Lotus effect shakes off dirt

The lotus, a flowering wetland plant native to Asia, may not at first glance be of interest to the nanotechnologist. But researchers at German chemical company BASF are developing a spray-on coating that mimics the way lotus leaves repel water droplets and particles of dirt. When developed, the spray will particularly suit rough surfaces such as leather, textiles, paper and masonry.

The leaves of lotus plants are coated with minute wax crystals around 1 nm in diameter, which repel water and droplets falling onto them bead up and, if the surface slopes slightly, will roll off. As a result, the surfaces stay dry even during a heavy shower. What's more, the droplets pick up small particles of dirt as they roll, so that the lotus leaves are 'self-cleaning'.

At the nanoscale, rough surfaces are more effective in repelling water than smooth ones as there is less contact between water and solid. This rough structure is also essential to the self-cleaning effect - on a smooth surface water droplets slide rather than roll, and do not pick up dirt particles to the same extent.

Wilhelm Barthlott, a botanist from the University of Bonn in Germany, first explained the phenomenon and now owns a patent and the Lotus Effect trademark. BASF's lotus-effect aerosol spray combines nanoparticles with water repelling polymers such as polypropylene, polyethylene and waxes. It also includes a propellant gas. As it dries, the coating develops a nanostructure through self-assembly.