14 research outputs found
Dynamique de réorientation et piégeage d'un cristal liquide nématique au contact d'une surface solide
No full textThe aim of this experimental work is to present the reorientationnal dynamics and the orientationnal pinning of a nematic liquid crystal on a solid surface.The first experimental part concerns the study of the reorientationnal dynamics of the 5CB liquid crystal, in its nematic phase, on SiO layers. The observation, under a polarizing optical microscope, of 5CB defect walls motion highlights a solid friction. This friction is opposed to the liquid crystal reorientation on the anchoring layers.In the second part, the precise study of the 5CB/SiO interface shows the existence of an orientationnal quenched disorder. This disorder is characterized by an original angular mapping method. Eventually, the role of the disorder on the dynamics is discussed to explain the nature of the friction torque.Ce travail expérimental aborde la dynamique réorientationnelle et le piégeage orientationnel d'un cristal liquide nématique au contact d'une surface solide.La première partie expérimentale porte sur l'étude de la dynamique de réorientation de cristal liquide 5CB en phase nématique, sur des surfaces de SiO. Le mouvement de parois de défauts du 5CB est observé, en microscopie optique polarisante, et permet notamment de mettre en évidence la présence d'une friction solide. Celle-ci s'oppose à la réorientation du cristal liquide sur les surfaces d'ancrages.Dans la seconde partie, l'étude fine de l'interface 5CB/SiO montre la présence d'un désordre figé orientationnel. Ce désordre est caractérisé par une méthode originale de cartographie angulaire. Nous discutons du rôle du désordre sur la dynamique, pour expliquer finalement la nature du couple de friction
Japon
No full textCe numéro de Perspective entend rendre compte de la richesse des études et des travaux que suscitent la création artistique et le patrimoine japonais. Loin de toute approche endogène ou essentialiste, il s’agit de considérer la thématique au-delà des frontières géographiques de l’archipel et de l’envisager à l’aune des dynamiques d’interactions économiques, culturelles et artistiques entretenues avec le reste du monde, les récits et les imaginaires qu’elle a nourris. La revue fait ainsi état de l’actualité de la discipline de l’histoire de l’art au Japon en proposant des grands débats portant sur les Jōmon, la question de la restauration des monuments historiques, ou encore sur les îles musées et les triennales d’art contemporain. Fidèle à sa ligne éditoriale, la revue ouvre ses pages à des contributions couvrant l’ensemble du spectre chronologique, traitant tant des cosmologies bouddhiques à l’époque médiévale, que de la construction de l’historiographie des avant-gardes, en passant par le renouveau des études sur la peinture populaire et les estampes érotiques, le marché de l’art, ou encore le jardin comme objet de déconstruction des stéréotypes de la culture japonaise. Ce numéro est en vente sur le site du Comptoir des presses d'universités. Comité de rédaction du volume Claire-Akiko Brisset, Hélène Bayou, Philippe Bonnin, Jean-Michel Butel, Mathieu Capel, Jean-Sébastien Cluzel, Michael Lucken, Matthew McKelway, Christophe Marquet, Christian Merlhiot, Manuela Moscatiello, Laurent Nespoulous, Sylviane Pagès, Mary Picone, Julien Rousseau, Cécile Sakai, Marie-Anne Sarda, Veerle Thielemans, Clélia Zerni
Rotational Solid Friction of a Nematic Liquid Crystal
No full textInternational audienceLiquid crystal defects are used as probes to study the local reorientation dynamics of the nematic surface director on SiOx alignment layers. The tracking of the defect's motion reveals the presence of solid friction forces, unexpected in this complex viscous fluid. We identify the director pinning due to a surface quenched disorder as a possible mechanism that gives rise to the measured solid friction
Ultraweak azimuthal anchoring of a nematic liquid crystal on a planar orienting photopolymer
No full textInternational audienceThe search of weak anchoring is an important issue for a whole class of liquid crystal displays. In this paper we present an orienting layer showing unreached weak planar azimuthal anchoring for 4-n-pentyl-4′-cyanobiphenyl nematic liquid crystal (5CB). Azimuthal extrapolation lengths as large as 80μm are easily obtained. Our layers are made with the commercial photocurable polymer Norland optical adhesive 60. The anisotropy of the film is induced by the adsorption of oriented liquid crystal molecules under a 2Tmagnetic field applied parallel to the surfaces. We use the width of surfaceπ-walls and a high-field electro-optical method to measure, respectively, the azimuthal and the zenithal anchorings. The azimuthal anchoring is extremely sensitive to the ultraviolet (UV) dose and it also depends on the magnetic field application duration. On the opposite, the zenithal anchoring is only slightly sensitive to the preparation parameters. All these results are discussed in terms of the adsorption/desorption mechanisms of the liquid crystal molecules on the polymer layer and of the flexibility of the polymer network
Hierarchical wrinkling in a confined permeable biogel
No full textInternational audienceConfined thin surfaces may wrinkle due the growth of an excess of material. Usually the wavelength is set by elasticity or gravity, but here we explore new selection mechanisms based on hydro-dynamics. First, inspired by yoghurt-making processes, we use caseins, a family of milk proteins, as pH-responsive building blocks and the acidulent glucono-δ-lactone, to design a porous biogel film immersed in a confined buoyancy-matched viscous medium. Provided specific boundary conditions, but without any external stimulus, the biogel film spontaneously wrinkles in cascade. Second, using a combination of titration, rheology, light microscopy and confocal microscopy, we demonstrate that, during continuous acidification, the gel first shrinks and then swells, inducing wrinkling. Third, taking into account both Darcy flow through the gel and Poiseuille flow in the surrounding solvent, we develop a model that predicts correctly the wrinkling wavelength. Our results should be universal for acid-induced protein gels as they are based on pH-induced charge stabilisation/destabilisation, and therefore could set a benchmark to gain fundamental insights into wrinkled biological tissues, to texture food or to design surfaces for optical purposes
Listen to Africa : a call from LS Senghor
No full textTranslated from the French by Pierre de FontnouvelleOriginal French title: Léopold Sédar SenghorContents (pages vii-viii) -- Copyright acknowledgements (pages ix-x) -- Foreword (pages xi-xii) -- L.S. Senghor: the man and the author (pages xiii-xxvi) -- Part 1. Entering the Forest of Initiation: the obstacles (pages 1-37) -- Part 2. The night of initiation: the many forms of existential angst (pages 38-75) -- Part 3. The initiatory teaching: presence and transcendence (pages 76-133) -- Conclusion: tradition and universalism (pages 134-139) -- Notes (pages 140-150 -- Glossary (pages 151-152) -- Bibliography (pages 153-159) -- Index (pages 160-162
Episodic encoding is more than the sum of its parts: An fMRI investigation of multifeatural contextual encoding
No full textEpisodic memories are characterized by their contextual richness, yet little is known about how the various features comprising an episode are brought together in memory. Here we employed fMRI and a multidimensional source memory procedure to investigate processes supporting the mnemonic binding of item and contextual information. Volunteers were scanned while encoding items for which the contextual features (color and location) varied independently, allowing activity elicited at the time of study to be segregated according to whether both, one, or neither feature was successfully retrieved on a later memory test. Activity uniquely associated with successful encoding of both features was identified in the intra-parietal sulcus, a region strongly implicated in the support of attentionally mediated perceptual binding. The findings suggest that the encoding of disparate features of an episode into a common memory representation requires that the features be conjoined in a common perceptual representation when the episode is initially experienced
Porous silica beads produced by nanofluid emulsion freezing
No full textInternational audienceIt is shown that porous spherical particles can be obtained via the freezing of silica nanoparticle aqueous suspensions emulsified in a continuous oil phase. After two freezethaw cycles, nanoparticles turn aggregated into flocculated objects with microstructure that depends upon emulsion volume fraction and droplet size. For low volume fractions, regular microspheres are produced while for large ones, irregular beads with several tens of micrometer radius are formed. Electronic microscopy, mercury porosimetry and nitrogen adsorption are used to get insights into these porous particles typical radius, pore size distribution, surface area and pore network structure. All exhibit mesopores that result from inter-nanoparticle spacing after flocculation. An unexpected macroporous domain appears which is not observed when drying non-emulsified suspensions. This macroporosity is interpreted as the signature of dendrite formation during the undercooled period, right before freezing occurs. Beside this additional macroporosity, the protocols presented in this article constitute also promising emulsion-based routes for porous material synthesis with original geometry, chemical composition and porosity
A new experimental set-up for aerosol stability investigations in microgravity conditions
No full textInternational audienceThe temporal and spatial evolution of dispersed media is a fundamental problem in a wide range of physicochemical systems, such as emulsions, suspensions and aerosols. These systems are multiphasic and involve compounds of different densities. They are therefore subject to the influence of gravity which determines the sedimentation rate of their dispersed phase. This effect can be dominant and prevent a detailed study of the phenomena occurring between the constituents themselves, such as the coalescence of drops in emulsions, the evaporation of droplets or the flocculation in suspensions. In this context, the Centre National d'Etudes Spatiales (CNES) has recently supported the development of a new instrument to produce populations of droplets, a few micrometers in radius, under controlled conditions with the objective of allowing a detailed study of their properties in microgravity conditions. The principle of this instrument is to generate, by a fast compression/expansion of air, populations of water droplets and to track their evolution by optical scanning tomography in transmission mode within a volume of approximately 2 mm 3. Parabolic flight experiments have shown the possibility to generate and accurately follow the evolution of populations of several hundred droplets for more than 20 seconds. The first experimental results show that it is possible to study their evaporation kinetics or their motion when imposing Von Karman swirling flows. This work is part of the AEROSOL project of DECLIC-EVO supported by CNES and aims to help the understanding of cloud microphysics which remains a critical open problem in the context of global warming
