingled skeletal elements, although the automated version still needs improvement. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00414-016-1334-3) contains supplementary material, which is available to authorized users

La philosophie de Rorty, un paradigme de la pensée américaine ?

Proche à bien des égards de la pensée d’auteurs qui, en France se situent dans le sillage des réflexions de Tocqueville, Rorty semble aller plus loin. Il voit dans la sécularisation non pas simplement un mouvement qui se déroule, mais un parti pris qu’il s’agit de faire valoir comme proprement libéral. C’est en cela que la pensée de Rorty a peut-être quelque mal à être prise ici, « chez nous » pour ce qu’elle est ; qu’elle se trouve souvent dénaturée parce qu’on la présente par le détail. Du coup on manque ce qui en constitue la radicalité, à savoir l’unité systématique de l’appareil théorique qui est mis en œuvre. Car la caractérisation de la pensée de Rorty tient à l’effort constant qu’il met en œuvre pour déployer le lien entre philosophie de la connaissance et philosophie politique. En explicitant les diverses expressions de la pensée « spéculaire », Rorty nous invite à un réquisitoire sans appel de la pensée des modernes. On s’attachera dans cet article à l’énoncé de quelques traits marquants de cette remise en question du fondement sur lequel s’est édifiée la vision de l’éducation scolaire en France. On analysera plus particulièrement ce qu’emporte l’élimination de tout extérieur et supérieur qui puisse dans un même élan justifier le savoir et constituer la communauté des citoyens. Car, si l’on prend au sérieux la pensée de Rorty, c’est bien le socle même sur lequel nous construisons notre action éducative qui se trouve ébranlé. Mais dans ces conditions aussi la radicalité de la critique nous invite à interroger le pragmatisme comme conséquence inéluctable de la sécularisation démocratique

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

What works where and for whom? Farm Household Strategies for Food Security across Uganda

East Africa's smallholder agriculture is expected to be strongly affected by climate change, which, together with a growing population and pressure on natural resources, will result in an increasing challenge to achieve food security for households and regions. Policy makers need information on what works where for which farmers in order to guide their decision making and prioritise investment for agricultural interventions to increase food security. For this, we must better understand how smallholder farm strategies for achieving food security differ across regions and farm types and what drives these strategies. In this study we present new analyses at country and farm household level that quantify drivers of productivity and food security, and that can be used to prioritise agricultural interventions. Uganda was chosen as a case study because of data availability but the approach can be applied to other countries in sub-Saharan Africa. First, we quantified how food security and farm types varied across Uganda, and which key factors drive this variability. We used household level data from the Living Standard Measurement Study – Integrated Survey on Agriculture (LSMS-ISA) of the World Bank and developed an approach to map and quantitatively explain food security and agricultural land use across Uganda. The resulting maps showed where which crops and livestock activities are important for which types of farm households. Subsequently, the effects of agricultural interventions on food security of different farm types were assessed. Second, we used this information to select contrasting sites and farm households for detailed interviews, which aimed at identifying drivers of farmers' decision making, assessing farmers' vulnerability to climate change and how proposed interventions match with the farmers' socio-ecological niche. The spatial approach we developed is a novel way to use farm household level information to generate country-wide patterns in farming systems and their productivity. It generates useful information for a quantitative assessment of what might happen to the food security of smallholder farmers in Uganda under climate change and for a country-wide targeting of agricultural interventions that aim at mitigating the effects of climate change