Yield stress and elasticity influence on surface tension measurements

We have performed surface tension measurements on carbopol gels of different concentrations and yield stresses. Our setup, based on the force exerted by a capillary bridge on two parallel plates, allows to measure an effective surface tension of the complex fluid and to investigate the influence of flow history. More precisely the effective surface tension measured after stretching the bridge is always higher than after compressing it. The difference between the two values is due to the existence of a yield stress in the fluid. The experimental observations are successfully reproduced with a simple elasto-plastic model. The shape of successive stretching-compression cycles can be described by taking into account the yield stress and the elasticity of the gel. We show that the surface tension $\gamma_{LV}$ of yield stress fluids is the mean of the effective surface tension values only if the elastic modulus is high compared to the yield stress. This work highlights that thermodynamical quantities measurements are challenged by the fluid out-of-equilibrium state implied by jamming, even at small scales where the shape of the bridge is driven by surface energy. Therefore setups allowing deformation in opposite directions are relevant for measurements on yield stress fluids.Comment: 12 pages, 16 figures in Soft Matter 201

Structural and cooperative length scales in polymer gels

Understanding the relationship between the material structural details, the geometrical confining constraints, the local dynamical events and the global rheological response is at the core of present investigations on complex fluid properties. In the present article, this problem is addressed on a model yield stress fluid made of highly entangled polymer gels of Carbopol which follows at the macroscopic scale the well-known Herschel-Bulkley rheological law. First, performing local rheology measurements up to high shear rates ($\dot{\gamma} \geq 10^{2}$ s-1)and under confinement, we evidence unambiguously the breakdown of bulk rheology associated with cooperative processes under flow. Moreover, we show that these behaviors are fully captured with a unique cooperativity length $\xi$ over the whole range of experimental conditions. Second, we introduce an original optical microscopy method to access structural properties of the entangled polymer gel in the direct space. Performing image correlation spectroscopy of fluorophore-loaded gels, the characteristic size D of carbopol gels microstructure is determined as a function of preparation protocol. Combining both dynamical and structural information shows that the measured cooperative length $\xi$ corresponds to 2-5 times the underlying structural size D, thus providing a strong grounding to the “Shear Transformation Zones” modeling approach

Capillary rise of yield-stress fluids

In this letter the capillary rise of yield-stress fluids is investigated experimentally. Contrary to the case of simple fluids, the final height reached by yield-stress fluids weakly depends on the gap and is strongly affected by the yield stress and the hydrodynamic boundary conditions. Such dependences suggest that the capillary rise of yield-stress fluids results from a dynamical arrest rather than from a thermodynamic equilibrium. To rationalize these results, we propose and validate an extension of Jurin's law to such complex systems. Finally we show that measuring the final height allows for the determination of both the surface tension and the yield stress

Multicellular aggregates: a model system for tissue rheology

Morphogenetic processes involve cell flows. The mechanical response of a tissue to active forces is linked to its effective viscosity. In order to decouple this mechanical response from the complex genetic changes occurring in a developing organism, we perform rheometry experiments on multicellular aggregates, which are good models for tissues. We observe a cell softening behavior when submitting to stresses. As our technique is very sensitive, we were able to get access to the measurement of a yield point above which a creep regime is observed obtained for strains above 12%. To explain our rheological curves we propose a model for the cytoskeleton that we represent as a dynamic network of parallel springs, which will break under stress and reattach at null strain. Such a simple model is able to reproduce most of the important behavior of cells under strain. We highlight here the importance of considering cells as complex fluids whose properties will vary with time according to the history of applied stress

Quel littoral à l'horizon 2050 ? Un exercice de prospective participative en Languedoc-Roussillon

National audienceDe 2010 à 2012, la DREAL et la préfecture de la région Languedoc-Roussillon ont lancé un exercice de prospective sur l’avenir du littoral régional à l’horizon 2050. Cet article rend compte de cet exercice, fondé sur une démarche participative coanimée par les services de l’État et un groupe de chercheurs. Après un bref rappel du contexte dans lequel fut lancé l’exercice, les auteurs présentent la démarche suivie (choix et hiérarchisation des variables structurantes, construction des micro et des macroscénarios). Ils analysent ensuite les aspects partenariaux et participatifs de cette démarche : relations avec le comité de pilotage, coconstruction des scénarios, profil des participants. Enfin, ils présentent les résultats qui en ont découlé au travers, d’une part des cinq scénarios élaborés sur l’avenir du littoral Languedoc-Roussillon à l’horizon 2050 ; d’autre part des apprentissages et de l’impact de la démarche sur les participants, évalués à partir d’une enquête menée a posteriori auprès des acteurs impliqués — enquête dont les résultats témoignent de la perception très positive de cette démarche prospective collaborative et des dynamiques que celle-ci a pu contribuer à développer chez un certain nombre d’entre eux

Quel littoral à l'horizon 2050 ? Un exercice de prospective participative en Languedoc-Roussillon

National audienceDe 2010 à 2012, la DREAL et la préfecture de la région Languedoc-Roussillon ont lancé un exercice de prospective sur l’avenir du littoral régional à l’horizon 2050. Cet article rend compte de cet exercice, fondé sur une démarche participative coanimée par les services de l’État et un groupe de chercheurs. Après un bref rappel du contexte dans lequel fut lancé l’exercice, les auteurs présentent la démarche suivie (choix et hiérarchisation des variables structurantes, construction des micro et des macroscénarios). Ils analysent ensuite les aspects partenariaux et participatifs de cette démarche : relations avec le comité de pilotage, coconstruction des scénarios, profil des participants. Enfin, ils présentent les résultats qui en ont découlé au travers, d’une part des cinq scénarios élaborés sur l’avenir du littoral Languedoc-Roussillon à l’horizon 2050 ; d’autre part des apprentissages et de l’impact de la démarche sur les participants, évalués à partir d’une enquête menée a posteriori auprès des acteurs impliqués — enquête dont les résultats témoignent de la perception très positive de cette démarche prospective collaborative et des dynamiques que celle-ci a pu contribuer à développer chez un certain nombre d’entre eux

Alpha Waves as a Neuromarker of Autism Spectrum Disorder: The Challenge of Reproducibility and Heterogeneity

International audienceBACKGROUND: There is no consensus in the literature concerning the presence of abnormal alpha wave profiles in patients with autism spectrum disorder (ASD). This may be due to phenotypic heterogeneity among patients as well as the limited sample sizes utilized. Here we present our results of alpha wave profile analysis based on a sample larger than most of those in the field, performed using a robust processing pipeline. METHODS: We compared the alpha waves profiles at rest in children with ASD to those of age-, sex-, and IQ-matched control individuals. We used linear regression and non-parametric normative models using age as covariate forparsing the clinical heterogeneity. We explored the correlation between EEG profiles and the patient's brain volumes, obtained from structural MRI. We automatized the detection of the alpha peak and visually quality controled our MRI measurements. We assessed the robustness of our results by running the EEG preprocessing with two different versions of Matlab as well as Python. RESULTS:A simple linear regression between peak power or frequency of the alpha waves and the status or age of the participants did not allow to identify any statistically significant relationship. The non-parametric normative model (which took account the non-linear effect of age on the alpha profiles) suggested that participants with ASD displayed more variability than control participants for both frequency and amplitude of the alpha peak (p < 0.05). Independent of the status of the individual, we also observed weak associations (uncorrected p < 0.05) between the alpha frequency, and the volumes of several cortical and subcortical structures (in particular the striatum), but which did not survive correction for multiple testing and changed between analysis pelines. DISCUSSIONS: Our study did not find evidence for abnormal alpha wave profiles in ASD. We propose, however, an analysis pipeline to perform standardized and automatized EEG analyses on large cohorts. These should help the community to address the challenge of clinical heterogeneity of ASD and to tackle the problems of reproducibility

Severe traumatic brain injury, frontal lesions, and social aspects of language use: A study of French-speaking adults

International audienceThe purpose of this study was to gain insight into the social (pragmatic) aspects of language use by French-speaking individuals with frontal lesions following a severe traumatic brain injury. Eleven participants with traumatic brain injury performed tasks in three areas of communication: production (interview situation), comprehension (direct requests, conventional indirect requests, and hints), and metapragmatic knowledge. The results of the patients pointed out some strengths (turn-taking in production, and request comprehension, including hints and the speaker's intention) and some weaknesses (topic maintenance in production and metapragmatic knowledge). The patients' good comprehension of requests and their difficulty expressing metapragmatic knowledge suggest that they differ from controls in how they "explain the world": their knowledge of the event sequence was not based on verbally expressible knowledge about the relationship between the structural characteristics of a request utterance and those of its social production context. The pragmatic skills of persons with traumatic brain injury seem to vary across tasks: these individuals have specific strengths and weaknesses in different domains. In addition, marked interindividual differences were noted among the patients: three of them had only one weak point, topic maintenance. These interindividual differences were not systematically linked to performance on executive function tests, but lesion unilaterality (right or left) seems to help preserve patients' pragmatic and metapragmatic skills. The discussion stresses the need to take each patient's strengths and weaknesses into account in designing remediation programs

Cross‐sectional case‐control study of the relationships between pharyngeal compliance and heart rate variability indices in childhood obstructive sleep apnoea

International audienceA combination of noradrenergic and antimuscarinic agents reduces the apnea-hypopnea index (AHI) in adult patients with obstructive sleep apnoea (OSA) via reduced upper airway collapsibility, suggesting that a shift in the sympathovagal balance improves OSA. The objectives of our present case-control study were to assess heart rate variability (HRV) indices in the stages of sleep in children with and without OSA to evaluate OSA-induced sleep HRV modifications and to assess whether increased collapsibility measured during wakefulness is associated with reduced sympathetic activity during non-rapid eye movement (NREM) sleep. Three groups of 15 children were matched by sex, age, z-score of body mass index and ethnicity: non-OSA (obstructive AHI [OAHI] <2 events/hr), mild (OAHI ≥2 to <5 events/hr) or moderate-severe (OAHI ≥5 events/hr) OSA. Pharyngeal compliance was measured during wakefulness using acoustic pharyngometry. HRV indices (time and frequency domain variables) were calculated on 5-min electrocardiography recordings from polysomnography during wakefulness, NREM and REM sleep in periods free of any event. As compared to children without OSA, those with OSA (n = 30) were characterised by increased compliance and no physiological parasympathetic tone increase in REM sleep. Children with increased pharyngeal compliance (n = 21) had a higher OAHI due to higher AHI in NREM sleep, whereas their sympathetic tone was lower than that of those with normal compliance (n = 24). In conclusion, children with increased pharyngeal compliance exhibit decreased sympathetic tone associated with increased AHI in NREM sleep. Therapeutics directed at sympathovagal balance modifications should be tested in childhood OSA