Gravitational oscillations of a liquid column

We report gravity oscillations of a liquid column partially immersed in a bath of liquid. We stress in particular some peculiarities of this system, namely (i) the fact that the mass of this oscillator constantly changes with time; (ii) the singular character of the beginning of the rise, for which the mass of the oscillator is zero; (iii) the sources of dissipation in this system, which is found to be dominated at low viscosity by the entrance (or exit) effects, leading to a long-range damping of the oscillations. We conclude with some qualitative description of a second-order phenomenon, namely the eruption of a jet at the beginning of the rise.Comment: 22 pages, pdf. Submitted to Physics of Fluid

Définition d'un panier de biens et services

L'objectif de cette étude est de proposer une méthode de construction d'un panier de biens et services de proximité. Ce panier de biens sera utilisé pour analyser les disparités d'accès aux équipements de proximité dans les aires urbaines, selon le quartier dans lequel on habite, et selon la catégorie sociale à laquelle on appartient. Cette analyse sera diffusée dans un autre rapport d'études.Le panier de biens est constitué d'enregistrements du fichier Sirene de l'Insee, géolocalisés à l'Iris. Les activités économiques sont choisies pour représenter au mieux les biens et services fréquentés par les individus dans leur vie quotidienne.Ce rapport d'étude retranscrit les analyses et choix faits pour cette proposition de panier de biens.observation urbaine, équipement de proximité

Explosions at the water surface

International audienceWe study the shape and dynamics of cavities created by the explosion of firecrackers at the surface of a large pool of water. Without confinement, the explosion generates a hemispherical air cavity which grows, reaches a maximum size and collapses in a generic w-shape to form a final central jet. When a rigid open tube confines the firecracker, the explosion produces a cylindrical cavity that expands without ever escaping the free end of the tube. We discuss a potential flow model, which captures most of these feature

A universal law for capillary rise in corners

International audienceWe study the capillary rise of wetting liquids in the corners of different geometries and show that the meniscus rises without limit following the universal law: h(t)/a ≈ (ɣt/na)⅓, where ɣ and n stand for the surface tension and viscosity of the liquid while a =√γ /ρɣ g is the capillary length, based on the liquid density p and gravity g. This law is universal in the sense that it does not depend on the geometry of the corner. © 2011 Cambridge University Press

Coating of a textured solid

International audienceWe discuss how a solid textured with well-defined micropillars entrains a film when extracted out of a bath of wetting liquid. At low withdrawal velocity V, it is shown experimentally that the film exactly fills the gap between the pillars; its thickness hd is independent of V and corresponds to the pillar height hp. At larger velocity, hd slowly increases with V and tends towards the Landau-Levich-Derjaguin (LLD) thickness h LLD observed on a flat solid. We model the entrainment by adapting the LLD theory to a double layer consisting of liquid trapped inside the texture and covered by a free film. This model allows us to understand quantitatively our different observations and to predict the transition between hp and hLLD. © 2011 Cambridge University Press

Magnetic control of Leidenfrost drops

International audienceWe show how a magnetic field can influence the motion of a paramagnetic drop made of liquid oxygen in a Leidenfrost state on solids at room temperature. It is demonstrated that the trajectory can be modified in both direction and velocity and that the results can be interpreted in terms of classical mechanics as long as the drop does not get too close to the magnet. We study the deviation and report that it can easily overcome 180 degrees and even diverge under certain conditions, leading to situations where a drop gets captured. In the vicinity of the magnet, another type of trapping is observed, due to the deformation of the drop in this region, which leads to a strong energy dissipation. Conversely, drops can be accelerated by moving magnets (slingshot effect)

Flexible scraping of viscous fluids

International audienceWe study the thickness h(d) of the liquid film left on a wet surface after scraping it with an elastic wiper (length L, rigidity B) moved at a velocity V. The scraper is clamped vertically at a given distance above the substrate, and h(d) is maximal when the tip of the scraper is just tangent to the surface. We show experimentally and theoretically that this maximum thickness is hmax similar or equal to 0.33L (eta VL2/B)(3/4) where eta is the liquid viscosity. The deposition law is found to be sensitive to the shape of the wiper: the film thickness can also be tuned by using wipers with a permanent curvature, and varying this curvature