Drops and bubbles in wedges

International audienceWe investigate experimentally the spontaneous motion of drops and bubbles confined between two plates forming a narrow wedge. Such discoidal objects migrate under the gradient in interfacial energy induced by the non-homogeneous confinement. The resulting capillary driving force is balanced by viscous resistance. The viscous friction on a drop bridging parallel plates is estimated by measuring its sliding velocity under gravity. The viscous forces are the sum of two contributions, from the bulk of the liquid and from contact lines, the relative strength of which depends on the drop size and velocity and the physical properties of the liquid. The balance of capillarity and viscosity quantitatively explains the dynamics of spontaneous migration of a drop in a wedge. Close the tip of the wedge, bulk dissipation dominates and the migrating velocity of drops is constant and independent of drop volume. The distance between the drop and the tip of the wedge is thus linear with time t, x(t) ∼ t 0 − t, where t 0 is the time at which the drop reaches the tip of the wedge. Far away from the apex, contact lines dominate the friction, the motion is accelerated toward the tip of the wedge and velocities are higher for larger drops. In this regime, it is shown that x(t) ∼ (t 0 − t) 4/13. The position and time of the crossover between the two dissipation regimes are used to write a dimensionless equation of motion. Plotted in rescaled variables, all experimental trajectories collapse to the prediction of our model. In contrast to drops, gas bubbles in a liquid-filled wedge behave as non-wetting objects. They thus escape the confinement of the wedge to reduce their surface area. The physical mechanisms involved are similar for drops and bubbles, so that the forces acting have the same mathematical structures in both cases, except for the sign of the capillary driving force and a numerical factor. We thus predict and show experimentally that the trajectories of drops and bubbles obey the same equation of motion, except for a change in the sign of t 0 − t

Roughness gradient induced spontaneous motion of droplets on hydrophobic surfaces: A lattice Boltzmann study

The effect of a step wise change in the pillar density on the dynamics of droplets is investigated via three-dimensional lattice Boltzmann simulations. For the same pillar density gradient but different pillar arrangements, both motion over the gradient zone as well as complete arrest are observed. In the moving case, the droplet velocity scales approximately linearly with the texture gradient. A simple model is provided reproducing the observed linear behavior. The model also predicts a linear dependence of droplet velocity on surface tension. This prediction is clearly confirmed via our computer simulations for a wide range of surface tensions.Comment: 6 pages, 8 figure

Impalement transitions in droplets impacting microstructured superhydrophobic surfaces

Liquid droplets impacting a superhydrophobic surface decorated with micro-scale posts often bounce off the surface. However, by decreasing the impact velocity droplets may land on the surface in a fakir state, and by increasing it posts may impale droplets that are then stuck on the surface. We use a two-phase lattice-Boltzmann model to simulate droplet impact on superhydrophobic surfaces, and show that it may result in a fakir state also for reasonable high impact velocities. This happens more easily if the surface is made more hydrophobic or the post height is increased, thereby making the impaled state energetically less favourable.Comment: 8 pages, 4 figures, to appear in Europhysics Letter

Programming stiff inflatable shells from planar patterned fabrics

Lack of stiffness often limits thin shape-shifting structures to small scales. The large in-plane transformations required to distort the metrics are indeed commonly achieved by using soft hydrogels or elastomers. We introduce here a versatile single-step method to shapeprogram stiff inflated structures, opening the door for numerous large scale applications, ranging from space deployable structures to emergency shelters. This technique relies on channel patterns obtained by heat-sealing superimposed flat quasi-inextensible fabric sheets. Inflating channels induces an anisotropic in-plane contraction and thus a possible change of Gaussian curvature. Seam lines, which act as a director field for the in-plane deformation, encode the shape of the deployed structure. We present three patterning methods to quantitatively and analytically program shells with non-Euclidean metrics. In addition to shapes, we describe with scaling laws the mechanical properties of the inflated structures. Large deployed structures can resist their weight, substantially broadening the palette of applications.Comment: 6 pages, 4 figures and Supplementary Information (14 pages, 3 figures

Dominations et résistances au travail. Enquête sur l'expérience corporelle des ouvrières et ouvriers du nettoyage.

This thesis is about relations of dominance and the resistances they cause in cleaning work. More precisely, our subject is the corporeal experience of the cleaning workers. The study is based on an immersion of the author, who worked as a cleaner in three companies – two cleaning companies and a nursery. The data collected during this immersion are completed by interviews and quantitative data analysis. We can consider cleaners as a specific professional group, which is based on a shared social reality. The thesis then highlights an objective structure of the work, relations of dominance, bodies which are re-defined, re-affirmed, or contested, using particularities of cleaning work, especially the fact that cleaning means producing an absence (of dirt). Some cleaners consider themselves « professionals » of cleaning, but a majority do not identify much to their work. Indeed, those individuals are dealing with some wider mecanisms. It is impossible to understand the actions and representations of cleaning workers without considering the fact they are also members of the working class, and evolve in a world which is not ruled only by their specific occupation.Cette thèse s'intéresse aux relations de domination et aux résistances qu'elles provoquent dans le travail de nettoyage. Plus précisément, c'est l'expérience corporelle que font les ouvriers et ouvrières du nettoyage qui est au centre de ce travail. L'enquête s'appuie sur une immersion de l'auteur, qui a travaillé comme nettoyeur dans trois entreprises – deux entreprises de nettoyage et une crèche. Les données recueillies au cours de cette immersion sont complétées par des entretiens et une analyse de données quantitatives. Il est possible de considérer les nettoyeurs et les nettoyeuses comme un groupe professionnel particulier, constitué sur la base d'une réalité sociale partagée. La thèse met alors au jour une structure objective du travail, des relations de domination, des corps qui se redéfinissent, sont réaffirmés ou contestés, en fonction des particularités du nettoyage et tout particulièrement de la nature productrice d'une absence (de sale) de cette activité. Certains nettoyeurs et nettoyeuses se considèrent comme des « professionnels » du secteur, tandis qu'une majorité ne s'identifie que peu par rapport à son travail. Car ces individus sont également aux prises avec des mécanismes plus larges. Il est impossible de saisir le sens des actions et des représentations des ouvriers et ouvrières du nettoyage sans considérer qu'ils appartiennent aux classes populaires, et évoluent au sein d'un monde dont le fonctionnement est loin de n'être régi que par l'activité de travail salarié spécifique qu'ils exercent

Faciliter l'interaction orale en anglais pour sciences humaines et sociales : la revue de presse

Dans ce compte rendu d’expérience de pratique pédagogique en centre de langues, Bénédicte Reyssat nous montre à quel point une activité pédagogique d’apparence anodine, une revue de presse réalisée à l’oral par des étudiants en binôme, peut apporter des réponses concrètes à des questions directement liées à l’enseignement de l’anglais dans le domaine des sciences humaines et sociales (SHS). La première tient à la nature même de ce domaine qui, parce qu’il porte sur des objets très divers, sup..

Flow field around a confined active droplet

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