Amphiphobic thin films by sol-gel route: wetting performances, functional behavior and stability

Wettability is a fundamental property of a solid surface, whose control plays an important role in many different industrial sectors from the ceramic one to the aerospace, naval or maritime, to that of electronics devices, pipes lines and so on. In last years, a lot of studies have focused on the possibility of mimicking the high ability easily found in nature to repel water or other non-polar fluids trying to replicate it on synthetic materials. This concept draws inspiration from the hierarchical structure presented by various living organisms (i.e Lotus leaf, gecko, etc) that rely on a combination of surface morphology and chemistry. Synthetic, super-hydrophobic surfaces, with a dramatically improved repellence to water and great application potential, should present static contact angle with water higher than 150° in order to exhibit enhanced self-cleaning attitudes. However, current knowledge suggests that surfaces with low contact angle hysteresis - defined as the difference between the advancing and receding contact angle in dynamic sliding movements of a fluid drop - are required to solve or reduce ice or fouling adhesion in materials withstanding adverse environmental conditions. In addition, design techniques coupling static and dynamic super-hydrophobicity with repellence to oils (termed as "amphiphobic" behaviour) would be extremely relevant to extend the application fields of many products. In this work, different typologies of materials - ceramics, metals, alloys - were processed in order to get special wettability of their surfaces by means of chemical and microstructural modifications at nano-scale: functional top layers, made up of thin, hybrid coatings and deposited by dip coating, provided materials with extreme repellence to water from the static and dynamic point of view. By this way, optically transparent, homogeneous, nanostructured organic/inorganic hybrid coatings, with a thickness in the 200-300 nm range, have been generated by sol-gel method (particle dimensions of about 30 nm), followed by thermal processing and introduction of low energy elements, such fluorine. Static contact angles with water as high as 178°±1° were obtained on ceramic tile, aluminium and alloys, the same materials presenting excellent dewetting phenomena, as certified by the contact angle hysteresis lower than 5°±1°. A good repellence against low surface tension fluids (oils and lubricants) was also obtained, this representing a significant improvement beyond the current state-of-the-art concerning amphiphobic products. Thin coatings were tested in order to investigate their mechanical resistance and durability to wearing phenomena, their anti-frost performances and resistance to chemical attacks. Up today, these results encourage to think that a new class of materials can be planned, bringing great convenience for different kind of industrial applications.