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International audience; We report on the response of semi-dilute shear-banding wormlike micelles to time-dependent flow protocols such as step stress and shear startup. We use global rheology coupled to direct optical visualisations in Taylor-Couette flow to provide a detailed 2D description of the spatio-temporal dynamics of the shear banding flow under imposed stress. We compare these dynamics to their extensively studied counterpart under imposed shear rate. We show that, for a given asymptotic state, the onset of banding is delayed and the transients are longer when the stress is used as control parameter. The way the banding structure is built up strongly depends on the control parameter. In the two cases, secondary flows develop on top of the shear-banding instability: after shear startup, the interface between bands is formed in the bulk of the system and migrates inwards to its stationary position in the gap of the Taylor-Couette device, before being destabilized due to elastic instability; under creep flow, the interface between bands forms at the inner wall and becomes wavy while it is still migrating outwards to its steady position, leading to permanent rearrangements of the flow structure until steady state is reached, potentially explaining longer transients. We also outline the possible mechanical signature of the onset of shear banding in terms of shape of the bulk mechanical signals, which is the subject of the second part of this work [Briole et al., submitted].

We suggest a minimal model to describe the evolution of daily deaths due to Covid-19 in the European Union (EU) and UK, without any epidemiological hypothesis. Assuming current lockdown conditions were to remain in place, as of May 3rd 2020, this ad hoc fitting model is forecasting a total of 204 586 deaths in the EU. We could currently be at 2/3 of the total casualty count, and reach an overall death rate of one over 2 500 people.

International audience; Wave resistance is the drag force associated to the emission of waves by a moving disturbance at a free fluid surface. In the case of capillary-gravity waves it undergoes a transition from zero to a finite value as the speed of the disturbance reaches a certain critical value. For the first time an experiment is designed in order to obtain the capillary-gravity wave resistance as a function of speed. The effect of viscosity is explored, and a magnetic fluid is used to extend the available range of critical speeds. The threshold values are in good agreement with the proposed theory. Contrary to the theoretical model, however, the measured wave resistance reveals a non-monotonic speed dependence after the threshold.

The common secondary school teaching practice is to use linear regression to deal with data from experiments on the thin lens equation. As we have shown in a first paper, this practice is open to criticism in many aspects. In this second article, we present an alternative: to calculate the focal length for each pair of distances measured, to represent these values and to calculate their mean and standard deviation. More scientifically justified and bringing out the variability inherent in any measurement, it is much more in line with the physics and chemistry curriculum of secondary schools. In addition to providing a measure of the focal length, this approach also allows, via a Monte-Carlo simulation, to judge the adequacy of the model to the experiment. The calculations remain simple, which allows them to be performed indifferently with a spreadsheet or using the Python™ programming language., La pratique scolaire courante consiste à utiliser la régression linéaire pour traiter les données d’expériences sur la relation de conjugaison. Comme nous l’avons montré dans un premier article, cette pratique est critiquable sur de nombreux aspects. Dans ce deuxième article, nous présentons une alternative : celle de calculer la focale pour chaque couple de distances relevées, de représenter ces valeurs et d’en calculer moyenne et écart-type. Plus justifiée scientifiquement et faisant émerger la variabilité inhérente à toute mesure, elle s’inscrit bien davantage dans les programmes de spécialité physique-chimie de première. En sus de fournir une mesure de la distance focale, cette approche permet également, via une simulation Monte-Carlo, de juger de l’adéquation du modèle à l’expérience. Les calculs restent simples, ce qui permet de les réaliser indifféremment avec un tableur ou en langage Python™.

We present a simple five-parameters function that is able to reasonably describe the evolution of the number of daily death due to COVID-19 in every country of the European Union. By extrapolation, assuming current lockdown conditions were to remain in place, this ad hoc fitting model is forecasting a total of about 200 000 deaths in the EU. Following this estimate, the current (May 3rd) number of death is expected to amount to two third of the total future casualty count., Nous présentons une fonction à cinq paramètres capable de raisonnablement décrire l'évolution du nombre de morts journaliers attribués au COVID-19, dans chacun des pays de l'Union Européenne. En extrapolant, et en supposant que les conditions du confinement soient maintenues, l'ajustement ad hoc de cette fonction prévoit un total d'environ 200 000 victimes dans l'UE. En suivant cette estimation, le nombre de morts actuels (le 3 mai 2020) constituerait deux tiers du total du nombre de victimes à venir.

International audience; Les indicateurs économiques et financiers sont les instruments de bord des marchés. Ce guide se propose de faire le tri parmi les indicateurs les plus utiles et d'offrir les clés de lecture correspondantes. L'ouvrage porte sur 50 indicateurs regroupés en 22 thèmes. Chaque indicateur est traité selon une grille prédéfinie : synthèse, définition, histoire de l'indicateur, mode d'utilisation et d'interprétation (pertinence pour l'investisseur, position à adopter). Ouvrage de référence et de consultation, il permet d'apprendre à analyser et à utiliser les indicateurs dans une perspective d'analyse financière ou de choix d'un portefeuille d'investissement.