Lotus Effect

The lotus effect refers to the very high water repellency (superhydrophobicity) exhibited by the leaves of the lotus flower (Nelumbo). Dirt particles are picked up by water droplets due to a complex micro- and nanoscopic architecture of the surface, which minimizes adhesion. This effect can easily be demonstrated in many other plants, for example tropaeolum, cane and columbine, and on the wings of certain insects.

Due to their high surface tension, water droplets tend to minimize their surface trying to achieve a spherical shape. On contact with a surface, adhesion forces result in wetting of the surface. Either complete or incomplete wetting may occur depending on the structure of the surface and the fluid tension of the droplet. The cause of self-cleaning properties is the hydrophobic water-repellent double structure of the surface. This enables the contact area and the adhesion force between surface and droplet to be significantly reduced resulting in a self-cleaning process.

Source: en.wikipedia.org/wiki/Lotus_effect - 04.06.2010