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International audience; The leading front of a collectively migrating epithelium often destabilizes into multicellular migration fingers where a cell initially similar to the others becomes a leader cell while its neighbours do not alter. The determinants of these leader cells include mechanical and biochemical cues, often under the control of small GTPases. However, an accurate dynamic cartography of both mechanical and biochemical activities remains to be established. Here, by mapping the mechanical traction forces exerted on the surface by MDCK migration fingers, we show that these structures are mechanical global entities with the leader cells exerting a large traction force. Moreover, the spatial distribution of RhoA differential activity at the basal plane strikingly mirrors this force cartography. We propose that RhoA controls the development of these fingers through mechanical cues: the leader cell drags the structure and the peripheral pluricellular acto-myosin cable prevents the initiation of new leader cells.

International audience; 1 Neural crest cells (NCCs) are a population of multipotent cells that migrate extensively during vertebrate development. Alterations to neural crest ontogenesis cause several diseases, including cancers and congenital defects, such as Hirschprung disease, which results from incomplete colonization of the colon by enteric NCCs (ENCCs). We investigated the influence of the stiffness and structure of the environment on ENCC migration in vitro and during colonization of the gastrointestinal tract in chicken and mouse embryos. We showed using tensile stretching and atomic force microscopy (AFM) that the mesenchyme of the gut was initially soft but gradually stiffened during the period of ENCC colonization. Second-harmonic generation (SHG) microscopy revealed that this stiffening was associated with a gradual organization and enrichment of collagen fibers in the developing gut. Ex-vivo 2D cell migration assays showed that ENCCs migrated on substrates with very low levels of stiffness. In 3D collagen gels, the speed of the ENCC migratory front decreased with increasing gel stiffness, whereas no correlation was found between porosity and ENCC migration behavior. Metalloprotease inhibition experiments showed that ENCCs actively degraded collagen in order to progress. These results shed light on the role of the mechanical properties of tissues in ENCC migration during development. Neural crest cells (NCCs) are essential to vertebrate development. This highly migratory and multipotent population of cells gives rise to craniofacial structures, cardiac and neuroendocrine derivatives, melanocytes, glial cells and neurons of the peripheral nervous system, as well as the intrinsic innervation of the digestive tract, the enteric nervous system (ENS). Different methods have been used to visualize NCCs and their derivatives, ranging from Le Douarin's historical chick-quail neural tube graft method

International audience

The forthcoming work is divided into two distinct parts. The first one deals with self-propelled particles systems. We start by studying the one particle in an external potential case. We derive the nonequilibrium properties of the Active Ornstein-Ulhenbeck Particle model at small persistence time in the presence of thermal noise and the stationary measure of a run-and-tumble particle around a hard spherical obstacle at large persistence time. We then focus on the collective behavior of such systems. From an analytical standpoint, not much is known given their high degree of complexity that combines those of out-of-equilibrium physics to those of strongly correlated liquids. Since the mid-eighties and Frisch’s & al. work, we have known that equilibrium fluids can be studied exactly in the limit where the dimension of the embedding space becomes infinite. The mathematical gains are then considerable: not only the free energy can be obtained analytically but also transport coefficients. These ideas later had a groundbreaking influence on the mean-field theory of the glass transition which is naturally expressed in infinite dimension. Here, the goal is to use the large dimension limit to gain theoretical insights into the behavior of active systems. The equations of the dynamical mean field theory are first studied in the dilute limit and we quantify the connections between the mean-square-displacement and the effective propulsion speed. To go beyond the dilute limit, we then propose an approximate resummation scheme of the Born-Bogolioubov-Green-Kirkwood-Yvon hierarchy of correlation functions. The latter allows us to account for various properties observed in finite dimensional systems, in particular for the Motility Induced Phase Separation and for the linear decrease with density of the effective self-propulsion speed of active hard spheres. We also introduce the concept of effective amplitude of potential interactions. We then show that this amplitude vanishes at the same density as the effective propulsion speed which is also that of the dynamical glass transition of an equilibrium colloidal system with equivalent structure. These results draw interesting links between the glass transition of equilibrium systems and the vanishing of the effective self-propulsion speed which is a stationary property of a unique active system. The specificity underlined by this approximate resummation is the presence of multibody interactions in the steady state measure. Unlike its equilibrium counterpart, it cannot be written as a product over the pairs in the system. The importance of these multibody interactions in the phase behavior of active systems was recently demonstrated in dimension 3. We keep exploring this idea by studying the phase diagram of the unified colored noise approximation of the AOUP dynamics. We show that it displays two regions of phase coexistence that the sole pair interactions are unable to account for. The second part of the manuscript deals with the extension of stochastic calculus to path integration and generalizes results recently obtained in the one-dimensional case. After explaining why it is in general impossible to use the rules of stochastic calculus to change variables within continuous time path integrals we show how to modify these rules consequently. We finally propose a higher-order discretization scheme extending that of Stratonovich and making the rules of standard differential calculus compatible with path integration., Ce travail de thèse se divise en deux parties distinctes. La première est consacrée aux systèmes actifs de particules autopropulsées. Nous commençons par étudier le cas d’une particule dans un potentiel. Nous analysons les déviations par rapport à la dynamique d’équilibre de celle d’une particule active propulsée par un processus d’Ornstein-Ulhenbeck (AOUP) à petit temps de persistance et en présence de bruit thermique ainsi que les propriétés stationnaires d’une particule autopropulsées autour d’un obstacle sphérique dans la limite de grand temps de persistance. Nous nous intéressons ensuite aux propriétés collectives de ce systèmes. D’un point de vue analytique, leur compréhension est pour l’instant entravée par leur diffculté intrinsèque qui combine celles des systèmes hors d’équilibre à celles des liquides fortement corrélés. Depuis le milieu des années 1980, nous savons que les fuides d’équilibres peuvent être étudiés analytiquement dans la limite où la dimension de l’espace ambiant devient infinie. Les gains mathématiques sont alors considérables : non seulement l’énergie libre peut être calculée exactement mais aussi les coeffcients de transport. Ces idées eurent ensuite une influence majeure dans la théorie de la transition vitreuse en champ moyen. L’objectif ici est d’utiliser la limite de grande dimension dans le cas actif. Nous étudions d’abord les équations de la théorie de champ moyen dynamique dans la limite diluée, ce qui nous permet de quantifier la relation entre le déplacement quadratique moyen et la vitesse effective d’autopropulsion. Pour étudier les propriétés des systèmes actifs au-delà de la limite diluée nous proposons ensuite un schéma approché de resommation de la hiérarchie de Born-Bogolioubov-Green-Kirkwood-Yvon des fonctions de corrélation. Celui-ci permet de rendre compte de nombreuses propriétés observées dans les systèmes ac-tifs de dimension finie, en particulier de la transition de phase induite par la motilité et de la décroissance linéaire de la vitesse effective d’autopropulsion des sphères dures actives avec la densité. Ces travaux nous conduisent à introduire le concept d’amplitude effective des interactions potentielles. Nous montrons alors que celle-ci s’annule à la même densité que la vitesse effective d’autopropulsion qui est aussi la densité de transition vitreuse dynamique d’un système colloïdal d’équilibre de structure équivalente. Ces résultats dressent un parallèle intéressant entre la transition vitreuse des systèmes d’équilibre et l’annulation de la vitesse effective d’autopropulsion des systèmes actifs qui est une propriété de la mesure stationnaire d’un système unique. La spécificité soulignée par cette resommation approchée est la présence d’interactions multicorps dans la mesure stationnaire. Contrairement au cas des liquides classiques d’équilibre, celle-ci ne peut en effet pas s’écrire sous la forme d’un produit sur les paires du système. L’importance de ces interactions multicorps dans le diagramme des phases des systèmes actifs a récemment été soulignée en dimension 3. Nous continuons d’explorer cette idée en dimension infinie en étudiant le diagramme des phases de l’approximation dite de bruit coloré unifié de la dynamique AOUP. Nous montrons que celui-ci présente deux régions de coexistence de phase que les interactions de paires seules ne peuvent expliquer. La deuxième partie de cette thèse porte sur des extensions du calcul stochastique dans les intégrales de chemin et généralise des résultats récemment établis dans le cas de processus unidimensionnels. Après avoir expliqué pourquoi il est en général impossible d’utiliser les règles du calcul stochastique pour changer de variable au sein des intégrales de chemin en temps continu nous montrons comment modifier ces dernières enconséquence. Enfin, nous proposons une discrétisation d’ordre supérieur étendant celle de Stratonovich et rendant utilisable le calcul différentiel au sein des intégrales de chemin.

We investigate the behavior of self-propelled particles in infinite space dimensions by comparing two powerful approaches in many-body dynamics: the Fokker-Planck equation and dynamical mean-field theory. The dynamics of the particles at low densities and infinite persistence time is solved in the steady-state with both methods, thereby proving the consistency of the two approaches in a paradigmatic out-of-equilibrium system. We obtain the analytic expression for the pair distribution function and the effective self-propulsion to first order in the density, confirming the results obtained in a previous paper and extending them to the case of a non-monotonous interaction potential. Furthermore, we obtain the transient behavior of active hard spheres when relaxing from equilibrium to the nonequilibrium steady-state. Our results show how collective dynamics is affected by interactions to first order in the density, and point out future directions for further analytical and numerical solutions of this problem., 25 pages, 5 figures

International audience

Research on cerium oxide nanoparticles (nanoceria) has captivated the scientific community due to their unique physical and chemical properties, such as redox activity and oxygen buffering capacity, which made them available for many technical applications, including biomedical applications. The redox mimetic antioxidant properties of nanoceria have been effective in the treatment of many diseases caused by reactive oxygen species (ROS) and reactive nitrogen species. The mechanism of ROS scavenging activity of nanoceria is still elusive, and its redox activity is controversial due to mixed reports in the literature showing pro-oxidant and antioxidant activity. In lieu of its current research interest, it is critical to understand the behaviour of nanoceria in the biological environment and provide answers to some of the critical and open issues. This review critically analyses the status of research on the application of nanoceria to treat diseases caused by ROS. It reviews the proposed mechanism of action and shows the effect of surface coatings on its redox activity. It also discusses some of the crucial issues in deciphering the mechanism and redox activity of nanoceria and suggests areas of future research., 43 pages; 6 figures; 3 tables

Modern dune fields are valuable sources of information for the large-scale analysis of terrestrial and planetary environments and atmospheres, but their study relies on understanding the small-scale dynamics that constantly generate new dunes and reshape older ones. Here we designed a landscape-scale experiment at the edge of the Gobi desert, China, to quantify the development of incipient dunes under the natural action of winds. High-resolution topographic data documenting 42~months of bedform dynamics are examined to provide a spectral analysis of dune pattern formation. We identified two successive phases in the process of dune growth, from the initial flat sand bed to a meter-high periodic pattern. We focus on the initial phase, when the linear regime of dune instability applies, and measure the growth rate of dunes of different wavelengths. We identify the existence of a maximum growth rate, which readily explains the mechanism by which dunes select their size, leading to the prevalence of a 15~m-wavelength pattern. We quantitatively compare our experimental results to the prediction of the dune instability theory using transport and flow parameters independently measured in the field. The remarkable agreement between theory and observations demonstrates that the linear regime of dune growth is permanently expressed on low-amplitude bed topography, before larger regular patterns and slip faces eventually emerge. Our experiment underpin existing theoretical models for the early development of eolian dunes, which can now be used to provide reliable insights into atmospheric and surface processes on Earth and other planetary bodies., 36 pages, 19 figures (supplementary material included)

National audience; Récemment, des chercheurs de l’Université de Paris Saclay ont trouvé des nanotubes de carbone dans les poumons d’enfants asthmatiques vivant en région parisienne : plus précisément dans les cellules des voies respiratoires. Ces nanotubes pourraient provenir de la pollution de l’air et entraîner des effets négatifs sur la santé. Sachant qu’ils sont similaires à ceux présents dans les gaz d’échappement des véhicules, dans les poussières prélevées à différents endroits de la ville mais aussi dans l’air ambiant, ces nanotubes de carbone focalisent d’autant plus l’attention des chercheurs qu’ils ouvrent des perspectives prometteuses, dans le domaine médical notamment. Mais que deviennent-ils une fois qu’ils sont dans l’organisme humain ? Comment se dégradent-ils ? Y a-t-il des mécanismes spécifiques de prise en charge de ces nanoparticules, en fonction de leur forme ou de leur composition ? L’objectif principal du projet CycCar est de comprendre la prise en charge au niveau du lysosome de nanotubes de carbone et de graphène en milieu biologique.

Fabric softeners are widely used to make clothes soft and impart nice smell. Household conditioners are water-based formulations consisting of quaternary ammonium surfactants assembled in vesicles. On the other hand, formulations used for textiles finishing are typically silicone oil emulsions. Here, these technologies are combined in a novel industrial formulation which contains both surfactant vesicles and silicone oil. In particular, we insert an amino-modified silicone oil into a recently developed softener comprising vesicles of an esterquat and guar polymers. Two strategies based on the use of esterquat or guar gum as stabilizers for the silicone incorporation in the fabric softener are proposed. Dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryoTEM), optical and fluorescent microscopy are applied to study the stability and the insertion of the silicone oil into the softener. It is shown that silicone oil is stabilized by the surfactant and the guar gum in nano- or micro-metric droplets. The vesicles structure and properties are not affected by the silicone oil while the formulation stability is preserved. These findings suggest that the proposed route for adding silicone oil into a topical conditioner is a promising route for multipurpose fabric softener development., Comment: 19 pages, 7 figures, 55 references

Strong acoustic streaming can be generated inside a microchannel near sharp-edge structures. In this study, three Sharp-Edge Acoustic Streaming (SEAS) micromixers with multiple sharp edge patterns actuated by piezoelectric transducers are investigated. Direct Numerical Simulation (DNS) is used to numerically solve the multi-physics phenomenon involving acoustics, fluid dynamics and mass transfer. Experiments are carried out to validate the numerical results by visualization, as well as to evaluate micromixing performance with Iodide–Iodate Reactions. Influence of the sharp edge pattern (i.e. the spacing between individual structures, the number of sharp edges), channel throughput as well as acoustic intensity are studied. The shape of flow streamlines first unveils the interaction between acoustic streaming and main flow, which is shown to be a key for mixing enhancement. Following this, an optimal structure is found among the three mixers which allows achieving a decrease of micromixing time from 0.28 s to 0.03 s. Finally, a comparison with literature on passive mixers confirms the micromixing performance of SEAS mixer in terms of micromixing time at low Reynolds flow.

The combination of multiple functionalities in a single material is an appealing strategy for the de-velopment of smart materials with unique features. In this work, we present the preparation of thermoresponsive magnetic nanoparticles and their one-dimensional assembly into transient micro-filaments. The material is based on 9.4 nm iron oxide nanoparticles grafted with poly(N-n-propylacrylamide) via multiphosphonic acid anchoring sites. The hybrid nanoparticles present a low critical solution temperature (LCST) transition between 21 {\deg}C and 28 {\deg}C, depending on the pH and the ionic strength. When heated above the LCST in defined conditions, the nanoparticles ag-gregate and respond to an external magnetic field. An intrinsic characteristic of the thermorespon-sive particles is an asymmetric transition between cooling and heating cycles, that was favorably exploited to build one-dimensional permanent microstructures, such as magnetic microfilaments and cilia. In summary, we present the development of a nanoplatform responsive to multiple stimu-li, including temperature, magnetic field, pH and ionic strength and its transformation into magneti-cally active microfilaments that could find potential applications in remotely controlled devices., Comment: 18 pages, 8 figures

Concentrated fabric conditioners are water-based formulations containing 10 wt. % of cationic surfactants that are deposited on textile fibers during the rinse cycle in a washing machine to make them smoother and softer to touch. In topical formulations, the concentration of cationic surfactants is reduced by half, this reduction being compensated by the addition of environment-friendly polysaccharides. Using atomic force microscopy (AFM), quartz crystal microbalance with dissipation (QCM-D) and ellipsometry, the deposition of formulations with or without polysaccharides on model cellulose substrates is studied. We found that 180 nm long cellulose nanocrystals (CNCs) deposited by spin-coating on amorphous silicon dioxide or on quartz crystal sensors provide a good model of cellulose surfaces. QCM-D results reveal strong electrostatic interactions of the surfactant vesicles and polysaccharides with the CNC layer. In the presence of polysaccharides, the adsorbed quantities are less (by 60%) than the sum of the respective amounts of each component, the structure of the interface being however more homogenous and rigid. This outcome suggests that surface techniques coupled with CNC coated substrates are promising for studying interactions of current formulations with cellulose surfaces., Comment: 21 pages 5 figures

The use of bio-based Phase Change Materials (bio-PCM) is an effective mean to enhance the building’s Thermal Energy Storage (TES) capacity with low-cost and eco-friendly materials. The present study encompasses the results of thermal, physico-chemical and mechanical characterizations of a novel composite bio-PCM developed in the laboratory to improve the thermal comfort in the building’s indoor environments. We tested a non-cocoa Vegetable Fat (VF) recovered from confectionery manufacturing industry. The VF is incorporated into a mineral board prepared from Natural Clay (NC) and Cellulose Fibers (CF). Phase change temperatures and TES capacity of the composite at melting and freezing cycles are measured with Differential Scanning Calorimetry (DSC) to be at 34.83 °C, 22.34 °C and 60.41 J/g, 62.39 J/g respectively. The Scanning Electron Microcopy (SEM) showed that the VF was evenly distributed into the microstructure of the mineral board. Results of thermal stability tests (more than 2000 cycles) confirmed that the composite bio-PCM is thermally and chemically stable. The compressive strength of the supporting material was slightly reduced after the incorporation of the bio-PCM, but it remains within the accepted limits for non-structural building applications. This research on thermal and mechanical properties has revealed that this homemade PCM panel could be a potential candidate for an application in buildings of hot and dry climates.

Fabric conditioners are household products used to impart softness and fragrance to tex-tiles. They are colloidal dispersions of cationic double chain surfactants that self-assemble in vesicles. These surfactants are primarily derived from palm oil chemical modification. Reducing the content of these surfactants allows to obtain products with lower environmental impact. Such a reduction, without adverse effects on the characteristics of the softener and its performance, can be achieved by adding hydrophilic biopolymers. Here, we review the role of guar biopolymers modified with cationic or hydroxyl-propyl groups on the physicochemical properties of the formulation. Electronic and optical microscopy, dynamic light scattering, X-ray scattering and rheology of vesicles dispersion in absence and in presence of guar biopolymers are analyzed. Finally, the deposition of the new formulation on cotton fabrics is examined through scanning electron microscopy and a new protocol based on fluorescent microscopy. With this methodology it is possible to quantify the deposition of surfactants on cotton fibers. The results show that the approach followed here can facilitate the design of sustainable home-care products., Comment: 22 pages, 10 figures, 56 references

Bioinspired flexible blades have been recently shown to significantly improve the versatility of horizontal-axis wind turbines, by widening their working range and increasing their efficiency. The aerodynamic and centrifugal forces bend the blade along its chord, varying the pitch angle by means of non-consuming mechanisms. Here we introduce a general method based on a universal scaling, which finds the optimal soft materials for the blades to maximize the overall turbine efficiency or rotational power, for any required geometry of classical horizontal-axis turbines. The optimization problem, which depends on various parameters, such as the wind velocity, the rotation rate, the density, the rigidity and the geometry of the blade, is reduced to only two dimensionless parameters: the Cauchy number and the centrifugal number. The blade-element momentum theory is coupled to a torsion spring-based model for the blade deformation. Taking into account realistic incoming wind-velocity distributions in the North Sea and a large wind-turbine geometry, we found a significant increase of the total harvested power, up to +35%. In addition, the optimal soft material corresponding to the maximal efficiency over the entire working range for a given wind turbine geometry is, within the limits of small blade deformations, scale-independent. Thus experiments on small wind turbines are a possible way to determine the optimal soft materials for larger ones. These flexible blades are found to be between 5% and 20% lighter than the current rigid blades., Comment: 15 pages, 9 figures

International audience; We consider microscopic models of active particles whose velocities, rotational diffusivities, and tumbling rates depend on the gradient of a local field, which is either externally imposed or depends on all particle positions. Despite the fundamental differences between active and passive dynamics at the microscopic scale, we show that a large class of such tactic active systems admit fluctuating hydrodynamics equivalent to those of interacting Brownian colloids in equilibrium. We exploit this mapping to show how taxis may lead to the lamellar and micellar phases observed for soft repulsive colloids. In the context of chemotaxis, we show how the competition between chemoattractant and chemorepellent may lead to a bona-fide equilibrium liquid-gas phase separation in which a loss of thermodynamic stability of the fluid signals the onset of a chemotactic collapse.

This chapter summarises a series of studies on the critical responses of beginning teachers when presented with questionable explanations in physics. Based on a typology of these responses and some hypotheses on the main factors of critical passivity, suggestions are made to facilitate the activation of a critical attitude in this population. The document recommends enlightening the pedagogical choices of explanations through a multi-criteria analysis. The final discussion draws attention to the crucial importance of the simplicity of an explanation as a criterion of choice for beginning teachers, even at the expense of its coherence.

Strong acoustic streaming in a liquid has been previously observed near sharp-edge structures inside a microchannel. Thanks to its non-invasive characteristic, powerful disturbance into a laminar flow and low-cost equipment requirement, such a sound-driven second-order mean flow has promising potential applications in Process Intensification(PI). Unlike MHz-level ultrasonic wave, the wavelength of the audible acoustic wave in the current study ($\lambda$ ~0.5~m ) is much larger than the characteristic dimensions of the microchannel (0.5~mm), which means the vibration of fluids has the same phase state. In this study, a Y-type (Polydimethylsiloxane, PDMS) micromixer with sharp-edges patterns is pre-fabricated and bonded onto a glass slide. A piezoelectric transducer is stuck next to the channel and it provides an acoustic field at a frequency range of 2 to 3 kHz. The streaming is characterized under various acoustic/flow conditions. Based on fluorescence visualisation analysis and Iodide-Iodate method together with Internal Exchange Model (IEM), both macro- and micro-mixing performance with different amplitudes of acoustic field and sharp edges are quantitatively investigated. It is showed that with the streaming flow around an array of sharp edges or even a single one, two separate flows across the width of the channel can quickly merge into each other with strong $v\_\omega$ acoustic vibration in the channel. In terms of micromixing, under acoustic streaming excitation, the segregation index decreases sharply to 0.01 (good mixing) while that without acoustic streaming is 0.06 (bad mixing). Micromixing time based on IEM decreases by a factor of 10: 0.04s with acoustic streaming instead of 0.3s without streaming. Meanwhile, the mixing enhancement by acoustic streaming is very sensitive to the throughput of the channel. Under weak acoustic field and high throughput, the mixing improvement becomes weak or even negligible.

Plasmodium falciparum gametocytes, the sexual stage responsible for malaria parasite transmission from humans to mosquitoes, are key targets for malaria elimination. Immature gametocytes develop in the human bone marrow parenchyma, where they accumulate around erythroblastic islands. Notably though, the interactions between gametocytes and this hematopoietic niche have not been investigated. Here, we identify late erythroblasts as a new host cell for P falciparum sexual stages and show that gametocytes can fully develop inside these nucleated cells in vitro and in vivo, leading to infectious mature gametocytes within reticulocytes. Strikingly, we found that infection of erythroblasts by gametocytes and parasite-derived extracellular vesicles delay erythroid differentiation, thereby allowing gametocyte maturation to coincide with the release of their host cell from the bone marrow. Taken together, our findings highlight new mechanisms that are pivotal for the maintenance of immature gametocytes in the bone marrow and provide further insights on how Plasmodium parasites interfere with erythropoiesis and contribute to anemia in malaria patients.

Extracellular vesicles (EVs) of endocytic origin are known as exosomes. These vesicles are released by cells and are found in biofluids, such as saliva, urine, and plasma. These vesicles are made up of small RNA, DNA, proteins, and play a vital role in many physiological processes. In the central nervous system (CNS), they participate in various physiological processes such as stress of nerve cells, communication between the cells, synaptic plasticity, and neurogenesis. The role of exosomes in depression needs to be explored further. It is known that exosomes can cross the blood brain barrier (BBB), which is made up of glial cells astrocytes. One of the advantages of these vesicles is that they are able to transfer macromolecules like DNA, protein, mRNAs, and miRNAs to recipient cells. This review focuses on the potential role of exosomes in depression and their utilization as a treatment option or diagnostic tool of depression.

Acoustic waves can generate steady streaming within a fluid owing to the generation of viscous boundary layers near walls, of typical thickness $\delta$. In microchannels, the acoustic wavelength $\lambda$ is adjusted to twice the channel width $w$ to ensure a resonance condition, which implies the use of MHz transducers. Recently though, intense acoustic streaming was generated by acoustic waves of a few kHz (hence with $\lambda \gg w$), owing to the presence of sharp-tipped structures of curvature radius at the tip $r_c$ smaller than $\delta$. The present study quantitatively investigates this sharp-edge acoustic streaming via the direct resolution of the full Navier-Stokes equation, using Finite Element Method. The influence of $\delta$, $r_c$ and viscosity $\nu$ on the acoustic streaming performance are quantified. Our results suggest choices of operating conditions and geometrical parameters, via dimensionless quantities $r_c/\delta$ and $\delta/w$ and provide guidelines on how to obtain strong, optimal sharp-edge acoustic streaming., Comment: 33 pages, 11 figures

Massive foam formation in aquatic environments is a seasonal threat that drastically impacts the stability of marine ecosystems. Because liquid foams are known to filter passive solid particles, with large particles remaining trapped by confinement in the network of liquid channels and small particles being freely advected by the gravity-driven flow, we hypothesized that a similar e↵ect could explain the major shifts in phytoplankton populations observed during foaming episodes. The model unicellular motile algae Chlamydomonas reinhardtii (CR) was incorporated in a bio-compatible foam, and the number of cells escaping the foam at the bottom was measured in time. Comparing the escape dynamics of living and dead CR cells, we found that dead cells are totally advected by the liquid flow towards the bottom of the foam, as expected since the CR diameter remains smaller than the typical foam channel diameter. In contrast, living motile CR cells escape the foam at a significantly lower rate: after two hours, up to 60 % of the injected cells may remain blocked in the foam, while 95 % of the initial liquid volume in the foam has been drained out of the foam. Microscopic observation of the swimming CR cells in a chamber mimicking the cross-section of foam internal channels revealed that swimming CR cells accumulate near channels corners. A theoretical analysis based on the probability density measurements in the micro chambers have shown that this trapping at the microscopic scale contributes to explain the macroscopic retention of the microswimmers in the foam.

Inhaled nanoparticles (< 100 nm) reaching the deep lung region first interact with the pulmonary surfactant, a thin lipid film lining the alveolar epithelium. To date, most biophysical studies have focused on particle induced modifications of the film interfacial properties. In comparison, there is less work on the surfactant bulk properties, and on their changes upon particle exposure. Here we study the viscoelastic properties of a biomimetic pulmonary surfactant in the presence of various engineered nanoparticles. The microrheology technique used is based on the remote actuation of micron-sized wires via the application of a rotating magnetic field and on time-lapse optical micros-copy. It is found that particles strongly interacting with lipid vesicles, such as cationic silica (SiO2, 42 nm) and alumina (Al2O3, 40 nm) induce profound modifications of the surfactant flow proper-ties, even at low concentrations. In particular, we find that silica causes fluidification, while alumi-na induces a liquid-to-soft solid transition. Both phenomena are described quantitatively and ac-counted for in the context of colloidal physics models. It is finally suggested that the structure and viscosity changes could impair the fluid reorganization and recirculation occurring during breath-ing., 19 pages 6 figures, accepted at ACS Nano 11 December 2019

International audience; The Lowry approach (1977) sets the framework for evaluating the meteorological effects of the urban heat island (UHI), by describing it as the superposition of "background", "local" and "urban" climates. In this paper, by adapting this framework to the study of UHI countermeasures, we propose a statistical method suited for assessing their effects in the field. The framework demonstrates that direct comparisons between case and control sites cannot isolate the impacts of UHI countermeasure. It also shows that the interstation differences before and after countermeasure implementation cannot be considered as statistically independent. Consequently, statistical procedures suited for handling dependent observations are necessary such as a linear mixed or fixed effects model. As a case study, experimental data from pavement-watering experiments conducted in Paris since 2013 are used, with the goal of assessing its cooling effects for two different watering strategies. With the fixed effects model, long-lasting statistically-significant effects are found. Results indicate beneficial thermal effects for pedestrians with reductions of UTCI-equivalent temperature up to 2°C, and duration of statistically-significant effects directly linked to the watered surface area. The method is limited by the number of measurements that must be gathered both before and after the UHI countermeasure implementation.

International audience; The rapid cell growth that is associated with the formation of new lateral organs in the shoot apical meristem was linked to an increase in cell wall elasticity but not viscosity in the plant model Arabidopsis thaliana. To investigate the generality of this puzzling relationship, we explored in seven plant species, covering a wide diversity across land plants, the changes in mechanical properties of the cell walls that occur during organ formation. We show that, despite the considerable variation in cell wall composition among the species tested, a drop in cell wall stiffness systematically accompanied primordia formation. We also observed that meristem activity correlates with cell wall elasticity in three species. Thus it seems that cell wall elasticity and growth rate in the meristem are correlated across the land plants.

International audience; Simulations of flows containing non-spherical particles (fibres or ellipsoids) rely on the knowledge of the equation governing the particle motion in the flow. Most models used nowadays are based on the pioneering work of Jeffery (1922), who obtained an equation for the motion of an ellipsoidal particle immersed in a Newtonian fluid, despite the fact that this model relies on strong assumptions: negligible inertia, unconfined flow, dilute regime, flow unperturbed by the presence of the suspended particle, etc. In this work, we propose a dumbbell-based model aimed to describe the motion of an inertial fibre or ellipsoid suspended in a Newtonian fluid. We then use this model to study the orientation kinematics of such particle in a linear shear flow and compare it to the inertialess case. In the case of fibres, we observe the appearance of periodic orbits (whereas inertialess fibres just align in the flow field). For spheroids, our model predicts an orbit drift towards the flow-gradient plane, either gradually (slight inertia) or by first rotating around a moving oblique axis (heavy particles). Multi-Particle Collision Dynamics (MPCD) simulations were carried out to assess the model predictions in the case of inertial fibres and revealed similar behaviours.

In the field of nanomedicine, superparamagnetic nanoparticles are one of the most studied nanomaterials for theranostics. In this study, a one-pot synthesis of magnetic nanoparticles is presented, with an increased control on particle size from 10 to 40 nm. Monitoring of vacuum level is introduced here as a crucial parameter for achieving a fine particle morphology. The magnetic properties of these nanoparticles are highly affected by disorders or mismatches in crystal structure. A prolonged oxidation step is applied to the obtained nanoparticles to transform the magnetic phases into a pure maghemite one, confirmed by high-resolution X-ray photoelectron spectroscopy analysis, by Mössbauer spectrometry and, indirectly, by increased performances in magnetization curves and in relaxation times. Afterward, the attained nanoparticles are transferred into water by a nonderivatized dextran coating. Thermogravimetric analysis confirms that polysaccharide molecules replace oleic acid on the surface by stabilizing the particles in the aqueous phase and culture media. Preliminary in vitro test reveals that the dextran-coated nanoparticles are not passively internalized from the cells. As a proof of concept, a secondary layer of chitosan assures a positive charge to the nanoparticle surface, thus enhancing cellular internalization.

Magnetic nanoparticles (MNPs) internalized within stem cells have paved the way for remote magnetic cell manipulation and imaging in regenerative medicine. A full understanding of their interactions with stem cells and of their fate in the intracellular environment is then required, in particular with respect to their surface coatings. Here, we investigated the biological interactions of MNPs composed of an identical magnetic core but coated with different molecules: phosphonoacetic acid, polyethylene glycol phosphonic carboxylic acid, caffeic acid, citric acid, and polyacrylic acid. These coatings vary in the nature of the chelating function, the number of binding sites, and the presence or absence of a polymer. The nanoparticle magnetism was systematically used as an indicator of their internalization within human stem cells and of their structural long-term biodegradation in a 3D stem cell spheroid model. Overall, we evidence that the coating impacts the aggregation status of the nanoparticles and subsequently their uptake within stem cells, but it has little effect on their intracellular degradation. Only a high number of chelating functions (polyacrylic acid) had a significant protective effect. Interestingly, when the nanoparticles aggregated prior to cellular internalization, less degradation was also observed. Finally, for all coatings, a robust dose-dependent intracellular degradation rate was demonstrated, with higher doses of internalized nanoparticles leading to a lower degradation extent.

International audience; Article type: Full length article Three-dimensional structural evolution of the cuttlefish Sepia officinalis shell from embryo to adult stagesThe cuttlefish shell is an internal structure with a composition and general organization unique among molluscs. Its formation and the structure-function relation are explored during Sepia officinalis development, using Computerized Axial Tomography Scanning (CAT-scan) 3D analyses coupled to physical measurements and modelling. In addition to the evolution of the overall form, modifications of the internal structure were identified from the last third embryonic stages to adult. Most of these changes can be correlated to life cycle stages and environmental constraints. Protected by the capsule during embryonic life, the first internal chambers are sustained by isolated pillars formed from the dorsal to the ventral septum. After hatching, formation of pillars appears to be a progressive process from isolated points to interconnected pillars forming a wall-delineated labyrinthine structure. We analysed the interpillar space, the connectivity and the tortuosity of the pillar network. The labyrinthine pillar network is complete just prior to the wintering migration, probably to sustain the need to adapt to high pressure and to allow buoyancy regulation. At that time, the connectivity in the labyrinth is compensated by an increase in tortuosity, most probably to reduce liquid diffusion in the shell. Altogether these results suggest adjustment of internal calcified structure development to both external forces and physiological needs.

Gold nanoraspberries were synthesized by a seed-mediated synthesis with polyethylene glycol-functionalized bisphosphonates. The original structure shifted the optical absorption to infrared, revealing very efficient photothermal properties within the 2nd biological transparency window and leading to cancer cell necrosis at moderate intracellular doses and low (safe) laser power.

International audience; Figure: From left to right: (a) drop on a soft substrate (water on PDMS), (b) structure near contact line, (c) non-linear simulations of the shear stress distribution below the contact line, whose intensity is encoded by the colours. Gel layers bound to a rigid substrate are used in cell culture to control differentiation and migration and to lower the friction and tailor the wetting of solids. Their thickness, often considered a negligible parameter, affects cell mechanosensing or the shape of sessile droplets [1]. We show first that the adjustment of thickness provides control over energy dissipation during the spreading of flowing matter on a gel layer [2]. We combine experiments and theory to uncover a scaling law that predicts the dynamic contact angle between the three phases as a function of the properties of the coating and the velocity of the contact line. We also demonstrate that thickness gradients can guide moving droplets. Our study shows that thickness is another parameter, besides surface energy and substrate mechanics, to tune liquid spreading and wetting on compliant coating, with potential applications to dew collection and free-surface film flow control. In a second step, we investigate numerically and theoretically the problem when the slope of the gel surface becomes non-negligible, the material reaching a regime of finite deformations, with non-linearities of geometrical origin. We show that both statics and dynamics are completely modified, a singular force displacing the well-known Neumann balance of forces at contact line [3]. We provide a general framework allowing to develop approximate analytical calculations, and give general laws governing the static and dynamic contact angle in this complex, non-linear regime. [1] J. Dervaux and L. Limat, "Contact lines on soft solids with uniform surface tension: analytical solutions and double transition for increasing deformability", Proc. R. Soc. A 471, 20140813 (2015) [2] M. Zhao, J. Dervaux, T. Narita, F. Lequeux, L. Limat, and M. Roché, "Geometrical control of dissipation during the spreading of liquids on soft solids"

The role of pulmonary surfactant is to reduce the surface tension in the lungs and to facilitate breathing. Surfactant replacement therapy (SRT) aims at bringing a substitute by instillation into the airways, a technique that has proven to be efficient and lifesaving for preterm infants. Adapting this therapy to adults requires to scale the administered dose to the patient body weight and to in-crease the lipid concentration, whilst maintaining its surface and flow properties similar. Here, we exploit a magnetic wire-based microrheology technique to measure the viscosity of the exogenous pulmonary surfactant Curosurf in various experimental conditions. The Curosurf viscosity is found to increase exponentially with lipid concentration following the Krieger-Dougherty law of colloids. The Krieger-Dougherty behavior also predicts a divergence of the viscosity at the liquid-to-gel transition. For Curosurf the transition concentration is found close to the concentration at which it is formulated (117 g L-1 versus 80 g L-1). This outcome suggests that for SRT the surfactant rheological properties need to be monitored and kept within a certain range. The results found here could help in producing suspensions for respiratory distress syndrome adapted to adults. The present work also demonstrates the potential of the magnetic wire microrheology tech-nique as an accurate tool to explore biological soft matter dynamics., Comment: 16 pages 6 figures submitted

Innovative synthesis routes revolutionized nanomaterial combination and design possibilities resulting in a new generation of fine-tuned nanoparticles featuring exquisite shape and constitution control. However, there is still room for improvement when it comes to the development of multi-functional nanoparticle agents merging a plurality of therapeutic functions to tackle tumors simultaneously by synergic mechanisms. Herein, we report the design of an optimized nanohybrid for cancer tri-therapy featuring a maghemite (γ-Fe2O3) nanoflower-like multicore nanoparticle conceived for efficient magnetic hyperthermia (MHT) and a spiky copper sulfide shell (IONF@CuS) with a high near-infrared (NIR) absorption coefficient suitable for photothermal (PTT) and photodynamic therapy (PDT). Methods: Spiky-like IONF@CuS nanohybrids were obtained through a straightforward and scalable water-based template sacrificial synthesis, which allows the shell shape control by tuning polyvinylpyrrolidone (PVP) concentration. A comprehensive characterization of nanohybrid size, shape and structural properties was carried out by combining complementary TEM, SEM, HR-TEM, EELS, XRD and NTA. The all-in-one therapeutic multi-functionality was assessed on cancer cells and on tumor-bearing nude mice. Results: Tests carried out on IONF@CuS nanohybrid aqueous dispersion demonstrated their impressive efficiency to convert light (conversion coefficient = 42 ± 6 %) and magnetic stimulation (SAR ~ 350 W g-1) into heat as well as to induce concurrent reactive oxygen species (ROS) formation upon laser irradiation. Such capabilities were further confirmed in cellular environment by in vitro tests and at the organism level by in vivo tests in a murine tumor model. Notably, complete tumor regression was obtained for the PTT mode at low Cu concentration. Overall, these results allowed determining windows of applicability for each therapy individually or in combination. Conclusions: Altogether, the obtained data evidence the successful synthesis of a unique tri-therapeutic nanoparticle featuring highly relevant assets for clinical translation such as reduced nanoparticle administered dose, reduced laser power exposure, reduced magnetic field frequency, and the possibility of serial heating cycles and therapy monitoring by photoacoustic (PA) and magnetic resonance imaging (MRI). Furthermore, the integration of the dual heating capability (MHT + PTT) with the PDT insult offers a unique asset to tackle tumors by multiple cytotoxic strategies in order to improve the therapeutic outcome in a broader spectrum of clinical conditions.

International audience; Emerging advances in extracellular vesicle (EV) research brings along new promises for tailoring clinical treatments in order to meet specific disease features of each patient in a personalized medicine concept. EVs may act as regenerative effectors conveying endogenous therapeutic factors from parent cells or constitute a bio-camouflaged delivery system for exogenous therapeutic agents. Physical stimulation may be an important tool in the field of EVs for personalized therapy by powering EV production, loading and therapeutic properties. Physically-triggered EV production is inspired by naturally occurring EV release by shear stress in blood vessels. Bioinspired physically-triggered EV production technologies may bring along high yield advantages combined to scalability assets. Physical stimulation may also provide new prospects for high-efficient EV loading. Additionally, physically-triggered EV theranostic properties brings new hopes for spatio-temporal controlled therapy combined to tracking. Technological considerations related to EV-based personalized medicine and the input of physical stimulation on EV production, loading and theranostic properties will be overviewed herein.

International audience; Targeting TRAIL (Tumor necrosis factor (TNF)-Related Apoptosis-Inducing Ligand) receptors for cancer therapy remains challenging due to tumor cell resistance and poor preparations of TRAIL or its derivatives. Herein, to optimize its therapeutic use, TRAIL was grafted onto iron oxide nanoclusters (NCs) with the aim of increasing its pro-apoptotic potential through nanoparticle-mediated magnetic hyperthermia (MHT) or photothermia (PT). Methods: The nanovector, NC@TRAIL, was characterized in terms of size, grafting efficiency, and potential for MHT and PT. The therapeutic function was assessed on a TRAIL-resistant breast cancer cell line, MDA-MB-231, wild type (WT) or TRAIL-receptor-deficient (DKO), by combining complementary methylene blue assay and flow cytometry detection of apoptosis and necrosis. Results: Combined with MHT or PT under conditions of "moderate hyperthermia" at low concentrations, NC@TRAIL acts synergistically with the TRAIL receptor to increase the cell death rate beyond what can be explained by the mere global elevation of temperature. In contrast, all results are consistent with the idea that there are hotspots, close to the nanovector and, therefore, to the membrane receptor, which cause disruption of the cell membrane. Furthermore, nanovectors targeting other membrane receptors, unrelated to the TNF superfamily, were also found to cause tumor cell damage upon PT. Indeed, functionalization of NCs by transferrin (NC@Tf) or human serum albumin (NC@HSA) induces tumor cell killing when combined with PT, albeit less efficiently than NC@TRAIL. Conclusions: Given that magnetic nanoparticles can easily be functionalized with molecules or proteins recognizing membrane receptors, these results should pave the way to original remote-controlled antitumoral targeted thermal therapies.

International audience; Many photosynthetic microorganisms are able to detect light and move toward optimal intensities. This ability, known as phototaxis, plays a major role in ecology by affecting natural phytoplankton mass transfers and has important applications in bioreactor and artificial microswimmers technologies. Here we show that this property can be exploited to generate macroscopic fluid flows using a localized light source directed toward shallow suspensions of phototactic microorganisms. Within the intensity range of positive phototaxis, algae accumulate beneath the excitation light where collective effects lead to the emergence of radially symmetric convective flows. These flows can thus be used as hydrodynamic tweezers to manipulate small floating objects. At high cell density and layer depth, we uncover a new kind of instability wherein the viscous torque exerted by self-generated fluid flows on the swimmers induces the formation of traveling waves. A model coupling fluid flow, cell concentration and orientation finely reproduces the experimental data.

International audience; The transition from Tayor vortex flow to wavy-vortex flow is revisited. The self-sustaining process (SSP) of Waleffe [Phys. Fluids 9, 883 (1997)] proposes that a key ingredient in transition to turbulence in wall-bounded shear flows is a three-step process involving rolls advecting streamwise velocity, leading to streaks which become unstable to a wavy perturbation whose nonlinear interaction with itself feeds the rolls. We investigate this process in Taylor-Couette flow. The instability of Taylor-vortex flow to wavy-vortex flow, a process which is the inspiration for the second phase of the SSP, is shown to be caused by the streaks, with the rolls playing a negligible role, as predicted by Jones [J. Fluid Mech. 157, 135 (1985)] and demonstrated by Martinand et al. [Phys. Fluids 26, 094102 (2014)]. In the third phase of the SSP, the nonlinear interaction of the waves with themselves reinforces the rolls. We show this both quantitatively and qualitatively, identifying physical regions in which this reinforcement is strongest, and also demonstrate that this nonlinear interaction depletes the streaks.

We investigate here the static finite deformation of two (or more) three-dimensional nonlinear elastic solids which merge in a third medium, defining a triple line ( tl ). The total energy, accounting for elastic, surface and possible gravity potentials is then minimized numerically in order to solve two physical problems of interest: (i) a soft incompressible axisymmetric drop at rest on a stiffer substrate and (ii) a stiff drop at rest upon a softer substrate, both in situations of strong substrate surface tension effects. Because of the interplay between surface and bulk properties, the behavior of the tl is related to another smaller length-scale (typically micrometre scale) than the elastic solids length-scale (typically millimetre scale). A unified deformability parameter relating all scales is introduced and an adaptive finite element discretization of the domain is used. A first example approached by our model is the elasto-wetting problem in which a very soft drop sits on a stiffer elastic solid. The numerical predictions of our model are compared with the analytical results of the linear elastic theory. In particular, our computations show that Laplace pressure , together with the asymmetry of the solid surface tensions (between the wet and dry part of the interface), well account for the rotation of the cusp of the ridge. In the second example, motivated by indentation problems of a nonlinear elastic substrate by a stiff sphere, we analyse the interplay of the solid surface tensions and elasticity at small-scale. The numerical results also show that gravity might have an important effect and must be included in any micro-scale modelling.

We present a wave-memory driven system that exhibits intermittent switching between two propulsion modes in free space. The model is based on a point-like particle emitting periodically cylindrical standing waves. Submitted to a force related to the local wavefield gradient, the particle is propelled, while the wave field stores positional information on the particle trajectory. For long memory, the linear motion is unstable and we observe erratic switches between two propulsive modes : linear motion and diffusive motion. We show that the bimodal propulsion and the stochastic aspect of the dynamics at long time are generated by a Shil'nikov chaos. The memory of the system controls the fraction of time spent in each phase. The resulting bimodal dynamics shows analogies with intermittent search strategies usually observed in living systems of much higher complexity., Comment: 14 pages, 4 figures

International audience; In vivo, the primary molecular mechanotransductive events mechanically initiating cell differentiation remain unknown. Here we find the molecular stretching of the highly conserved Y654-β-catenin-D665-E-cadherin binding site as mechanically induced by tissue strain. It triggers the increase of accessibility of the Y654 site, target of the Src42A kinase phosphorylation leading to irreversible unbinding. Molecular dynamics simulations of the β-catenin/E-cadherin complex under a force mimicking a 6 pN physiological mechanical strain predict a local 45% stretching between the two α-helices linked by the site and a 15% increase in accessibility of the phosphorylation site. Both are quantitatively observed using FRET lifetime imaging and non-phospho Y654 specific antibody labelling, in response to the mechanical strains developed by endogenous and magnetically mimicked early mesoderm invagination of gastrulating Drosophila embryos. This is followed by the predicted release of 16% of β-catenin from junctions, observed in FRAP, which initiates the mechanical activation of the β-catenin pathway process.

Providing appropriate means for heat generation by low intratumoral nanoparticle concentrations is a major challenge for cancer nanotherapy. Here we propose RGD-tagged magnetosomes (magnetosomes@RGD) as a biogenic, genetically engineered, inorganic platform for multivalent thermal cancer treatment. Magnetosomes@RGD are biomagnetite nanoparticles synthesized by genetically modified magnetotactic bacteria thanks to a translational fusion of the RGD peptide with the magnetosomal protein MamC. Magnetosomes@RGD thus combine the high crystallinity of their magnetite core with efficient surface functionalization. The specific affinity of RGD was first quantified by single-cell magnetophoresis with a variety of cell types, including immune, muscle, endothelial, stem and cancer cells. The highest affinity and cellular uptake was observed with PC3 prostatic and HeLa uterine cancer cells. The efficiency of photothermia and magnetic hyperthermia was then compared on PC3 cells. Unexpectedly, photothermia was far more efficient than magnetic hyperthermia, which was almost totally inhibited by the cellular environment. RGD targeting was then assessed in vivo at tumor site, in mice bearing PC3 tumors. As a result, we demonstrate that targeted magnetic nanoparticles could generate heat on a therapeutic level after systemic administration, but only under laser excitation, and successfully inhibit tumor progression.

International audience; The transport of particles in very confined channels in which single file diffusion occurs has been largely studied in systems where the transverse confining potential is smooth. However, in actual physical systems, this potential may exhibit both static corrugations and time fluctuations. Some recent results suggest the important role played by this nonsmoothness of the confining potential. In particular, quite surprisingly, an enhancement of the Brownian motion of the particles has been evidenced in these kinds of systems. We show that this enhancement results from the commensurate effects induced by the underlying potential on the vibrational spectra of the chain of particles, and from the effective temperature associated with its time fluctuations. We will restrict our derivation to the case of low temperatures for which the mean squared displacement of the particles remains smaller than the potential period.

Cerium oxide nanoparticles are known to catalyze the decomposition of reactive oxygen species such as superoxide radical and hydrogen peroxide. Herein, we examine the superoxide dismutase (SOD) and catalase (CAT) mimetic catalytic activities of nanoceria and demonstrate the existence of generic behaviors. For particles of size 4.5, 7.8, 23 and 28 nm, the SOD and CAT catalytic activities exhibit the characteristic shape of a Langmuir isotherm as a function of cerium concentration. Results show that the catalytic effects are enhanced for smaller particles and for the particles with the largest Ce3+ fraction. The SOD-like activity obtained from the different samples is found to superimpose on a single master curve using the Ce3+ surface area concentration as a new variable, indicating the existence of particle independent redox mechanisms. For the CAT assays the adsorption of H2O2 molecules at the particle surface modulates the efficacy of the decomposition process and must be taken into account. We design an amperometry-based experiment to evaluate the H2O2 adsorption at nanoceria surfaces, leading to the renormalization of the particle specific area. Depending on the particle type the amount of adsorbed H2O2 molecules varies from 2 to 20 nm-2. The proposed scalings are predictive and allow determining SOD and CAT catalytic properties of cerium oxide solely from physico-chemical features., Comment: 33 pages, 6 figures, 7 sections in Supplementary Informations, 62 references

International audience

International audience; In order to give insights into how anisotropic nano-objects interact with living cell membranes, and possibly self-assemble, we designed magnetic nanorods with average size around 100 nm x 1µm by assembling iron oxide nanocubes within a polymeric matrix under a magnetic field. We then explored the nano-bio interface at the cell membrane under the influence of a rotating magnetic field. We observed a complex structuration of the nanorods intertwined with the membranes. Unexpectedly, after a magnetic rotating stimulation, the resulting macrorods were able to rotate freely for multiple rotations, revealing the creation of a bio-magnetic torsion pendulum.

T cells are mechanosensitive but the effect of stiffness on their functions is still debated. We characterize herein how human primary CD4+ T cell functions are affected by stiffness within the physiological Young’s modulus range of 0.5 kPa to 100 kPa. Stiffness modulates T lymphocyte migration and morphological changes induced by TCR/CD3 triggering. Stiffness also increases TCR-induced immune system, metabolism and cell-cycle-related genes. Yet, upon TCR/CD3 stimulation, while cytokine production increases within a wide range of stiffness, from hundreds of Pa to hundreds of kPa, T cell metabolic properties and cell cycle progression are only increased by the highest stiffness tested (100 kPa). Finally, mechanical properties of adherent antigen-presenting cells modulate cytokine production by T cells. Together, these results reveal that T cells discriminate between the wide range of stiffness values found in the body and adapt their responses accordingly. DOI: http://dx.doi.org/10.7554/eLife.23190.001, eLife digest Our immune system contains many cells that play various roles in defending the body against infection, cancer and other threats. For example, T cells constantly patrol the body ready to detect and respond to dangers. They do so by gathering cues from their surroundings, which can be specific chemical signals or physical properties such as the stiffness of tissues. Once the T cells are active they respond in several different ways including releasing hormones and dividing to produce more T cells. Tissue stiffness varies considerably between different organs. Furthermore, disease can lead to changes in tissue stiffness. For example, tissues become more rigid when they are inflamed. The stiffness and other physical properties of the surfaces that T cells interact with affect how the cells respond when they detect a threat, but few details are known about exactly how these cues tune T cell responses. Saitakis et al. studied how human T cells respond to artificial surfaces of varying stiffness that mimic the range found in the body. The experiments show that T cells that interact with stiff surfaces become more active than T cells that interact with softer surfaces. However, some responses are more sensitive to the stiffness of the surface than others. For example, the ability of the T cells to release hormones was affected by the whole range of stiffnesses tested in the experiments, whereas only very stiff surfaces stimulated the T cells to divide. These findings show that T cells can detect the stiffness of surfaces in the body and use this to adapt how they respond to threats. Future challenges will be to find out how T cells sense the physical properties of their surroundings and investigate whether cell and tissue stiffness affects immune responses in the body. This will help us to understand how T cells fight infections and other threats, and could be used to develop new ways of boosting these cells to fight cancer and other diseases. DOI: http://dx.doi.org/10.7554/eLife.23190.002

When does a drop, deposited on a cold substrate, stop spreading? Despite the practical relevance of this question, for example, in airplane icing and 3D metal printing, the detailed mechanism of arrest in solidifying spreading drops has remained debated. Here, we consider the spreading and arrest of hexadecane drops of constant volume on two smooth wettable substrates: copper with a high thermal conductivity and glass with a low thermal conductivity. We record the spreading radius and contact angle in time for a range of substrate temperatures. The experiments are complemented by a detailed analysis of the temperature field near the rapidly moving contact line, by means of similarity solutions of the thermo-hydrodynamic problem. Our combined experimental and theoretical results provide strong evidence that the spreading of solidifying drops is arrested when the liquid at the contact line reaches a critical temperature, which is determined by the effect of kinetic undercooling., SCOPUS: ar.j, info:eu-repo/semantics/published

Clusters of magnetic nanoparticles have received considerable interest in various research fields. Their capacity to generate heat under an alternating magnetic field has recently opened the way to applications such as cancer therapy by hyperthermia. This work is an attempt to investigate the collective effects of interacting dipoles embedded in magnetic nano-particles (MNP) to predict their thermal dissipation with a liquid. We first present a general approach, based on the tracking of the microscopic dipole fluctuations, to access to the dissipation spectra of any spatial distribution of MNPs. Without any other assumption that the linear response regime, it is shown that increasing the particle concentration (dipolar interactions) dramatically diminishes and blueshifts the dissipation processes. This effect originates in a predominance of the coupling energy over the Brownian torques, which create a long-range ordering that saturates the response of the system to an external field. Consequently, the particle density is of fundamental importance to the control of the absorption of electromagnetic energy and its subsequent dissipation in the form of heat.

International audience; no abstract