water repellents
Introduction
The lotus effect refers to the self-cleaning properties that are the result of very high water repellency (superhydrophobic), as exhibited by the leaves of the lotus flower (Nelumbo).[1]
Dirt particles are picked up by water droplets due to the micro and nanoscopic architecture on the surface, which minimizes the adhesion of the droplet to that surface.
Superhydrophobicity and self-cleaning properties are also found in other plants, such as Tropaeolum (nasturtium), Opuntia, Alchemilla, cane, and also in the wings of some insects.[2].
The phenomenon of superhydrophobicity was first studied by Dettre and Johnson in 1964[3] using raw hydrophobic surfaces. Their work developed a theoretical model based on experiments with glass beads coated with telomere paraffin or PTFE. The self-cleaning property of micro-superhydrophobic nanostructured surfaces was studied by the German botanists Barthlott and Ehler in 1977,[4] who described the superhydrophobic properties as self-cleaning and described for the first time the "lotus effect". Some superhydrophobic materials, such as perfluoroalkyl and perfluoropolyether, were developed by Brown in 1986 for the management of chemical and biological fluids.[5] Other biotechnological applications have emerged since the 1990s.[6][7][8][9][10].
Working principle
Due to their high surface tension, water droplets tend to minimize their surface area by trying to achieve a spherical shape. In contact with a surface, adhesion forces result in wetting of the surface. Any complete or incomplete wetting can occur depending on the surface structure and fluid tension of the droplet.[11]
The cause of the self-cleaning properties is the water repellent double hydrophobic structure of the surface.[12] This allows the contact area and adhesion force between the surface and the droplets to be significantly reduced resulting in a self-cleaning process.[13][14][15]
This double hierarchical structure was formed from a characteristic epidermis (the outermost layer called the cuticle) and the waxes that cover it. The epidermis of the lotus plant has papillae with 10 to 20 microns in height and 10 to 15 microns in width in which the so-called epicuticular waxes are imposed. These overlapping waxes are hydrophobic and form the second layer of the double structure.