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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.