Wetting on Anisotropically Patterned and Rough Surfaces

Since Young in 1805 described in words the trigonometric relations between the contact angle and the forces acting on a droplet in mechanical equilibrium on a sulid surface, many advances in the description of several aspects of wetting behavior have been done. Besides the recent years developements in the field of micropatterning allowed the production surfaces with chemical and geometrical regular patterns, which make possible a direct test of theoretical models. Beyond the patterns characterized by a global isotropic disposition if the surface asperities and heterogeneities, patterns constituted of series of parallel stripes or reliefs have been produced, introducing an anisotropic element in the substrate. Recently many works focused on the characterization of the anisotropic behavior of droplets on those surfaces. However there is not a complete theory describing the anisotropy of droplets in these conditions. Furthermore most part of previous works study the anisotropy on regular patterns made by micrometric channels. To give a general description of those aspects of the anisotropic behavior which are independent by the nature of the micrometric regular pattern, and to focus on the influence of different wettabilities, in this thesis we studied the anisotropic wetting of droplets sitting on the top of single posts, characterized by flat surfaces and sharp corners, and made with different materials. The anisotropy was quantified by measuring the contact angles and base elongations in the two principal symmetry axis. Measurements were obtained by a homemade apparatus, and the analysis software has been entirely developed in this thesis. The main finding is that the contact angle difference and the base eccentricity show the same relation within the experimental errors regardless of surface wettability. These measurements were complemented by numerical simulations with the Lattice Boltzmann method, which showed a good agreement with experimental results. We also developed a simple geometrical model, valid for small eccentricities which reproduces qualitatively experimental and numerical data. In addition, during this thesis I characterized the wetting properties of thin (isotropic) films of nanostructured titania, and related them to the morphological parameters of the substrates.