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.