Europhys. Lett., 74 (2), pp. 306–312 (2006) DOI: 10.1209/epl/i2005-10523-2 EUROPHYSICS LETTERS 15 April 2006 Bouncing transitions on microtextured materials M. Reyssat 1 , A. P´ epin 2 , F. Marty 3 , Y. Chen 2,4 and D. Qu´ er´ e 1 1 Laboratoire de Physique de la Mati` ere Condens´ ee, FRE 2844 du CNRS Coll` ege de France - 75231 Paris Cedex 05, France 2 Laboratoire de Photonique et Nanostructures (LPN) - Route de Nozay 91460 Marcoussis, France 3 Groupe ESIEE, SMM, Cit´ e Descartes - BP 99, 93162 Noisy le Grand, France 4 Ecole Normale Sup´ erieure - 24 rue Lhomond, 75231 Paris Cedex 05, France received 18 November 2005; accepted in final form 14 February 2006 published online 22 March 2006 PACS. 68.08.Bc – Wetting. PACS. 68.03.-g – Gas-liquid and vacuum-liquid interfaces. 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 whole to be hydrophobic. Then, drops are indeed observed to float on the posts [2–4], whose size, distance and height can be varied using photolithography and deep reactive ion etching techniques. On such substrates, the more dilute the pillars, the larger the proportion of air below the drop, and the higher the contact angle. However, if the pillars are too dilute, they cannot sustain anymore the drop, which falls on the bottom surface. Because the surface roughness is moderate in this limit, the contact angle decreases and becomes close to 90 ◦ (its value on a flat hydrophobic surface); on the other hand, the contact angle hysteresis is dramatically increased (owing to the pinning of the liquid inside the cavities), reaching values as high as 100 ◦ [5,6]. The water-repellent state is of course the floating one, and it is useful, on a given substrate, to quantify the robustness of this state. This can be done by pressing on the drop c  EDP Sciences Article published by EDP Sciences and available at http://www.edpsciences.org/epl or http://dx.doi.org/10.1209/epl/i2005-10523-2