Influence of Wetting Properties on Diffusion in a Confined Fluid

We briefly discuss how the wetting properties of a fluid/solid interface can indirectly influence the diffusion properties of fluid confined between two solid walls. This influence is related to the variability of the hydrodynamic boundary conditions at the interface, which correlates to the wetting properties.Comment: Paper presented at the ILL workshop "Dynamics in confinment", Grenoble, January 2000 (http://www.ill.fr

Mécanisme de mélange par convection intermittente dans un nuage de bulles confinées

Ce travail s'intéresse au mélange d'un traceur passif peu diffusif dans un nuage homogène de bulles en ascension dans une cellule Hele-Shaw. Le nombre de Reynolds du mouvement relatif des bulles est élevé. L'écoulement peut être considéré comme bidimensionnel et possède une agitation du liquide très particulière, liée principalement aux sillages des bulles ([1], [2]). Nous avons réalisé des expériences de mélange en injectant un traceur fluorescent au sein du nuage de bulles pendant un temps fini. Une technique de mesure LIF originale et adaptée à cet écoulement à bulles a été développée. Elle consiste à éclairer avec un laser un volume de 0,5 mm³, à différentes distances de l'injecteur, et à observer avec une fibre optique la lumière de ce volume qui est transmise à un spectromètre permettant d'analyser la lumière fluorescée. Il est donc possible de remonter à la concentration locale à une fréquence de 250 Hz (résolution temporelle de la mesure). La figure 1.a montre une évolution typique de la concentration du colorant en un point situé à 150 mm au dessus de l'injection. Dans un premier temps, le traceur arrivant dans le volume de mesure, la concentration augmente, la diminution de concentration qui suit, se fait de manière exponentielle ce qui montre que le mélange en cellule Hele-Shaw n'est pas un processus diffusif de type Fickien [3]. De plus, nous observons à une échelle temporelle beaucoup plus courte des fluctuations de concentration très marquées. Cette intermittence est principalement due aux mécanismes intrinsèques au mélange en cellule de Hele-Shaw. Ce mélange se fait principalement par séquences de capture - transport - largage de colorant par les sillages des bulles (figure 1.b). Un modèle de mélange convectif intermittent a été développé et reproduit bien les expériences

No more!: the right of women to live a life free of violence in Latin America and the Caribbean

Includes bibliographyThe report No more! The right of women to live a life free of violence in Latin America and the Caribbean1 is the result of a joint effort by specialist United Nations organizations and entities in Latin America and the Caribbean. To fulfil this mission inspired by resolution 58/185 of the United Nations General Assembly of December 2003,2 the organizations represented in the thirteenth meeting of specialist bodies and other organizations of the United Nations system on the advancement of women in Latin America and the Caribbean (Mar del Plata, Argentina, September 6, 2005) agreed to implement an inter-institutional study on violence against women in all its forms under the coordination of ECLAC. In the same manner as the Secretary General's report relating to the in-depth study of all forms of violence against women, the eradication is sought of one of the most widespread crimes along with an end to the accompanying impunity. Its dissemination and debate throughout all levels of society will help to raise social awareness providing authorities with the resources and instruments needed for its elimination. Just like the Secretary General's report, the regional report will make it clear that to eradicate violence it must unequivocally become a central objective of public agendas: as a human rights issue in the first place, and as an obstacle to development in the second. Advances must be made toward public policies which underline the States's duty of diligence to protect women from violence. The political will must be accompanied with sufficient human, technical and financial resources to articulate and deepen existing efforts for prevention, attention and sanction alike.It is not lack of experience, lack of models or social indifference that explains the weaknesses analyzed in the present document. The principal obstacles lie in the vacillating will of the institutional sphere. The three powers of the State show weakness and a lack of technical, financial and human resources. Secondly, there is the persistence of cultural factors which invade all the spheres of social life, legitimating violence.The negative synergy between institutional violence and the patriarchal culture encourage and promote situations of impunity. These factors are both cause and consequence of the lack of power of women, adolescent girls and female children. Good practices identified in the region show that this circle of impunity can be broken thanks to the establishment of multi-sectoral strategies and with the full participation of women as citizens. The exponential growth of reports, the greater social visibility of forms of violence - including gender violence - added to the low legitimacy of the public institutions mean the programmes that were successful in the early stages of the battle against violence, today require a firm change of direction based on a strong political commitment to bring about institutional reforms in the realms of justice and security and public policies framed in the international legislation on the human rights of women

Mixing induced by a bubble swarm rising through incident turbulence

This work describes an experimental investigation on the mixing induced by a swarm of high Reynolds number air bubbles rising through a nearly homogeneous and isotropic turbulent flow. The gas volume fraction $\alpha$ and the velocity fluctuations $u'_0$ of the carrier flow before bubble injection are varied, respectively, in the ranges $0\leq \alpha \leq 0.93\%$ and 2.3 cm/s $\leq u'_0 \leq 5.5$ cm/s, resulting in a variation of the bubblance parameter $b$ in the range [0, 1.3] ($b = \frac{V_r^2 \alpha}{{u'}_0^2}$, where $V_r$ is the relative rising velocity). Mixing in the horizontal direction can be modelled as a diffusive process, with an effective diffusivity $D_{xx}$. Two different diffusion regimes are observed experimentally, depending on the turbulence intensity. At low turbulence levels, $D_{xx}$ increases with gas volume fraction $\alpha$, while at high turbulence levels the enhancement in $D_{xx}$ is negligible. When normalizing by the time scale of successive bubble passage, the effective diffusivity can be modelled as a sole function of the gas volume fraction $\alpha^* \equiv \alpha/\alpha_c$, where $\alpha_c$ is a theoretically estimated critical gas volume fraction. The present explorative study provides insights into modeling the mixing induced by high Reynolds number bubbles in turbulent flows.Comment: 8 pages, 6 figures (submitted manuscript

Étude des propriétés de transport et de mélange dans les écoulements à bulles

Les réacteurs chimiques impliquant une phase liquide et une phase gazeuse sont couramment utilisés dans l'industrie pétrochimique et biologique car les écoulements à bulles ont de très bonnes propriétés de transfert et de mélange. Cela permet de mêler intimement différents composés et d'optimiser les réactions chimiques. Néanmoins, les mécanismes et les phénomènes mis en jeu dans le mélange au sein d'un écoulement à bulles restent encore mal connus. Ce travail a donc consisté à identifier les différents mécanismes de mélange en écoulement à bulles pour réviser le modèle physique de transport des espèces chimiques. Afin de distinguer et séparer les différents mécanismes, le mélange d'un traceur passif a été étudié dans différentes configurations expérimentales. Premièrement, l'étude du mélange dans un écoulement à bulles fortement confiné dans une cellule de Hele-Shaw a permis de mettre en évidence le mélange par capture du traceur dans les sillages. Ce mécanisme de mélange, fortement intermittent et convectif, s'est révélé être incompatible avec un processus purement diffusif. Deuxièmement, l'étude du mélange dans un essaim de bulles homogène tridimensionnel a été entreprise. Au contraire du cas confiné, le mélange, qui est causé par l'agitation induite par les bulles dans le liquide, est bien de nature diffusive. Nous avons donc pu mesurer les coefficients de diffusion effectifs en fonction de la fraction volumique de gaz. Ces coefficients sont différents dans les directions verticale et horizontale, ce qui traduit le caractère anisotrope du mélange. De plus, ils deviennent constants au-delà d'une certaine valeur de fraction volumique. Pour finir, nous avons considéré le mélange dans un essaim inhomogène de bulles, où se développe une boucle de recirculation du liquide. Dans le cas d'une recirculation modérée, la dispersion du traceur peut être estimée en combinant le mélange résultant de l'agitation des bulles avec l'advection par le mouvement moyen du fluide

Early Jurassic (latest Toarcian) brachiopods from the northeastern margin of the Western Tethys (Central Iran) and their paleobiogeographical significance

Brachiopod fauna from central Iran, recorded in the upper part of the Shemshak Group and attributed to the upper Toarcian (Pseudoradiosa-Aalensis zones), are reported for the first time in Iran. The assemblage recognized includes six different taxa: Homoeorhynchia sepahanensis nov. sp., formally described in this paper, Globirhynchia subobsoleta, Pseudogibbirhynchia sp., Tetrarhynchiidae sp. indet., Monsardithyris? aff. haresfieldensis, and Zeilleria cf. leckenbyi. Analysis of faunal affinities with other paleobiogeographical regions shows a free connection the central Iranian brachiopod fauna with wide areas of the northern shelf margin of the Tethys Ocean. This is due to an apparent disruption of bioprovinciality inferred for the late Toarcian-earliest Aalenian, congruent with a connection through the northern seaway across the peri-Laurasian epicontinental platforms.The present research is partially supported by Research Group VIGROB-167 (University of Alicante)

Temperature dependence of the superconducting gap anisotropy in Bi2_{2}Sr2_{2}Ca1_{1}Cu2_{2}O8+x_{8+x}

We present the first detailed data of the momentum-resolved, temperature dependence of the superconducting gap of $Bi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x}$, complemented by similar data on the intensity of the photoemission superconducting condensate spectral area. The gap anisotropy between the $\Gamma-\bar{M}$ and $\Gamma-X$ directions increases markedly with increasing temperature, contrary to what happens for conventional anisotropic-gap superconductors such as lead. Specifically, the size of the superconducting gap along the $\Gamma-X$ direction decreases to values indistinguishable from zero at temperatures for which the gap retains virtually full value along the $\Gamma-\bar{M}$ direction.Comment: APS_REVTEX. 19 pages, including 8 figures, available upon request. UW-Madison preprin

Critical review on the mechanisms of maturation stress generation in trees

International audienceTrees control their posture by generating asymmetric mechanical stress around the periphery of the trunk or branches. This stress is produced in wood during the maturation of the cell wall. When the need for reaction is high, it is accompanied by strong changes in cell organization and composition called reaction wood, namely compression wood in gymnosperms and tension wood in angiosperms. The process by which stress is generated in the cell wall during its formation is not yet known, and various hypothetical mechanisms have been proposed in the literature. Here we aim at discriminating between these models. First, we summarize current knowledge about reaction wood structure, state and behaviour relevant to the understanding of maturation stress generation. Then, the mechanisms proposed in the literature are listed and discussed in order to identify which can be rejected based on their inconsistency with current knowledge at the frontier between plant science and mechanical engineering

Observation of a van Hove Singularity in Bi2Sr2Ca1Cu2O8+xBi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x} with Angle Resolved Photoemission

We have performed high energy resolution angle-resolved photoemission studies of the normal state band structure of oxygen overdoped $Bi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+x}$. We find that there is an extended saddle point singularity in the density of states along $\Gamma-\bar{M}-Z$ direction. The data also indicate that there is an asymmetry in the Fermi surface for both the $\Gamma-\bar{M}-Z$ and perpendicular directions.Comment: APS_Revtex. 28 pages, including 16 figures, available upon request. UW-Madison preprint#

Structure of liquid and glassy methanol confined in cylindrical pores

We present a neutron scattering analysis of the density and the static structure factor of confined methanol at various temperatures. Confinement is performed in the cylindrical pores of MCM-41 silicates with pore diameters D=24 angstrom and D=35 angstrom. A change of the thermal expansivity of confined methanol at low temperature is the signature of a glass transition, which occurs at higher temperature for the smallest pore. This is an evidence of a surface induced slowing down of the dynamics of the fluid. The structure factor presents a systematic evolution with the pore diameter, which has been analyzed in terms of excluded volume effects and fluid-matrix cross-correlation. Conversely to the case of Van der Waals fluids, it shows that stronger fluid-matrix correlations must be invoked most probably in relation with the H-bonding character of both methanol and silicate surface.Comment: version March 12 200