Shopkeepers, army generals and car manufacturers have long marvelled at the lotus plant. They do not admire the aquatic flower's striking beauty or its remarkable hardiness (lotus seeds freed from the pyramids reportedly germinated) but its leaves, its very clean leaves.

Japanese researchers have now developed artificial, water-repellant films that mimic lotus leaves' self-cleaning abilities -- perhaps promising clearer shop windows, portable satellite receivers and windscreens.

Lotus (Nelumbo) leaves keep themselves spotless because countless tiny projections, coated with water-repellent wax, cover their surface. Water cannot spread out on these leaves so it rolls around as droplets, removing dirt and grime as it goes. Chemists and engineers have attempted to copy this 'lotus-effect' (or 'super-hydrophobicity') for decades.

Now, Toshiya Watanabe of the University of Tokyo, Japan, and colleagues say they have developed the first transparent, super-hydrophobic films that self-clean over long lifetimes. The films provide "a great opportunity" to develop lotus-effect coatings for various industrial items, the researchers report in Langmuir1. The new technology might stop problematic water films building up on flat surfaces.

Lotus leaves' water-repellency depends as much on the bumpy leaf surface as on its waxy coating. Roughness reduces the ability of water to spread out, so droplets snag and gather up into almost spherical beads. Such surfaces have very high 'contact angles': a measure of the tendency for liquids to spread over or wet the surface. The lower the contact angle, the more the liquid spreads over the solid. Complete wetting occurs at zero degrees.

Artificial hydrophobic surfaces must have contact angles greater than 150 degrees to earn a 'super' prefix. The biggest problem is usually keeping the film surface clean -- grimy stains reduce the surface contact angle, allowing water to spread out.

Lotus leaves do not have this problem because they continuously replace their waxy layer. "Since the metabolic mechanism of lotus leaves is difficult to duplicate, practical applications of super-hydrophobic surfaces have been limited," Watanabe's group say.

The new film cleans itself -- but the researchers are not entirely sure how. Mixing titanium oxide into the film significantly alters how quickly stains build up. This is probably because titanium oxide reacts with dirt particles under ultraviolet light.

But other researchers have previously shown that this 'photo-catalysis' process wrecks artificial water-repellent surfaces, bringing contact-angle and hydrophobicity crashing down. So how do the new films escape this fate? The answer seems to lie in the titanium oxide concentration.

To investigate, the team covered glass plates with films containing different amounts of titanium oxide and left them on the roof of a Tokyo building for over two months. Stains quickly built up on films containing twenty per cent titanium oxide but those with just two per cent stayed clean and water-repellant, outperforming films with no titanium oxide at all.

The researchers cannot explain exactly how the lower amount of titanium oxide offers cleaning protection without reducing film contact angle, although they say that the relative size of the particles involved is probably important.

Tsutomu Minami who works on super-hydrophobic films at Osaka Prefecture University in Sakai, Japan, agrees that the new technology should be useful, but adds that methods to make self-cleaning films are now well known. "I guess [the new film] is a combination of the two," he says.