Wetting properties of flat-top periodically structured superhydrophobic surfaces

In recent years, superhydrophobic surfaces have attracted a considerable amount of attention from the academic and industrial communities. This growing interest is mainly caused by the fundamental physico-chemical theoretical aspects that remain obscure and the number of promising practical applications emerging in a wide range of fields, such as textiles, self-cleaning coatings, and micro-fluidic systems. Superhydrophobic surfaces exhibit simultaneously high static water contact angles and low resistance to liquid motion on the surface (i.e. low contact angle hysteresis). These properties result from the combination of the chemical hydrophobicity of the topmost layers of the surface and its roughness, the latter being the dominant factor. In this PhD work, advantage has been taken of recent advances in micro-/nano-processing technologies to fabricate microstructured surfaces with specific and controlled roughness. This has enabled systematic experimental investigations to be carried out to address some of the still unanswered questions relating to superhydrophobic phenomena. Silicon wafers were microstructured by photolithography in order to obtain periodical distributions of well-defined flat-top obstacles. A gas-phase silanization process was used to cover the prepared microstructured surfaces with a hydrophobic dense mono-layer of perfluorodecyltrichlorosilane. Several series of samples in which each roughness parameter (distance between obstacles, obstacle height, obstacle size, obstacle shape, etc.) was individually varied were fabricated, and static and dynamic contact angle variations as a function of each parameter were studied. The results obtained by water contact angle measurements were compared to the classic Wenzel and Cassie models. The first assumes that the liquid wets the asperities of the rough substrate completely (referred to as wetted state), whilst the second describes the liquid as sitting on a mixture of air and solid (referred to as composite state). By systematically varying a given parameter, a transition between the composite and the wetted regime was observed. Simple thermodynamic considerations based on the energy minimization of the drop-substrate system showed that Cassie and Wenzel contact angles correspond to two energy minima of the system. Experimentally, the Cassie regime is observed when the Cassie angle is the absolute energy minimum, while when the Wenzel angle is the absolute energy minimum, either the Wenzel regime or a metastable Cassie state is observed. The existence of these metastable states is explained theoretically by the energy barrier that the system has to overcome to reach the Wenzel state when a drop is gently deposited on the surface, and good agreement with experimental data is demonstrated. Water and n-hexadecane dynamic contact angle measurements performed in two different configurations, i.e. with a "negligible" and a "NON-negligible" extra-pressure exerted on the drop-substrate system,