Application Of Click Chemistry In The Fabrication Of Cactus-Like Hierarchical Particulates For Sticky Superhydrophobic Surfaces

The wettability of solid surfaces is an important property, and it is well-known that the wetting properties of materials are governed by the combination of two factors:the inherent chemical nature and the surface texture. In recent years, superhydrophobic surfaces, with a water contact angle (CA) greater than 150°, have received much attention not only for their significance to fundamental research but also for their important applications in fields ranging from self-cleaning materials to microfluidic devices. There exists two kinds of extremely superhydrophobic cases in nature, that is, "sliding" superhydrophobic lotus leaves with ultralow water sliding resistance and "sticky" superhydrophobic gecko feet with high adhesive force.As is inspired by the finding of gecko foot structure, nano-and submicron-silica materials (14 nm and 300 nm in diameters) were used to fabricate dual-sized structured surfaces via two simple methods. We believe that nano-structures and dual-sized rough structures have a significant impact on the surface wetting properties.1. LBL self-assembly methodA silicon substrate was surface modified with 3-aminopropyltrimethoxysilane (3-APTMS), rendering an amine-terminated surface. Treated with hydrochloric acid, the surface was positively charged and was able to adsorb negatively charged silica particles through electrostatic interactions. Large silica particles (300 nm in diameter) were assembled six times on the protonated 3-APTMS modified silica surface to produce a surface with microscale roughness. After a subsequent modification of the particulate film with 3-APTMS and acid treatment, fumed silica particles (14 nm in diameter) were adsorbed on the particulate film to construct a finer structure on the coarse one, leading to a surface with a burr-like morphology. Heat treatment was then performed to remove possible amine groups left on the surface and to enhance the mechanical property of the particulate film. A hydrophobic surface (CA=145.0±1.5°) was obtained after modification with 7-[(trifluoromethoxyphenylazo)phenoxy]allylene (CF3AZO) via click reaction. And its preparation method, surface structure and wetting properties were studied; the relatively optimum concentration and time of the self-assemble process were obtained.2. Click chemistry methodWhen fabricating azide-functionalized hierarchical structures, alkyne-functionalized silica nanoparticles ca.300 nm in size were used as core and the azide-functionalized primary aggregates were directly clicked onto the sphere surface. It is convenient to control the size of hierarchical particulates and to tune their surface roughness by adjusting the cycles of click reaction.Dual-sized surface roughness, which biomimics the surface topology of sticky superhydrophobic gecko feet, originates from well-defined silica-based cactus-like particulates that are covalently bonded to an alkynyl-treated substrate. After surface modification with fluorinated azobenzene, the resulting hairy hierarchical structure coatings show the static water contact angle as high as 151.6±1.5°and high adhesion to water and outstanding chemical stability. And its preparation method, surface structure and wetting properties were studied.Furthermore, the wetting regime of the hierarchical structures for water droplets was also demonstrated. We believe that the densely packed nanoscale aggregates are the key contributor to the observed high adhesion, presumably by generating large van der Waals'forces from the large surface area in very close contact with water.