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| Research Centre China
The wettability of solid surfaces is a very important property governed by both chemical composition and the geometrical microstructure of the surface. Super-hydrophobic surfaces, with a water contact angle greater than 150, have attracted much interest because of potential practical applications. In general, super-hydrophobic surfaces can be produced in two ways: one is to create a rough surface, and the other is to modify the surface with low surface free energy materials such as silicon compounds. And this property is usually enhanced by surface roughness, especially by fractal structures. Now, aligned carbon nanotubes (ACNTs) film with super-hydrophobic and super-lipophobic properties has been successively prepared. The water contact angle is 158. In order to enhance the super-hydrophobic properties, different nanotubes patterns are also fabricated. For example, lotus-like ACNTs film and honeycomb-like ACNTs film, which contain both microstructures and nanostructures together, have been prepared. The water contact angles are 166 and 163, but the sliding angles are 8 and 5, respectively, showing the improvement of super-hydrophobic property. In addition, super-hydrophobic polyacrylonitrile nanofibres are successively produced with the water contact angle of 173. This improvement mainly relies on the surface structure, so there is no need to use fluorinated compound furthermore. In conclusion, super-hydrophobic surface can be prepared by controlling the surface structure with nanotechnology and a new thought is brought about to substitute fluorinated compound at this aspect. Technology 2: Super-Hydrophobic Surfaces with Super Switcher Property The wettability of solid surfaces is a very important property, and is governed by both the chemical composition and geometrical microstructure of the surface. The generation by UV illumination of a super-hydrophilic TiO2 surface with a contact angle (CA) for water of 0° has attracted significant attention. This material has already been successfully applied as a transparent super-hydrophilic coating with anti-fogging and self-cleaning properties. Currently, super-hydrophobic surfaces with water CA higher than 150° are arousing much interest because they will bring great convenience in daily life as well as in many industrial processes. Various phenomena, such as snow sticking, contamination or oxidation, and current conduction, are expected to be inhibited on such a surface. Conventionally, super-hydrophobic surfaces have been produced mainly in two ways. One is to create a rough structure on a hydrophobic surface, and the other is to modify a rough surface by materials with low surface free energy. Up to now, many methods have been developed to produce rough surfaces, including solidification of melted alkylketene dimmer, plasma polymerization/etching of polypropylene in the presence of polytetrafluoroethylene, microwave plasma-enhanced chemical vapour deposition of trimethylmethoxysilane, anodic oxidization of aluminium, immersion of porous alumina gel films in boiling water, mixing of a sublimation material with silica or boehmite, phase separation, and moulding. To obtain super-hydrophobic surfaces, coating with low surface energy materials such as fluoroalkylsilane is often necessary. While the water CA has commonly been used as a criterion for the evaluation of hydrophobicity of a solid surface, this alone is insufficient to assess the sliding properties of water droplets on the surface. A fully super-hydrophobic surface should exhibit both high CA and low sliding angle. Their recent studies on lotus and rice leaves reveal that a super-hydrophobic surface with both a large CA and small sliding angle needs the cooperation of micro- and nanostructures, and the arrangement of the microstructures on this surface can influence the way a water droplet tends to move. These results from the natural world provide a guide for constructing artificial super-hydrophobic surfaces and designing surfaces with controllable wettability. Accordingly, super-hydrophobic surfaces of aligned carbon nanotube films, aligned polymer nanofibres and differently patterned aligned carbon nanotube films have been fabricated. Furthermore, the cooperation between surface micro- and nanostructures and surface modification of poly (N-isopropylacrylamide) gave reversible switching between superhydrophilicity and superhydrophobicity in a narrow temperature range of about 10°C. Poly (N-isopropylacrylamide) will switch from being hydrophilic to being mildly hydrophobic when the temperature is raised from 25° to 40°C. At the lower temperatures, the C=O and N-H groups are partnered by water molecules, and intermolecular hydrogen bonding dominates; when the temperature is raised, intramolecular hydrogen bonding takes over, and the chains adopt a more compact form, so that to eject the water molecules. The transition can be enhanced by depositing the polymer onto patterned silicon substrates. As the pattern size was decreased (finer grooves), an increase in the range of contact angles achieved on switching. This method can be extended to other stimuli-responsive surfaces and have potential applications in many industrial domains including functional textiles, intelligent microfluidic switches, controllable drug release, and thermal-responsive filters. |
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