Geometric depolarization in patterns formed by backscattered light

We formulate a framework for the depolarization of linearly polarized backscattered light based on the concept of geometric phase, {\it i.e} Berry's phase. The predictions of this theory are applied to the patterns formed by backscattered light between crossed or parallel polarizers. This theory should be particularly adapted to the situation in which polarized light is scattered many times but predominantly in the forward direction. We apply these ideas to the patterns which we obtained experimentally with backscattered polarized light from a colloidal suspension.Comment: 3 pages and 3 figure

Reply to Comment by S. J. Cox and D. Weaire on Free Drainage of Aqueous Foams: Container Shape Effects on Capillarity and Vertical Gradients

Cox and Weaire [1] rightly emphasize that our solution of the drainage equation for the “Eiffel Tower” geometry does not treat the boundary conditions. There should be a no- flow condition at the top, and, after leakage begins, the liquid fraction should be pegged to εc ≈ 0.36 at the bottom. They then show how approximating the no-flow conditions at the top can improve agreement with numerical solution. But as argued in [2], we maintain that the neglect of capillarity coming from boundary conditions at the bottom dominates, and that this cannot explain our measurements. At short times, capillarity can delay the onset of leakage, and at long times it can counter gravity and retain liquid in the foam indefinitely; in either case, leakage is slower than our approximate solution, contrary to experiment. Therefore, we speculated that the discrepancy arose from neglect of coarsening, whereby the average bubble size increases via gas diffusion from smaller to larger bubbles. This is an important puzzle because, while the drainage equation successfully predicts forced-drainage experiments, it fails dramatically for free-drainage experiment

Yielding and flow of solutions of thermoresponsive surfactant tubes: tuning macroscopic rheology by supramolecular assemblies

International audienceIn this article, we show that stimuli-induced microscopic transformations of self-assembled surfactant structures can be used to tune the macroscopic bulk and interfacial rheological properties. Previously, we had described the formation of micron-sized 12-hydroxystearic acid tubes having a temperaturetunable diameter in the bulk, and also adsorbing at the air-water interface. We report now a detailed study of the bulk and interfacial rheological properties of this solution of thermoresponsive tubes as a function of temperature. In the bulk, the structural modifications of tubes with temperature lead to sharp and non-monotonous changes of rheological behavior. As well, at the air-water interface, the interfacial layer is shifted several times from rigid-like to fluid-like as the temperature is increased, due to morphological changes of the adsorbed interfacial layer. The temperature-induced variations in the fatty acid supramolecular organization and the richness in structural transitions at this microscopic level lead to unique rheological responses in comparison with conventional surfactant systems. Also, this study provides new insights into the required packing conditions for the jamming of anisotropic soft objects and highlights the fact that this system becomes glassy under heating. Due to these unique macroscopic properties both in the bulk and at the interface, this simple system with stimuli-responsive viscoelasticity is of interest for their potential applications in pharmacology or cosmetic formulations

Influence of bubble size and thermal dissipation on compressive wave attenuation in liquid foams

International audienceAcoustic or blast wave absorption by liquid foams is especially efficient and bubble size or liquid fraction optimization is an important challenge in this context. A resonant behavior of foams has recently been observed, but the main local dissipative process is still unknown. In this paper, we evidence the thermal origin of the dissipation, with an optimal bubble size close to the thermal boundary layer thickness. Using a shock tube, we produce typical pressure variation at time scales of the order of the millisecond, which propagates in the foam in linear and slightly non-linear regimes

Free Drainage of Aqueous Foams: Container Shape Effects on Capillarity and Vertical Gradients

The standard drainage equation applies only to foam columns of constant cross-sectional area. Here, we generalize to include the effects of arbitrary container shape and develop an exact solution for an exponential, Eiffel Tower , sample. This geometry largely eliminates vertical wetness gradients, and hence capillary effects, and should permit a clean test of dissipation mechanisms. Agreement with experiment is not achieved at late times, however, highlighting the importance of both boundary conditions and coarsening

Creating a magnetic resonance imaging ontology.

International audienceThe goal of this work is to build an ontology of Magnetic Resonance Imaging. The MRI domain has been analysed regarding MRI simulators and the DICOM standard. Tow MRI simulators have been analysed: JEMRIS, which is developed in XML and C++, has a hierarchical organisation and SIMRI, which is developed in C, has a good representation of MRI physical processes. To build the ontology we have used Protégé 4, owl2 that allows quantitative representations. The ontology has been validated by a reasoner (Fact++) and by a good representation of DICOM headers and of MRI processes. The MRI ontology would improved MRI simulators and eased semantic interoperability

Instabilities in a Liquid-Fluidized Bed of Gas Bubbles

Gas bubbles in an aqueous foam can be unjammed, or fluidized, by introducing a forced flow of the continuous liquid phase at a sufficiently high rate. We observe that the resulting bubble dynamics are spatially inhomogeneous, exhibiting a sequence of instabilities vs increasing flow rate. First irregular swirls appear, then a single convective roll, and finally a series of stratified convection rolls each with a different average bubble size

Utilisation des ontologies dans le contexte de l'Imagerie par Résonance Magnétique

L imagerie médicale et en particulier l Imagerie par Résonance Magnétique (IRM) occupe une place de choix dans les décisions médicales. Malgré des techniques et des pratiques d examens comparables les industriels du secteur utilisent un vocabulaire différent pour décrire les événements de l expérience IRM. Les ontologies permettent de résoudre cette problématique. En les intégrant dans un système informatique nous avons choisi de proposer des solutions innovantes pour trois situations quotidiennes: l annotation d examen, la reconnaissance et la correction d artéfact et l aide à la prescription d examen. Les connaissances du domaine IRM sont issues de la littérature et de la pratique quotidienne. DICOM, élément incontournable à l échange de données en imagerie médicale, a été le point de départ de l élaboration de l ontologie. Les connaissances sur les artéfacts en IRM sont en partie issues d une collaboration avec l université de Texas A&M Temple. Pour l aide à la prescription d examen nous avons choisi une situation clinique représentative la demande d IRM dans le cadre des traumatismes du genou. Les statistiques proviennent d une étude réalisée dans le service de médecine du sport du CHU de Rennes. Les systèmes élaborés permettent aux utilisateurs d utiliser les ontologies sans y être confronté et permettent l analyse de l entête DICOM d une image, l annotation de l image, la comparaison d une image IRM pour la correction d artéfact et l aide à la prescription d IRM est sous la forme d un serveur web permettant à l utilisateur de connaître en fonction des signes cliniques présents, la probabilité d avoir une lésion lors de la réalisation de l IRM. Nous avons démontré la possibilité d utilisation des ontologies pour améliorer l exercice quotidien des praticiens. Les techniques utilisées montre la possibilité d utiliser les ontologies en les associant aux images et aux probabilités. Le système choisi, interface permettra l évolution vers une technologie de type webservice.Magnetic resonance imaging (MRI) is a key examination in medical decision making. Despite MRI technics are slightly similar, each industrial has developed his own vocabulary to describe the MRI experience. Ontologies have been developed to help in such situations. We have decided to create IT solutions using ontology for three daily radiological situations: exams annotation, MRI artifacts recognition and correction and exam appropriateness. The domain knowledge is extracted from literature and everyday practice. DICOM, as key element for data exchange in radiology, has been used to create the ontology. Concerning MRI artifacts, a part of the knowledge comes from a collaborative work with the university of Texas A&M Temple. Concerning exam appropriateness, we have chosen a representative clinical situation: interest of knee MRI in case of knee trauma. The statistical data are coming from a clinical study done in the CHU of Rennes. Our systems allow users to take benefits of ontology without facing it. They give a DICOM header analysis, proposed an image annotation, compare image to correct MRI artifacts and help physicians to judge MRI appropriateness in case of knee trauma. We have demonstrated that ontologies could be used to improve daily practice in radiology and that ontologies could be associated to image and statistical data. Future of this work could be a system transformation into a web service.RENNES1-Bibl. électronique (352382106) / SudocSudocFranceF

Fatal attraction: rare species in the spotlight

The exploitation of rare and endangered species can end in the species's extinction because the increased value people associate with rarity increases the economic incentive to exploit the last individuals, creating a positive feedback loop. This recently proposed concept, called the anthropogenic Allee effect (AAE), relies on the assumption that people do value rarity, but this remains to be established. Moreover, it also remains to be determined whether attraction to rarity is a trait confined to a minority of hobbyists (e.g. wildlife collectors, exotic pet owners) or characteristic of the general public. We estimated how much the general public valued rare species compared with common ones, using five different metrics related to personal investment: time spent, physical effort, unpleasantness, economic investment and risk. We surveyed the visitors of a zoo. To see the rare species, the visitors to the zoo invested more time in searching and contemplation, they were ready to expend more physical effort, they tolerated more unpleasant conditions, they were willing to pay more and, finally, they risked more to obtain (steal) a rare species. Our results provide substantial evidence of how the general public places more value on rare species, compared with common species. This confirms the AAE as an actual process, which in addition concerns a large part of the population. This has important consequences for the conservation of species that are rare now, or that could become so in the future