Mantle flow drives the subsidence of oceanic plates

International audienceThe subsidence of the seafloor is generally considered as a consequence of its passive cooling and densifying since its formation at the ridge and thus, as a function of lithospheric age only. However, the lithosphere is defined as the thermal boundary layer of mantle convection, which should thus determine its structure. We examined the evolution of the lithosphere structure and depth along trajectories representative of the underlying mantle flow. We show that along these flow lines, the seafloor depth varies as the square root of the distance from the ridge – as given by the boundary layer equation – along the entire plate, without any flattening. Contrary to previous models, no addidional heat supply is required at the base of the lithosphere

À la recherche des biens sous tutelle

La notion de biens sous tutelle (merit goods) introduite par Musgrave en 1957 définit des biens pour lesquels l’évaluation sociale ne provient pas uniquement des informations fournies par les préférences individuelles. L’utilisation d’un argument de type biens sous tutelle pour justifier l’intervention de l’État est très fréquente dans le domaine culturel, la santé, la protection de la biodiversité, etc. Pour autant, la légitimité économique de cet argument demeure controversée : qu’est-ce qui peut justifier que l’État s’érige en tuteur des préférences des individus?L’objet de cet article est de clarifier la place occupée par les biens sous tutelle en économie du bien-être. La première partie définit le concept et présente les problèmes qu’il pose à la théorie économique. Par la suite, nous montrons comment certaines extensions de la théorie, en particulier les avancées proposées par l’économie comportementale, contribuent à asseoir sa légitimité économique tant du point de vue des préférences individuelles que du choix social.Musgrave’s so-called merit goods, introduced in 1957, intend to deal with situations where the social weight or concern of a good differs from the information given by individual preferences. Merit goods are often referred to as a case for government intervention in education, health care or biodiversity protection policies. But theoretical roots of this concept seem a bit fuzzy and are, at best, very controversial. To put it crudely, what –if anything– can justify that government choices rule out individual choices? The paper deals with this issue. What role do merit goods play in economic theory?The first part of the paper defines the concept of merit goods and emphasizes the theoretical issues at stake. Afterward, we show how recent developments in economic theory, especially behavorial economics, provide a useful framework for the recognition of the concept, both as for individual decision making theory as for social choice

Air invasion in a granular layer immersed in a fluid: Morphology and dynamics

We investigate the morphology and dynamics of the region invaded by air injected at the bottom of an immersed granular bed. Previous experimental results point out the formation of a fluidized zone with a parabolic shape which does not depend, in the stationary regime, on the injection flow-rate. By tilting the experiment, we can tune the effective gravity in the system. We show that it does not affect significantly the morphology either. A numerical study made it possible to access the typical height and width of the structure, which are governed by the relative effects of gravity and capillarity. After a brief review on this subject, we propose first, new experimental observations on the air invasion regimes and on the morphology of the fluidized zone, in particular its growth dynamics; then, we complement the previous numerical study by considering the influence of the bottom boundary condition. In particular, we quantify the morphology of the invaded region when the gas is injected in the bulk, thus when air is likely to propagate downwards. These results are of practical importance in the prediction of the morphology of gas invasion in soils, from CO2 sequestration to pollutant propagation

Morphology of air invasion in an immersed granular layer

International audienceWe report a study of the paths formed by a finite volume of air gently injected at the base of an immersed granular material. A two-dimensional model, based on experimental observations, shows that the typical height and width of the region explored by the branched path depends not only on the injected volume V , but also on a dimensionless parameter χ which accounts for the relative effects of the gravity and capillarity. For a given injected volume V , larger gravity effects lead to taller and narrower structures; for a given χ , the typical height and width of the structure scale like V 1/2 and V 1/4 , respectively, while the typical gaseous fraction in the corresponding region increases accordingly like V 1/4 . Such results can be of practical importance: For instance, gas can be trapped on purpose in an underground natural container below a granular slurry. Our results can help in predicting if the gas is likely to reach the free surface and escape the system if the container presents a defect (hole or fracture)

Dynamics of crater formation in immersed granular materials

We report the formation of a crater at the free surface of an immersed granular bed, locally crossed by an ascending gas flow. In 2D, the crater consists of two piles which develop around the location of the gas emission. We observe that the typical size of the crater increases logarithmically with time, independently of the gas emission dynamics. We describe the related granular flows and give an account of the influence of the experimental parameters, especially of the grain size and of the gas flow

Granular friction: Triggering large events with small vibrations

International audienceTriggering large-scale motion by imposing vibrations to a system can be encountered in many situations, from daily-life shaking of saltcellar to silo unclogging or dynamic earthquakes triggering. In the well-known situation of solid or granular friction, the acceleration of imposed vibrations has often been proposed as the governing parameter for the transition between stick-slip motion and continuous sliding. The threshold acceleration for the onset of continuous slip motion or system unjamming is usually found of the order of the gravitational acceleration. These conclusions are mostly drawn from numerical studies. Here, we investigate, in the laboratory, granular friction by shearing a layer of grains subjected to horizontal vibrations. We show that, in contrast with previous results, the quantity that controls the frictional properties is the characteristic velocity, and not the acceleration, of the imposed mechanical vibrations. Thus, when the system is statically loaded, the typical acceleration of the vibrations which trigger large slip events is much smaller than the gravitational acceleration. These results may be relevant to understand dynamic earthquake triggering by small ground perturbations. Granular assemblies are athermal systems, often used as paradigms to study the dynamics of industrial or natural processes, such as fault gouge and earthquake nucleation 1–6. When submitted to mechanical vibrations, granular media can behave as thermalized fluids when the vibrations are large enough, or remain in a solid-like state for small enough vibrations. Such a transition between jammed and unjammed states has been reported to occur when the peak-acceleration characterizing the vibration is of the order of the gravitational acceleration 7,

Venting dynamics of an immersed granular layer

6 pagesInternational audienceAir is injected locally at the base of an immersed granular bed. The gas, which is forced to flow gently through thematerial, creates several paths between the grains.We observe that the latter gas venting results in the emission of bubbles in a localized region at the free surface. Additional experiments, performed in two dimensions, permit a direct visualization of the paths, and a theoretical approach shows that the typical size of the region at the free surface can be accounted for by a diffusionlike process. The diffusion coefficient is expressed as a function of the system parameters

Degassing cascades in a shear-thinning viscoelastic fluid

International audienceWe report the experimental study of the degassing dynamics through a thin layer of shear-thinning viscoelastic fluid when a constant air flow is imposed at its bottom. The fluid is an aqueous solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). Over a large range of parameters, the air is periodically released through a series of successive bubbles, hereafter named cascades. Each cascade is followed by a continuous degassing, lasting for several seconds, corresponding to an open channel crossing the fluid layer. The periodicity between two cascades does not depend on the injected flow rate. Inside one cascade, the properties of the overpressure signal associated with the successive bubbles vary continuously. The pressure threshold above which the fluid starts flowing, fluid deformation and pressure drop due to degassing through the thin fluid layer can be simply described by a Maxwell model

Marie-Antoinette Rouilly Le Chevallier

Graveur-peintre, peintre-graveur et dans les deux cas totalement et à part entière, Marie-Antoinette Rouilly Le Chevallier, née parisienne, est terriblement attachée aux lumières grises, humides et brumeuses des régions d’Île-de-France. Les mêmes éclats, les mêmes transparences et les mêmes sujets traités en peinture, en gravure et à travers ses dessins, se retrouvent et se complètent. Ici en peinture, la lumière explose comme l’éclat d’une lampe trop forte, là dans ses dessins, la lumière se..

Localized instability of a granular layer submitted to an ascending liquid flow

International audienceUsing a very simple experimental setup, we study the response of a thin layer of immersed granular material to an ascending liquid-flow; a pressure difference Delta P is imposed between the two horizontal free surfaces of a thin layer of glass beads, such that the liquid tends to flow upwards, and the resulting flow-rate v is measured. As generally observed in fluidized beds, the layer destabilizes when the pressure force exactly compensates the weight of the grains. At the free surface, one then observes the formation of a localized fountain of granular material the characteristic size of which is found to be proportional to the grain size and, surprizingly, independent of both the flow-rate and the thickness of the granular layer. Simple theoretical arguments account for the main experimental features