Understanding processes creating water repellent surfaces in nature and applying the derived knowledge to technical surfaces has been a long time quest, not only in material sciences. Water repellent surfaces in nature are seldom completely smooth but have a certain roughness. The majority of plants have distinctly microstructured leaves; some are additionally covered with nanoscale wax crystals. This combination of a twofold structure together with hydrophobic properties of these waxes creates an outstanding phenomenon: water forming spherical droplets. Such droplets have contact angles exceeding 140° and roll off angles of less than 10°. Commonly, surfaces having these properties are termed superhydrophobic.
Not only water cannot adhere properly to superhydrophobic surfaces even dirt, whether it is hydrophilic or even hydrophobic, is removed easily by running water. This is due to the reduced contact area of surface and particle, which results in the drop laying on the surface like a fakir on a bed of nails. Further information: www.lotus-effect.com
After the discovery and the analysis of the self-cleaning effect, the mechanism was applied to first technical prototypes. The technical conversion was patented and the trade mark Lotus-Effect® was introduced. In the mid-1990s a large cooperation project with industrial partners started. Since then several Lotus-Effect® products have been marketed: a facade paint and a rendering by Sto AG, a coating for glass surfaces by Ferro, a spray and a nano-particle powder (Aeroxide LE®) for multipurpose applications by Degussa.
Recently a different aspect of water repellent surfaces was investigated: underwater superhydrophobicity. Within this project aquatic floating plants and semiaquatic animals were examined in respect of surfaces holding an air film when submerged. Successfully transferred from its natural models textile prototypes proved to retain an air film for about 4 days. Aim of the underwater superhydrophobicity project is to develop technical surfaces for long time application in ships and pipelines, as an air film between surface and liquid produces high drag reductions and thus savings of fuel and energy. |