Making superhydrophobic splashes by surface cooling

We study experimentally the enhancement of splashing due to solidification. Investigating the impact of water drops on dry smooth surfaces, we show that the transition velocity to splash can be drastically reduced by cooling the surface below the liquid melting temperature. We find that at very low temperatures (below $-60 ^\circ \rm C$), the splashing behaviour becomes independent of surface undercooling and presents the same characteristics as on ambient temperature superhydrophobic surfaces. This resemblance arises from an increase of the dynamic advancing contact angle of the lamella with surface undercooling, going from the isothermal hydrophilic to the superhydrophobic behaviour. We propose that crystal formation can affect the dynamic contact angle of the lamella, which would explain this surprising transition. Finally, we show that the transition from hydrophilic to superydrophobic behaviour can also be characterized quantitatively on the dynamics of the ejecta

Contact Line Catch Up by Growing Ice Crystals

The effect of freezing on contact line motion is a scientific challenge in the understanding of the solidification of capillary flows. In this letter, we experimentally investigate the spreading and freezing of a water droplet on a cold substrate. We demonstrate that solidification stops the spreading because the ice crystals catch up with the advancing contact line. Indeed, we observe the formation and growth of ice crystals along the substrate during the drop spreading, and show that their velocity equals the contact line velocity when the drop stops. Modelling the growth of the crystals, we predict the shape of the crystal front and show that the substrate thermal properties play a major role on the frozen drop radiusComment: Physical Review Letters, 22 juin 202

Effervescence in champagne and sparkling wines: From bubble bursting to droplet evaporation

When a bubble reaches an air-liquid interface, it ruptures, projecting a multitude of tiny droplets in the air. Across the oceans, an estimated 1018 to 1020 bubbles burst every second, and form the so called sea spray, a major player in earth's climate system. At a smaller scale, in a glass of champagne about a million bubbles nucleate on the wall, rise towards the surface and burst, giving birth to a particular aerosol that holds a concentrate of wine aromas. Based on the model experiment of a single bubble bursting in simple liquids, we depict each step of this effervescence, from bubble bursting to drop evaporation. In particular, we propose simple scaling laws for the jet velocity and the top drop size. We unravel experimentally the intricate roles of bubble shape, capillary waves, gravity, and liquid properties in the jet dynamics and the drop detachment. We demonstrate how damping action of viscosity produces faster and smaller droplets and more generally how liquid properties enable to control the bubble bursting aerosol characteristics. In this context, the particular case of Champagne wine aerosol is studied in details and the key features of this aerosol are identified. We demonstrate that compared to a still wine, champagne fizz drastically enhances the transfer of liquid into the atmosphere. Conditions on bubble radius and wine viscosity that optimize aerosol evaporation are provided. These results pave the way towards the fine tuning of aerosol characteristics and flavor release during sparkling wine tasting, a major issue of the sparkling wine industry

Steady to unsteady dynamics of a vesicle in a flow

International audienceWe investigate the dynamics of a vesicle in a shear flow on the basis of the newly proposed advected field (AF) method [T. Biben and C. Misbah, Eur. Phys. J. E 67, 031908 (2003)]. We also solve the same problem with the boundary integral formulation for the sake of comparison. We find that the AF results presented previously overestimated the tumbling threshold due to the finite size of the membrane, inherent to the AF model. A comparison between the two methods shows that only in the sharp interface limit (extrapolating the results to a vanishing width) the AF method leads to accurate quantitative results. We extensively investigate the tank-treading to tumbling transition, and compare our numerical results to the theory of Keller and Skalak which assumes a fixed ellipsoidal shape for the vesicle. We show that this theory describes correctly the two regimes, at least in two dimensions, even for the quite elongated non-convex shapes corresponding to red blood cells (and therefore far from ellipsoidal), This theory is, however, not fully quantitative. Finally we investigate the effect of a confinement on the tank-treading to tumbling transition, and show that the tumbling regime becomes unfavorable in a capillary vessel, which should have strong effects on blood rheology in confined geometries

Agir pour réduire la vulnérabilité des peuplements de poissons

National audienc

Agir pour réduire la vulnérabilité des peuplements de poissons

National audienc

Chercheurs et gestionnaires d'espaces naturels protégés : des liens à construire

Les espaces naturels protégés (ENP) offrent des terrains privilégiés pour la recherche et l'expertise, dans un contexte marqué par la nécessité, pour les gestionnaires d'ENP, de réagir à des problèmes socio-écologiques complexes de manière scientifiquement fondée et, pour les chercheurs, de démontrer la pertinence sociale et environnementale de leurs travaux. Chercheurs et gestionnaires constituent des groupes professionnels distincts, dotés d'objectifs et de contraintes spécifiques, qui peuvent tirer parti de leurs différences pour améliorer leurs propres pratiques et poser ensemble de nouvelles questions. Toutes les recherches menées dans les ENP ne font pas et n'ont pas nécessairement vocation à faire l'objet de collaborations. Toutefois, le développement de collaborations qui soient considérées satisfaisantes par les deux parties répond au souhait et au besoin croissants des chercheurs et des gestionnaires de davantage et mieux travailler ensemble, dans le respect de leurs spécificités. Il requiert une meilleure interconnaissance entre les deux univers professionnels et la constitution d'une communauté transdisciplinaire à la frontière entre science et gestion. Les collaborations entre chercheurs et gestionnaires peuvent relever de plusieurs modèles : un modèle pratico-centré orienté prioritairement vers l'appréhension des questions des gestionnaires, un modèle sciento-centré orienté prioritairement vers la production de connaissances scientifiques fondamentales ou finalisées, et un modèle hybride qui vise également les deux objectifs