Wetting properties of flat-top periodically structured superhydrophobic surfaces

摘要: 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 fundamental physico-chemical theoretical aspects that remain obscure number promising practical applications emerging in wide range fields, such as textiles, self-cleaning coatings, micro-fluidic systems. Superhydrophobic exhibit simultaneously high static water contact angles low resistance to liquid motion on surface (i.e. angle hysteresis). These properties result combination chemical hydrophobicity topmost layers its roughness, latter being dominant factor. this PhD work, advantage has been taken advances micro-/nano-processing technologies fabricate microstructured with specific controlled roughness. enabled systematic experimental investigations be carried out address some still unanswered questions relating phenomena. Silicon wafers were photolithography order obtain periodical distributions well-defined flat-top obstacles. A gas-phase silanization process was used cover prepared hydrophobic dense mono-layer perfluorodecyltrichlorosilane. Several series samples which each roughness parameter (distance between obstacles, obstacle height, size, shape, etc.) individually varied fabricated, dynamic variations function studied. The results obtained measurements compared classic Wenzel Cassie models. first assumes wets asperities rough substrate completely (referred wetted state), whilst second describes sitting mixture air solid composite state). By systematically varying given parameter, transition regime observed. Simple thermodynamic considerations based energy minimization drop-substrate system showed correspond two minima system. Experimentally, observed when absolute minimum, while either or metastable state existence these states explained theoretically barrier overcome reach drop gently deposited surface, good agreement data demonstrated. Water n-hexadecane performed different configurations, i.e. "negligible" "NON-negligible" extra-pressure exerted system, permitted validity model proposed confirmed. They clearly evidenced region stability both regimes, particularly where metastable, really interesting one for all potential self-cleaning, non-sticking surfaces, since only are those characterized very hysteresis, main requirements surface. With reduction size obstacles micrometer range, an increase extent robustness Drop line fragmentation seems most important factor determining states. Results top-surface shape demonstrate length corrugations secondary factors. Studies influence asperity together preliminary but nanostructured substrates, allow proposal conclusions concerning determination macroscopic superhydrophobicity appears vanishes. typical ∼ 50 µm (where "typical size" refers width top-surface) can considered upper limit scale tens nanometers lower limit. comprised several hundred assumed suitable obtaining larger robustness. related appropriate height must sufficiently great prevent meniscus touching bottom vibration, inducing state.