Comments on "The Depth-Dependent Current and Wave Interaction Equations: A Revision"

Equations for the wave-averaged three-dimensional momentum equations have been published in this journal. It appears that these equations are not consistent with the known depth-integrated momentum balance, especially over a sloping bottom. These equations should thus be considered with caution as they can produce erroneous flows, in particular outside of the surf zone. It is suggested that the inconsistency in the equations may arise from the different averaging operators applied to the different terms of the momentum equation. It is concluded that other forms of the momentum equations, expressed in terms of the quasi-Eulerian velocity, are better suited for three dimensional modelling of wave-current interactions.Comment: Paper submitted to J. Phys. Oceanog

Coastal cliff ground motions and response to extreme storm waves

Coastal cliff erosion from storm waves is observed worldwide, but the processes are notoriously difficult to measure during extreme storm wave conditions when most erosion normally occurs, limiting our understanding of cliff processes. Over January–February 2014, during the largest Atlantic storms in at least 60 years with deepwater significant wave heights of 6–8 m, cliff-top ground motions showed vertical ground displacements in excess of 50–100 µm; an order of magnitude larger than observations made previously. Repeat terrestrial laser scanner surveys over a 2 week period encompassing the extreme storms gave a cliff face volume loss of 2 orders of magnitude larger than the long-term erosion rate. The results imply that erosion of coastal cliffs exposed to extreme storm waves is highly episodic and that long-term rates of cliff erosion will depend on the frequency and severity of extreme storm wave impacts

Nouvelles voies de synthèses de matériaux silicones

Cette thèse avait pour but de synthétiser un silicone hybride de forte masse molaire afin d'obtenir, après réticulation, un élastomère présentant une résistance accrue aux solvants et une très bonne tenue thermique. Une étude bibliographique préliminaire a permis d'identifier les différences de propriétés (résistances thermiques, aux solvants, propriétés de surface et mécaniques) des silicones fluorés conventionnels (groupements perfluorés en chaînes pendantes) et des silicones fluorés hybrides (groupements perfluorés au sein de la chaîne). Une première voie d'accès au silicone hybride par polyhydrosilylation a été testée, celle-ci a conduit à la formation d'un réticulat inexploitable dont nous avons étudié la formation. Nous nous sommes ensuite orientés vers une technique de polycondensation, suivant deux voies: l'une entre des groupements silanol catalysée avec un mélange TMG/TFA, l'autre entre des groupements alcoxysilane et hydrogénosilane en présence de tris(pentafluorophényl)borane (B(C6F5)3). Au cours du premier procédé, nous avons mis en évidence la formation d'un gélastomère du fait de l'organisation des briques de base sous forme de phases lamellaires. Le second procédé n'a pas permis d'obtenir des masses molaires suffisamment élevées pour prétendre former un élastomère possédant de bonnes propriétés mécaniques. Nous avons également mis au point un protocole alternatif à la synthèse classique des silicones hybrides en greffant directement des oléfines fluorées sur un PDMS commercial via les groupements méthyl en alpha du silicium. La dernière partie de ce manuscrit est consacrée à la synthèse de PDMS par polymérisation en émulsion, catalysée par le B(C6F5)3, en absence de tensio-actif ainsi qu'à l'identification du mécanisme réactionnel mis en jeuThe aim of this thesis was to synthesize high molecular mass hybrid silicones via reticulation, in order to obtain elastomers with good resistance to solvents and good thermal stability. The bibliographic section provides an overview of the various properties (solvent resistance, thermal stability, mechanical and thermal properties) of conventional fluorinated silicones (perfluorinated groups in hanging chains) and hybrid fluorinated silicones (perfluorinated groups within the chains). A first attempt to prepare hybrid silicone using polyhydrosilylation returned unusable crosslinked material of which the formation was studied. As an alternative, polycondensation between either silanol groups catalysed with a mixture TMG/TFA, or between alkoxysilane and hydrogenosilane groups with tris(pentafluorophenyl)borane (B(C6F5)3) were also studied. In the first instance, the formation of a gelastomer was observed due to the organization of building blocks in lamellar phases. The second pathway returned non-elastomeric materials with poor mechanical properties, mainly due to the very low molecular mass. Radical chemistry was subsequently used to synthesize hybrid silicones via direct grafting of fluorinated olefins on to a commercially available PDMS; this was achieved by utilizing the methyl groups in the alpha position of the silicon. Finally, PDMS was synthesized by emulsion polymerization catalysed with B(C6F5)3 and without surfactant, and the mechanism was identifiedMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Influence des facteurs sociaux sur le stade de découverte d une infection par le VIH et sur la réponse au traitement antirétroviral (étude rétrospective entre 2005 et 2007 dans un hôpital du nord-ouest parisien)

PARIS7-Xavier Bichat (751182101) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

Les économies sont-elles le produit des croyances des économistes?

International audienc

Cationic Polymerization of Hexamethylcyclotrisiloxane in Excess Water

International audienceRing-opening ionic polymerization of cyclosiloxanes in dispersed media has long been discovered, and is nowadays both fundamentally studied and practically used. In this short communication, we show some preliminary results on the cationic ring-opening polymerization of hexamethylcyclotrisiloxane (D3), a crystalline strained cycle, in water. Depending on the catalyst or/and surfactants used, polymers of various molar masses are prepared in a straightforward way. Emphasis is given here on experiments conducted with tris(pentafluorophenyl)borane (BCF), where high-molar polymers were generated at room temperature. In surfactant-free conditions, µm-sized droplets are stabilized by silanol end-groups of thus generated amphiphilic polymers, the latter of which precipitate in the course of reaction through chain extension. Introducing various surfactants in the recipe allows generating smaller emulsions in size with close polymerization ability, but better final colloidal stability, at the expense of low small cycles’ content. A tentative mechanism is finally proposed

Copolycondensation of regular functional silane and siloxane in aqueous emulsion using B(C6F5)3 as a catalyst

International audienceThe copolycondensation of siloxane and silane bearing hydrido and methoxy groups, resp., was carried out in a surfactant-free aq. emulsion, at room temp. and using B(C6F5)3 as a water-tolerant Lewis acid. Several characterization techniques, including triple detection SEC, 29Si NMR and MALDI-TOF, confirmed the generation of linear polymers (no macrocycles), of av. molar masses ranging between 30 and 80 000 g×mol-1, and bearing silanol end-groups. Colloidal evolution with time showed that a stable surfactant-free emulsion was generated before some coalescence occurs by decreasing the content of silanol groups suspected to stabilize the emulsion. The kinetics of reaction was followed by SEC, and revealed the generation of small cycles, along with linear oligomers, before converting them to poly(dimethylsiloxane) chains. A tentative mechanism, where ring-opening polymn. of the most stranded cyclosiloxane and polycondensation of polymer chains are considered, is proposed