Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2004, 43 (33), 4338—4341
Plastic with a Lotus Effect
It’s in the combination: a super-water-repellent polystyrene film made of microspheres and nanothreads
The lotus is admired for its beauty, but that isn’t all: engineers envy it for the fascinating ability of its leaves to clean themselves, known as the lotus effect. Never having to wash the car or windows again—that would be nice, but it’s still a long way off. Chinese researchers have now made a highly promising step in the right direction.
The secret of the perpetually clean lotus leaf lies in its special, extremely water-repellent ("superhydrophobic") surface structure. It is virtually impossible for water to wet the surface, it simply beads off, taking any clinging particles of dirt with it. This magic is a result of the contact angle, the angle between the surfaces of the leaf and the water droplet at the point of contact; the more water-repellent the surface, the larger the contact angle. The lotus leaf has an extremely rough, nanostructured surface. The water droplets lie on it as if on a bed of nails, but there is air between the nanopeaks and air is not at all wettable. This is thus equivalent to a contact angle of 180°.
A team led by Lei Jiang has now used polystyrene, a common plastic, to produce a thin plastic film with superhydrophobic properties. The electrohydrodynamic production method they use is also a very common technique. A solution of polystyrene in an organic solvent is sprayed through a nozzle. There is a high electrical voltage between the nozzle and the collector plate, which charges the stream of liquid and accelerates it toward the collector. The form of the polystyrene film that accumulates on the collector depends primarily on the concentration of the polystyrene solution. Concentrated solutions are so viscous that they form nanothreads as they shoot out of the nozzles. The nanothread film has contact angles of "only" 139°. In contrast, if a dilute solution is used, the liquid doesn’t form threads, but droplets. Evaporation of the solvent causes these droplets to solidify into porous microparticles whose surface is covered with nanopapillae. A film of these little porous particles is extremely rough and attains dream contact angles of 162°. Unfortunately, the particles come out of the film. This led to the idea of a composite material that incorporates the advantages of both textures; a finely balanced polystyrene concentration leads to the formation of both nanothreads and microspheres. The porous microspheres in the resulting film lead to a high contact angle (160.4°), while the nanothreads fix the spheres in a stable network. This simple method also seems to be suitable for the production of superhydrophobic films made of many other materials.