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Complex systems often exhibit shear banding—the coexistence of two different states characterized by their internal structuring and local shear rates. For some of them, the heterogeneous flow corresponds to the final steady-state response, while for others, shear banding can only be transient, the banding structure healing back to a homogeneous flow in the ultimate steady state after long-lived periods. In order to explain the diversity of observations, Moorcroft and Fielding have established general criteria for the onset of banding in time-dependent flows of complex systems, ranging from polymeric fluids to soft glassy materials [Moorcroft et al., Phys. Rev. Lett. 110, 086001 (2013)]. The proposed criteria are based on the time evolution of the bulk rheological response function of the system to a given time-dependent flow protocol and are associated with a specific signature in the mechanical response. In this contribution, we test the validity of these criteria in the case of two common time-dependent flow protocols: a step stress and a shear startup. Two types of fluids are examined. On the one hand, a wormlike micelles system exhibiting steady shear banding is studied experimentally, using rheometry coupled with direct visualizations and particle image velocimetry. On the other hand, we analyze previous literature on yield stress fluids exhibiting transient shear banding. Under conditions of a constant imposed stress, for both types of fluids, the onset of banding arises in a time window compatible with the Moorcroft–Fielding criterion. However, the mechanical signature, i.e., the shape of the bulk mechanical signal as a function of time, is not the one expected within some of the specific models with which the general Moorcroft–Fielding criteria were tested numerically. Under shear startup, both types of fluids behave differently. The criterion holds for yield stress fluids, the onset of banding arising just after the stress overshoot, as expected. On the contrary, for wormlike micelles, the window of instability is delayed, even if the overshoot clearly plays a crucial role in the nucleation of shear-induced structures. Regardless of the flow protocol or the system, wall slip seems to go hand in hand with banding indicating that it is a key ingredient to take into account.

We report on the response of semidilute shear-banding wormlike micelles to time-dependent flow protocols such as step stress and shear startup. We use global rheology coupled with direct optical visualizations in the Taylor–Couette flow to provide a detailed 2D description of the spatiotemporal dynamics of the shear banding flow under imposed stress. We compare these dynamics to their extensively studied counterpart under an 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 a 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 inward 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 outward to its steady position, leading to permanent rearrangements of the flow structure until a 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 Part II of this work [Briole et al., J. Rheol., 65, 1201–1217 (2021)].

International audience; Linear regression is used as an analysis tool in practical teaching sessions about the conjugation relation. In two previous articles, we have shown how this practice was open to criticism; we have suggested an alternative method, which consists in calculating the focal length for each pair of distances measured; this allows to deduce a measure of the focal length. However, our alternative, if it remains accessible in the first class, is not optimal. Here, we propose a more efficient method, but only accessible to the university level. Using a Monte-Carlo simulation, we show, in a general way, how to compare the performances of different analysis methods. Finally, we discuss the possible extension of our alternative to the study of other physical laws in experimental teaching sessions.; La régression linéaire est utilisée comme outil d’analyse en séance de travaux pratiques sur la relation de conjugaison. Dans deux articles précédents, nous avons montré combien cette pratique était critiquable ; puis nous avons proposé une méthode alternative, consistant à calculer la focale pour chaque couple de distances relevées ; cela permet d’en déduire une mesure de la distance focale. Cependant, notre alternative, si elle reste accessible en classe de première, n’est pas optimale. Ici, nous proposons une méthode plus efficiente, mais uniquement accessible auniveau universitaire. À l’aide d’une simulation Monte-Carlo, nous montrons, en toute généralité, comment comparer les performances de différentes méthodes d’analyse. Enfin, nous discutons de l’extension possible de notre alternative à l’étude d’autres lois physiques en séance de travauxpratiques.

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™.

National audience; Tout le programme en 13 chapitres et 336 pages : - 66 doubles pages d'activités au coeur de la science, avec plus de 25 interviews de chercheuses et chercheurs (climatologues, paléoanthropologues, économiste, démographe, linguiste, océanographe, etc.) et 9 activités pour faire travailler l'esprit critique sur les questions à l'interface sciences/société - 208 exercices dont 33 exercices méthode pour se préparer efficacement aux Épreuves communes de contrôle continu (E3C) du BAC.

International audience; Une expérience de résonance à ondes multiples est réalisée dans l’air avec des ondes ultrasonores de fréquence 40 kHz. Le dispositif expérimental est l’exact équivalent de l’interféromètre optique de Fabry-Pérot, les deux réflecteurs étant ici constitués de deux films en plastique tendus. Ce montage permet un nouveau type de mesure d’une longueur d’onde ultrasonore – et donc, à fréquence connue, de la célérité du son.

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.

In order to deal with strong inhomogeneities of the polymer stress in shear banding of wormlike micelles, rheological models usually include stress diffusion terms. These terms lift the degeneracy of steady flow solutions by selecting the steady-state positions of interfaces between bands and the values of the total mechanical stress. They are also responsible for slow transients of interface kinetics under imposed step shear flow. In our previous experiments, we estimated the typical lengthscale associated with these terms to be of the order of several nanometers, while several other experiments obtained significantly larger estimates. We report here a new set of experiments that reconcile our previous work with the rest of the literature and obtain a diffusive lengthscale of order of a few micrometers. Surprisingly, we find that this lengthscale is a strong function of the applied shear rate, decreasing rapidly across the stress plateau. We perform numerical simulations of several constitutive equations used to model shear-banded flows and conclude that they cannot predict this behavior. We argue that the strong dependence of the diffusive lengthscale on the applied shear rate should be incorporated into the classical constitutive models to obtain quantitative predictions.

International audience; S i les plaidoyers récents pour des formes d'enseignement des sciences plus proches des pra-tiques de référence ont beaucoup mis l'accent sur la démarche scientifique, cette contribution vise à montrer que cette posture engage de fait d'autres aspects essentiels de ces pratiques, comme une nécessaire mathématisation du réel. Homo, naturae minister et interpres, tantum facit et intelligit quantum de naturae ordine re vel mente observaverit : nec amplius scit, aut potest. Nec manus nuda, nec intellectus sibi permissus, multum valet ; instrumentis et auxiliis res perficitur; quibus opus est, non minus ad intellectum, quam ad manum. L'homme, interprète et ministre de la nature, n'étend ses connaissances et son action qu'à mesure qu'il découvre l'ordre naturel des choses, soit par l'observation, soit par la ré-flexion ; il ne sait et ne peut rien de plus. Ni la seule main ni l'intelligence abandonnée à elle-même ne peuvent beaucoup ; ce sont les instruments et les aides qui font presque tout ; ils ne sont pas moins nécessaires à l'intellect qu'à la main.

We present here new experimental results about the waves at the horizontal free surface of a magnetic fluid submitted to a normal magnetic field. The waves are generated by a small modulation at frequency of the vertical field . Using a shadowgraph method, we are able to measure the wavevector k of the 2D waves for a given value of and . The dispersion relation of the surface waves is established experimentally. On the other hand, we propose a theoretical derivation of the dispersion equation which includes a more complete treatment of the magnetic term than the previous works. Finally, we conclude that a linear and inviscid analysis is sufficient to fit well the experimental data, except in the vicinity of the critical field where a surface instability occurs.

We report on the flow dynamics of a wormlike micellar system (CPCl/NaSal/brine) undergoing a shear-banding transition using a combination of global rheology, 1D ultrasonic velocimetry and 2D optical visualisation. The different measurements being performed in a single Taylor–Couette geometry, we find a strong correlation between the induced turbid band observed optically and the high shear rate band. This correspondence reveals that fluctuations observed in the 1D velocity profiles are related to elastic instabilities triggered in the high shear rate band: 3D coherent (laminar) flow and 3D turbulent flow successively develop as the applied shear rate is increased. The specific characteristics of the resulting complex dynamics are found to depend on subtle changes in the sample, due to temporary light exposure. The CPCl molecules exhibit a photochemistry mainly influenced by the photo-induced cleavage of the pyridine ring that yields an unstable aldehyde enamine, which further decays by thermally activated processes. The products of the reaction possibly build up a lubrication layer responsible for pathological flow dynamics. Overall, our results bridge the gap between previous independent optical and local velocity measurements and explain most of the observed fluctuations in terms of a sequence of elastic instabilities which turns out to be widespread among semidilute wormlike micellar systems.

A method for determining the response of a contractile or expansible biological material to a variation of the rigidity of its environment comprising the steps of: a) placing a sample (100) of the biological material between a force measurement device (200, 500) and a rigid support (300, 600); b) choosing an effective rigidity value, keff, of the environment of the sample (100); c) maintaining the contacting area in a constant position by applying a force, F(t), where F(t) = ko.δ(t) and where δ(t) is the stretching of the elastic probe; - simultaneously translating the rigid support of a distance D(t), where D(t) = δ(t).(ko /keff); - determining the response of the biological material sample (100); d) repeating steps b) and c) when choosing an effective rigidity value different from former effective rigidity value when the sample (100) remains in situ.; L'invention porte sur un procédé de détermination de la réponse d'un matériau biologique contractile ou extensible à une variation de la rigidité de son environnement. Ce procédé comprend les étapes consistant à : a) placer un échantillon (100) du matériau biologique entre un dispositif de mesure de force (200, 500) et un support rigide (300, 600); b) choisir une valeur de rigidité efficace, keff, de l'environnement de l'échantillon (100); c) maintenir la zone de contact dans une position constante en appliquant une force, F(t) où F(t) = ko.δ(t) et où δ(t) représente l'étirement de la sonde élastique; déplacer en translation simultanément le support rigide d'une distance D(t), où D(t) = δ(t).(ko/keff); déterminer la réponse de l'échantillon de matériau biologique (100); d) répéter les étapes b) et c) en choisissant une valeur de rigidité efficace différente de la valeur de rigidité efficace précédente lorsque l'échantillon (100) reste in situ.

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.

We compare and synthesize the results of two microrheological experiments on the cytoskeleton of single cells. In the first one, the creep function J(t) of a cell stretched between two glass plates is measured after applying a constant force step. In the second one, a micrometric bead specifically bound to transmembrane receptors is driven by an oscillating optical trap, and the viscoelastic coefficient $G_e(\omega)$ is retrieved. Both $J(t)$ and $G_e(\omega)$ exhibit power law behavior: $J(t)= A(t/t_0)^\alpha$ and $\bar G_e(\omega)\bar = G_0 (\omega/\omega_0)^\alpha$, with the same exponent $\alpha\approx 0.2$. This power law behavior is very robust ; $\alpha$ is distributed over a narrow range, and shows almost no dependance on the cell type, on the nature of the protein complex which transmits the mechanical stress, nor on the typical length scale of the experiment. On the contrary, the prefactors $A_0$ and $G_0$appear very sensitive to these parameters. Whereas the exponents $\alpha$ are normally distributed over the cell population, the prefactors $A_0$ and $G_0$ follow a log-normal repartition. These results are compared with other data published in the litterature. We propose a global interpretation, based on a semi-phenomenological model, which involves a broad distribution of relaxation times in the system. The model predicts the power law behavior and the statistical repartition of the mechanical parameters, as experimentally observed for the cells. Moreover, it leads to an estimate of the largest response time in the cytoskeletal network: $\tau_m \approx 1000$ s., Comment: 47 pages, 14 figures // v2: PDF file is now Acrobat Reader 4 (and up) compatible // v3: Minor typos corrected - The presentation of the model have been substantially rewritten (p. 17-18), in order to give more details - Enhanced description of protocols // v4: Minor corrections in the text : the immersion angles are estimated and not measured // v5: Minor typos corrected. Two references were clarified

International audience; We present an experimental method allowing a direct measurement of the local rheological behavior of complex fluids. A magnetic probe is inserted into the bulk of the fluid and submitted to a controlled magnetic force or torque, which induces a mechanical perturbation of the fluid. The geometry of the perturbation can be varied using two kinds of probes: a magnetic bead submitted to a homogeneous magnetic force in one direction, and a magnetic needle that can turn inside the material under the effect of an applied magnetic torque. Two complex viscoelastic fluids are investigated. First, a surfactant solution, which has a linear mechanical behavior in the range of the applied stresses, is used to test and validate the experimental methodology. We then use the local probes to investigate a Laponite colloidal suspension, which exhibits nonlinear behavior such as thixotropy, shear thinning, and aging. In this latter fluid, we find an exponential growth of the rheological relaxation time versus the system age, a power-law dependence of the fluid viscosity on the applied stress, and a dynamical yield stress which saturates with the fluid aging time.

Die Erfindung betrifft ein neues Verfahren zur Detektion von Analyten bzw. Bindungseaktionen mittels Messung der Doppelbrechung, sowie die Verwendung der dazu notwendigen Verbindungen in der Analytik.; The invention relates to a new method for detecting analytes, especially binding reactions, by means of measurement of the birefringence, and also to the use of compounds which are necessary to the analysis.; La présente invention concerne un nouveau procédé de détection d'analytes, en particulier de réactions de liaison à l'aide de la mesure de la relaxation de la biréfringence, ainsi que l'utilisation des composés nécessaires à l'analyse.

International audience; We present here the experimental determination of the dispersion relation of surface waves of a ferrofluid submitted to a steady vertical magnetic field. The results are compared to a linear theory where all the magnetic characteristics of the fluid have been taken into account. The knowledge of the dispersion equation is then used to analyse the capillary-wave resistance, that is the drag force associated to the emission of waves by a moving disturbance at a free fluid surface. It undergoes a transition from zero to a finite value as the speed of the disturbance reaches a certain critical value. The effect of viscosity is explored, and a magnetic fluid is shown to allow controlling the critical speeds. Contrary to the theoretical model, however, the measured wave resistance reveals a non monotonic speed dependence after the threshold.

International audience; We describe the concept of a voltage generator based on the use of a magnetic fluid. A cell contains two immiscible liquids: the lower liquid boils at a low temperature and the upper is a magnetic liquid magnetized by a permanent magnet. By heating the bottom of the cell the lower liquid boils, and bubbles of the gas phase cross both liquid phases; consequently a flux variation is induced in a detection coil. Finally, the gas phase condenses in a recovery device in order to be injected back at the bottom of the cell. We obtain a voltage generator with a shape signal linked to the nucleation frequency and volume of the gas bubbles. This device has been built by secondary school pupils and requires the acquisition of knowledge of magnetism, hydrodynamics and thermodynamics.

International audience; We describe the concept of a voltage generator based on the use of a magnetic fluid. A cell contains two immiscible liquids: the lower liquid boils at a low temperature and the upper is a magnetic liquid magnetized by a permanent magnet. By heating the bottom of the cell the lower liquid boils and bubbles of the gas phase cross both liquid phases; consequently a flux variation is induced in a detection coil. Finally, the gas phase condenses in a recovery device in order to be injected back at the bottom of the cell. We obtain a voltage generator with a shape signal linked to the nucleation frequency and to the volume of the gas bubbles.

International audience; This work reports the first terrestrial laboratory study of thermal convection in a central body-force field. The force field is produced by gradients of magnetic field magnitude imposed on a spherical shell of heated, magnetizable fluid. Cells of thermal convection consistent with the notion of continental drift are detected using infrared imaging. Transitions of symmetry for the patterns of convection are observed in the range of Earth's Rayleigh number.

Wave resistance is the drag force associated to the emission of waves by a moving disturbance at a fluid free surface. In the case of capillary-gravity waves it undergoes a transition from zero to a finite value as the speed of the disturbance is increased. For the first time an experiment is designed in order to obtain the 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., 12 pages, 4 figures, 1 table, submitted to Physical Review Letters

We report here the collective behavior of droplets in the scale ${10}^{\ensuremath{-}6}--{10}^{\ensuremath{-}5}\mathrm{m}$ made of a concentrated magnetic fluid elongated and oriented by a magnetic field while rotating synchronously with respect to the carrier liquid. Distribution of droplet sizes is studied as a function of a magnetoviscous number ${N}_{\mathrm{mv}}$ that quantifies the competition between magnetic field and local vorticity. The liquid state and very low interfacial tension enable both breakup and coalescence processes, which are undergone by the droplet population to reach dynamic equilibrium. Theoretical analysis of a single drop motion is extended to the case of the drop assembly. Experiments combining rotation and field modulation show a regime of nonsteady rotation in good agreement with theory.

Analysis of high-resolution digital topography across 21 Plio-Pleistocene grabens in central and southern Tibet shows that high topography occurs along the graben margins. Profiles across these marginal highlands fit curves describing the flexure of an elastic plate under vertical loading. We interpret the features as flexural rift flank uplifts, resulting from regional isostatic compensation of low-density grabens. The short wavelengths of the uplifts (30–40 km) imply a low flexural rigidity for the plateau (D = 2−6 × 1020 N m), similar to values inferred for the Basin and Range of western North America. In addition, the grabens themselves are extremely narrow compared to typical continental rifts. Taken together, these observations suggest the existence of a low-viscosity lower crust and suggest that isostatic compensation is taking place within this ductile lower crust. Simple channel flow calculations suggest that crustal viscosities below ∼1022 Pa s are necessary to accommodate this compensation over reasonable timescales.

International audience; Increasing evidence suggests that mechanical cues inherent to the extracellular matrix may be as important as its chemical nature in regulating cell behavior. Here, the response of cells to the mechanical properties of the substrate is examined by culturing 3T3 fibroblastic cells and epithelial cells on surfaces composed of a dense array of flexible microfabricated pillars. We focus on the influence of substrate rigidity on the traction forces exerted by cells, and on cell adhesion and migration. We first measure these forces by monitoring the deflection of the pillars. Then, by varying their geometric parameters, we control the substrate stiffness over a large range from 1 to 200 nN μm−1. We show that the force–rigidity relationship exhibits a similar behavior for both cell types. Two distinct regimes are evidenced: first, a linear increase of the force with the rigidity and then a saturation plateau for the largest rigidities. We observe that the cell spreading area increases with increasing rigidity, as well as the size of focal adhesions. Substrates with an anisotropic rigidity allow us to determine that the migration paths of 3T3 cells are oriented in the stiffest direction in correlation with maximal traction forces. Finally, to compare the force measurements on micro-textured surfaces and continuous flexible gels, we propose an elastic model that estimates the equivalent Young's modulus of a micropillar substrate. This qualitative model gives comparable results for both experimental approaches.

International audience

International audience; The experimental illustration of the equation of thin lenses often leads to perform a linear regression, simultaneously to verify the validity of this law and to deduce the focal length of the lens studied. This method of analysis is problematic in many ways, and an alternative can be considered. In this first article, we analyze the French secondary school curriculum to try to reformulate the questions that arise in practical work. Then we examine the proposals of corresponding textbooks and show that the use of the linear regression is done without understanding its conditions of application. The notion of variability associated with the measurement, which is central to the expectations of the curriculum, does not emerge, nor does the idea of uncertainty. As a result, the interpretation of the results is always erroneous.; L’illustration expérimentale de la relation de conjugaison de lentilles minces conduit souvent à réaliser une régression linéaire, simultanément pour vérifier la validité de cette loi et pour en déduire la distance focale de la lentille étudiée. Cette méthode d’analyse est problématique à bien des égards, et une alternative peut être envisagée. Dans ce premier article, nous analysons les programmes de classe de première pour essayer de reformuler les questions qui se posent en travaux pratiques. Puis nous examinons les propositions des livres scolaires pour montrer que le recours à la régression linéaire se fait sans compréhension de ses conditions d’application. La notion de variabilité associée à la mesure, pourtant centrale dans les attendus du programme, n’émerge pas, pas plus que l’idée d’incertitude. Il en découle que l’interprétation des résultats est toujours erronée.