Bouncing transitions on microtextured materials

PACS. 68.08.-p – Liquid-solid interfaces. Abstract. – A drop of water thrown on a super-hydrophobic solid will often bounce off. Here we discuss the conditions to be fulfilled on the surface design (which provides super-hydrophobicity) to observe such a behavior. This allows us to precise how a material can be made water-repellent. We show in particular how the reduction of the scale of the microstruc-ture provides a robust water repellency, and describe some peculiarities of violent shocks on such surfaces. Super-hydrophobic surfaces are obtained by mixing chemistry (these surfaces are hy-drophobic) and physics (they are rough): the presence of a microtexture may enhance dramat-ically the natural hydrophobicity of the material, yielding contact angles larger than 160 ◦ [1]. On such solids, the contact angle hysteresis may be very small (less than 5◦), indicating that the liquid (water, here) hardly interacts with its substrate: the drops stand at the top of the microtextures so that there is mainly air below, which explains both large angles and small hysteresis [2]. A convenient way to check these ideas, and to tune finely the wetting behavior of such substrates, consists of decorating a flat solid with micrometric posts, and treating the