The Lotus Effect
Biomimetics is the process of imitation of the models, systems, and elements of nature for the purpose of solving complex human problems.The terms biomimetics come from Ancient Greek: βίος (bios), life, and μίμησις (mīmēsis), imitation. A closely related field in Biomimetics is Hydrophobic Nanotechnology.
The Lotus effect
Reminiscing our Biology class days in school, we all know that plants and leaves needs sunlight to grow. A task impossible to take place if leaves are dirty. That’s why through millions of years of evolution, nature has developed a process of self-cleaning leaves so photosynthesis can do its job.
The lotus effect refers to self-cleaning properties that are a result of very high water repellence (superhydrophobicity), as exhibited by the leaves of the lotus flower. Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to that surface. Superhydrophobicity and self-cleaning properties are also found in other plants, such as Tropaeolum (nasturtium), Opuntia (prickly pear), Alchemilla, cane, and also on the wings of certain insects.
The phenomenon of superhydrophobicity was first studied and experimented by Dettre and Johnson in 1964 using rough hydrophobic surfaces. Their work developed a theoretical model based on experiments Optimal Hydrophobic Contact Angles based on experiments with glass beads coated with paraffin or PTFE telomer. The self-cleaning property of superhydrophobic micro-nanostructured surfaces was studied by Barthlott and Ehler in 1977, who described such self-cleaning and superhydrophobic properties for the first time as the "lotus effect"; perfluoroalkyl and perfluoropolyether superhydrophobic materials were developed by Brown in 1986 for handling chemical and biological fluids. Other biotechnical applications have emerged since the 1990s giving birth to Hydrophobic Nanotechnology coatings for everyday surfaces.
The high surface tension of water causes droplets to assume a nearly spherical shape, since a sphere has minimal surface area, and this shape therefore has least surface energy. 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.
The importance of Hydrophobics in Nature
This effect is of a great importance for plants as a protection against pathogens like fungi or algae growth, and also for animals like butterflies, dragonflies and other insects not able to cleanse all their body parts. Another positive effect of self-cleaning is the prevention of contamination of the area of a plant surface exposed to light resulting in reduced photosynthesis.
When it was discovered that the self-cleaning qualities of superhydrophobic surfaces come from physical-chemical properties at the microscopic to nanoscopic scale, the discovery opened up the possibility of using this effect in manmade surfaces, by mimicking nature.
Some nanotechnologists have developed treatments, coatings, paints, roof tiles, fabrics and other surfaces that can stay dry and clean themselves by replicating in a technical manner the self-cleaning properties of plants, such as the lotus plant. This can usually be achieved using special fluorochemical or silicone treatments on structured surfaces or with compositions containing micro-scale particulates. Superhydrophobic coating comprising Teflon microparticles have been used on medical diagnostic slides for over 30 years.
Further applications have been marketed, such as self-cleaning glasses installed in the sensors of traffic control units on German autobahns and have also developed a spray for generating self-cleaning films on various substrata. Superhydrophobic coatings applied to microwave antennas can significantly reduce rain fade and the buildup of ice and snow.