Flow field around a confined active droplet

International audienc

Splendeur et misère de l'effet lotus

Hydrophobic surfaces can be made super-hydrophobic by creating a texture on them. This effect, sometimes referred to as the "fakir" effect, is due to air trapped in the structure : the textured substrate behaves as a composite surface made of solid and air, and displays enhanced hydrophobicity. We give evidence for this effect using forests of micro-pillars which we obtained by micro-fabrication techniques. This situation where the drop sits on the top of the micro-pillars is not always the most stable : the liquid can prefer to fill in the texture. We have studied the transition between these two states by different ways : by pressing on drops, by letting drops evaporate and by carrying impact experiments. We have also achieved surfaces displaying gradients in the density of micro-pillars and have discussed the possibility to observe spontaneous movement of droplets. At last, we have studied the wicking of well-defined surfaces by wetting liquids and have inferred dynamics laws of imbibition.Une surface hydrophobe peut être rendue hyperhydrophobe si on la rend rugueuse. Cet effet, que l'on appelle souvent l'effet "fakir", est dû au piégeage d'air dans la structure. Une goutte repose alors sur une surface composite faite de solide et d'air. Nous montrons cet effet sur des surfaces constituées de forêts de micropiliers que nous avons réalisées grâce à des techniques de microfabrication. La situation où la goutte repose sur le sommet des micropiliers n'est pas toujours la plus stable : le liquide peut préférer plutôt remplir la micro-texture. Nous avons étudié la transition de l'état "fakir" à l'état "planté" de différentes manières : par des expériences de presse, d'évaporation et d'impact. Nous avons également réalisé des surfaces à gradient de densité de plots et discuté la possibilité d'observer des mouvements spontanés de gouttes. Enfin, nous avons étudié l'imprégnation de liquides mouillants et en avons tiré des lois dynamiques de l'imprégnation à deux dimensions

Non-adhesive lotus and other hydrophobic materials

International audienc

Contact Angle Hysteresis Generated by Strong Dilute Defects

International audienc

Imbibition in geometries with axial variations

International audienceWhen surface wetting drives liquids to invade porous media or microstructured materials with uniform channels, the penetration distance is known to increase as the square root of time. We demonstrate, experimentally and theoretically, that shape variations of the channel, in the flow direction, modify this ''diffusive'' response. At short times, the shape variations are not significant and the imbibition is still diffusive. However, at long times, different power law responses occur, and their exponents are uniquely connected to the details of the geometry. Experiments performed with conical tubes clearly show the two theoretical limits

Wicking within forests of micropillars

International audienc

Wicking within forests of micropillars

International audienc

Drops onto gradients of texture

International audienc