Plunging cavities

International audienceWhen a wave breaks, the tip forms a liquid sheet which impinges the base and creates an air cavity which breaks into bubbles. Gomez-Ledesma, Kiger & Duncan (J. Fluid Mech., this issue, vol. 680, 2011, pp. 5-30) have conducted a nice experiment on this problem, enabling them to discuss both the inclination of the jet and the effect of its translation. This work has interesting links with other transient cavities. © 2011 Cambridge University Press

L'homme et l'animal dans le bassin du lac Tchad

L'analyse des relations liant un système pastoral et l'organisation sociale qui l'entretient présente, dans le cadre du bassin du lac Tchad, l'avantage de disposer de toute la gamme des situations agro-pastorales que l'on retrouve dispersées dans les autres domaines de l'Afrique sub-saharienne. Au niveau des aires pastorales ou des unités de déplacement, il est assez aisé de montrer, entre autres aspects, à quel point la mobilité des groupes influence la composition des cheptels possédés ou combien celle des parentèles de propriétaires limite le capital pastoral détenu. En revanche, en prenant en compte tout un système pastoral, les détermininismes liant les hommes et les troupeaux se distendent et semblent dépassés par la récurrence des organisations spatiales naturelles. Toutefois, si le jeu des contraintes d'ordre sous-continental paraît l'emporter, en particulier lors des crises climatiques ou politiques, tous les groupes de solidarité composant le tissu des cultures pastorales sont engagés dans des processus d'appropriation du pouvoir, dans le cadre d'un Etat-nation dépassant les finalités premières de ce que l'économie mondiale attend d'un système pastoral. (Résumé d'auteur

Beating the teapot effect

We investigate the dripping of liquids around solid surfaces in the regime of inertial flows, a situation commonly encountered with the so-called "teapot effect". We demonstrate that surface wettability is an unexpected key factor in controlling flow separation and dripping, the latter being completely suppressed in the limit of superhydrophobic substrates. This unforeseen coupling is rationalized in terms of a novel hydro-capillary adhesion framework, which couples inertial flows to surface wettability effects. This description of flow separation successfully captures the observed dependence on the various experimental parameters - wettability, flow velocity, solid surface edge curvature-. As a further illustration of this coupling, a real-time control of dripping is demonstrated using electro-wetting for contact angle actuation.Comment: 4 pages; movies at http://lpmcn.univ-lyon1.fr/~lbocque

Basin Focal Project Niger

Challenge Program Water and Food: Basin Focal Project NIGER Faced with increasing food and water insecurity as a result of climatic and anthropogenic (demography, land use) changes, the CGIAR Challenge Program for Water and Food commissioned research in 10 river basins to study the links between water, food and poverty. Looking at the Niger river basin, we carry out a diagnosis of the hydrologic and agronomic potential, before attempting to identify how good agricultural water management may reduce vulnerability in the region, and preserve local ecosystems. Major future threats and opportunities, as well as the influence of institutions on water and agricultural development are discussed. The study indicated that while many technical solutions are available and identified, institutional issues as well as generalized poverty undermine their sustained uptake by communities. Further research is required in order to determine how to overcome socio-economic and institutional issues