Lotus effect
As the lotus effect, also lotus effect, the low wettability of a surface is referred to as can be observed in the lotus plant. Water runs off in drops and takes all dirt particles on the surface. This is due to a complex micro-and nanoscopic architecture of the surface that minimizes the adhesion of dirt particles.
Other plants such as nasturtium (Tropaeolum ), common reed (Phragmites ), white cabbage (Brassica oleracea convar. Capitata var alba), water lettuce ( Pistia stratiotes ), lady's mantle ( Alchemilla ) or the columbine (Aquilegia ) show, as well as some animals (many insect wings ), this effect.
The self-cleaning ability of water-repellent micro - nanostructured surfaces was discovered in the 1970s and transferred to biomimetic -technical products since the mid- 1990s. These are marked with the brand name Lotus effect.
Principle of operation
Have water drops, due to their high surface tension which tends to minimize their surface area and therefore try to achieve a spherical shape. In case of contact with another surface adhesion forces act ( adhesion forces to the surface ), so that it comes to wet the same. Depending on the nature of the surface and the surface tension of the liquid may cause complete ( spreading agent ) or incomplete wetting.
The cause of self-purification is located in a hydrophobic ( water-repellent ) Double structure of the surface. Thus, the contact area and thus the adhesive force between the surface and lying on her particles and water droplets is reduced so much that it comes to self-clean. This double structure is of a characteristically shaped epidermis, the outermost layer is called the cuticle, and formed on their waxes are located. The epidermis of the lotus plant is about 10 to 20 microns high and 10 to 15 microns away from each other papillae, on the so-called epicuticular waxes are superposed. This superimposed waxes are hydrophobic and form the second part of the twin structure. Thus, water no longer has the possibility to get into the interstices of the sheet surface, with the result that the contact area between water and the surface is drastically reduced.
The hydrophobicity of surfaces is determined by the contact angle. The higher the contact angle, the more hydrophobic the surface. Surfaces < 90 ° are referred to as hydrophilic, those with a contact angle > with a contact angle 90 ° referred to as hydrophobic. In some plants contact angle of up to 160 ° ( superhydrophobicity ) can be reached. This means that only about 2-3 % of the drop surface with the surface of the plant are in contact, so it has an extremely poor wettability. By the double structure of the lotus plant, the leaves can achieve a contact angle of about 170 °, whereby a drop has a contact surface of only about 0.6%. The adhesion between the sheet surface and drops of water is so low that the water can easily run off. Superficial dirt - which also have only a small contact area - are transported and washed away by it. Even hydrophobic dirt particles are washed off the plant surface, because their adhesion to the plant surface is less than the water drops. The central role of the surface tension of aqueous solutions for minimizing the contact area is understood that the self-purification in this form can not occur in strongly wetting solvents, so such surfaces provide no protection against all kinds of paint and inks dar.
The biological significance of the lotus effect for the plant is in the protection against colonization by microorganisms, pathogens or germs, such as fungal spores, or before growth of algae. Similarly, this also applies to animals such as butterflies, dragonflies and other insects: With her legs she can not reach any part of their body to clean, and the more advantageous it is when moisture and dirt roll off independently. Another positive effect of self-cleaning is the prevention of debris that may reduce the incidence of light and thus the photosynthesis and close stomata.
Technical Applications
Self-cleaning surfaces are of increasing industrial and commercial importance. In this coveted property by superhydrophobic micro-and nanostructured surfaces is a purely physico- chemical phenomenon, and it can be applied to biomimetic technical surfaces. There are now world alone with this product approximately 600,000 buildings that are equipped with lotus surfaces.
The self-cleaning ability of water-repellent nano -structured surfaces was discovered in the 1970s by Wilhelm Barthlott. With respect to the technical theory for the self-cleaning effect Barthlott reported in international patent protection. Furthermore, products which go back to the developed by Barthlott technical theory for the self-cleaning effect, widely covered by international brands "lotus effect " or " Lotus-Effect ". Exclusive brand owner is the Sto AG in Stühlingen, producers, among others of the architectural paint " Lotusan ", which was introduced in 1999 by Sto AG is the first commercial product in implementing the teaching of Barthlott on the market.
Other applications include self-cleaning glass on the principle of lotus plant from Ferro GmbH, which are used for example to the Toll Collect cameras. The Degussa AG has developed prototypes of plastics and sprays.
In advertising, partially aware so-called "Easy-to -clean" surfaces to be confused with the self-cleaning surfaces according to the Lotus principle.
The Swiss company HeiQ Materials AG and Schoeller Textil AG have developed under the trade name " Barrier by HeiQ " and " Nanosphere " stain-resistant textiles. In October 2005, tests revealed the Hohenstein Research Institute that clothes the Nanosphere series can easily run even after repeated washing, tomato sauces, coffee and red wine. Another application can be found at self-cleaning awnings, tarpaulins and sails that are otherwise spoiled easily and difficult to clean.
By superhydrophobic surfaces are not wet expectant swimsuits and better gliding hulls can be developed. The swim suits have been used in international competitions and were banned again from 2010, as it significantly improved the times of swimmers.
History of Research
Although the phenomenon of self- cleaning lotus in Asia has been known for at least 2000 years ( lotus is, among other things, the symbol of purity in Buddhism ), the effect was studied with the use of scanning electron microscopy by the botanist Wilhelm Barthlott only since the early 1970s. The original work was carried out mainly at the nasturtium. The first scientifically -depth analysis was carried out on the lotus leaves ( W. Barthlott, C. Neinhuis ). Mid-1990s, these two authors also succeeded in the first implementation of technical prototypes and the first industrial collaborations. The methods are patented. Since the late 1990s, especially physicists and materials scientists have studied the phenomenon extensively, and there are now a very extensive literature and dozens of dependent patents. For the elucidation of the functional principle of the self-cleaning surfaces of lotus flower and its implementation in technical products the work of Wilhelm Barthlott numerous prizes were awarded (1997 Karl -Heinz Beckurts Price, 1998 nominated for the German Future Prize of the Federal President, in 1999 Philip Morris Research Award and the 1999 German environmental Award 2005 Innovation award of the Ministry of Education and Research and others).