Power laws in microrheology experiments on living cells: comparative analysis and modelling

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 clarifie

Les ferrofluides : ondes de surface, résistance de vague et simulation de la convection dans le manteau terrestre

Jury : M. Jean-Claude BACRI, M. Andreï CEBERS, M. Yves COUDER, M. Patrice FLAUD, M. Claude JAUPART, M. Elie RAPHAEL.Ferrofluids are stable colloidal solutions of nanoparticles. The influence of a magnetic can generate several hydrodynamic instabilities, either at the interfaces or in the bulk. After a short introduction to ferrofluids, three situations are examined. Surface waves are susceptible to be amplified by a normal magnetic field, leading to the Rosensweig instability. An experiment is designed in order to measure the dispersion relation that is otherwise obtained by linear analysis. The shadowgraph method that is used allows to observe a pre-transitional deformation, as well as azimutal wavelets close to the magnetic field threshold. The wave resistance is the force that puts up against the movement of an object at the surface of a fluid, caused by the emission of surface waves. If its size is much smaller that the capillary length, the wave resistance is to be zero below a critical speed, the minimum velocity of capillary-gravity waves. Beyond, it is an increasing function of the movement speed. An experiment has been designed in order to observe those recent theoretical predictions stemming from linear analysis. Fluids with different viscosities have been employed. With ferrofluids, we show that it is possible to reduce at will the critical velocity. In any case, the critical phenomenon is observed in accordance with the theory. However, the wave resistance seems to be a non-monotonic function of speed. Lastly, the original properties of ferrofluids allow conceiving a magneto-caloric experiment in central geometry, similar to the Rayleigh-Bénard convection. By the use of appropriate scaling laws, a laboratory model is elaborated that simulates the earth mantle convection: the Rayleigh number is in the order of a million. The temperature field is observed by infrared imaging. Moving convection cells are observed, and the Nusselt number is obtained as a function of the Rayleigh number.Les ferrofluides sont des colloïdes stables de nanoparticules. L'action d'un champ magnétique est à même d'engendrer diverses instabilités hydrodynamiques, qui se produisent aux interfaces ou en volume. Après une courte introduction aux ferrofluides, trois situations sont examinées. Les ondes de surface sont amplifiées par un champ magnétique normal à l'interface, conduisant à l'instabilité de Rosensweig. Une expérience est réalisée pour mesurer la relation de dispersion obtenue théoriquement par l'analyse linéaire. La méthode ombroscopique employée permet de visualiser une déformation prétransitionnelle de l'interface, ainsi que l'apparition d'ondes azimutales au voisinage du champ critique de l'instabilité. La résistance de vague est la force qui s'oppose au déplacement d'un objet en surface,causée par l'émission d'ondes de surface. Si sa taille est très petite devant la longueur capillaire, la résistance de vague est nulle en deçà d'une vitesse critique, puis fonction croissante de la vitesse de déplacement. Une expérience a été réalisée afin d'observer ces prédictions théoriques récentes. Des fluides de différentes viscosités ont été employés. Avec des ferrofluides, il est possible de réduire a volo la vitesse critique. Dans tous les cas, le phénomène critique est bien observé conformément à la théorie. Cependant, la résistance de vague paraît être une fonction non-monotone de la vitesse. Enfin, les propriétés originales des ferrofluides permettent de concevoir une expérience de convection magnéto-calorique en géométrie centrale, en tous points semblable à la convection de Rayleigh-Bénard. Moyennant l'utilisation de lois d'échelle appropriées, un modèle de laboratoire est élaboré qui simule la convection dans le manteau terrestre!: le nombre de Rayleigh avoisine le million. Le champ de température est observé par thermographie infrarouge. Des cellules de convection mobiles sont observées, et le nombre de Nusselt est obtenu en fonction du nombre de Rayleigh

A simple function to fit the time evolution of daily reported COVID-19 deaths in EU countries

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

Contribution à la rénovation des programmes de physique (lycée / collège) La nécessaire place des mathématiques dans l'enseignement de la physique

International audienceS 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

La relation de conjugaison et la régression linéaire. Deuxième partie : alternative

International audienceThe 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™

Shear-banding fluid(s) under time-dependent shear flows : Spatiotemporal dynamics (Part I )

International audienceWe 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]

Single atoms with 6000-second trapping lifetimes in optical-tweezer arrays at cryogenic temperatures

We report on the trapping of single Rb atoms in tunable arrays of optical tweezers in a cryogenic environment at approximately 4 K. We describe the design and construction of the experimental apparatus, based on a custom-made UHV-compatible closed-cycle cryostat with optical access. We demonstrate the trapping of single atoms in cryogenic arrays of optical tweezers, with lifetimes up to 6000 s, despite the fact that the vacuum system has not been baked out. These results open the way to large arrays of single atoms with extended coherence, for applications in large-scale quantum simulation of many-body systems and, more generally, in quantum science and technology.K.N.S. acknowledges funding from the Studienstiftung des deutschen Volkes. S.P. is partially supported by the Erasmus+ program of the European Union (EU). D.B. acknowledges support from the Ramón y Cajal program (Grant No. RYC2018-025348-I). This project has received funding from the Région Île-de-France through the Major Interest Domain (DIM) “Science and engineering in Ilede-France Region for Quantum Technologies” (SIRTEQ) project CARAQUES and from the EU Horizon 2020 research and innovation program, under Grant Agreement No. 817482 (Programmable Atomic Large-Scale Quantum Simulation, or PASQuanS).Peer reviewe

Relaxation der doppelbrechung magnetischer nanopartikel während bindungsreaktionen

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.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.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

Shear-banding fluid(s) under time-dependent shear flows. Part II: A test of the Moorcroft–Fielding criteria

International audienceComplex 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 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., 2013, 110, 086001]. 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 visualisations and particle image velocimetry. On the other hand, we analyse previous literature on yield stress fluids exhibiting transient shear banding. Under creep flow, 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 the 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