The lotus effect refers to the very high water repellence (superhydrophobicity) resulting in self-cleaning properties, as exhibited by the leaves of the lotus flower (Nelumbo). Dirt particles are picked up by water droplets due to a complex micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to said surface. Superhydrophobic and self-cleaning properties can also easily be demonstrated in many other plants, for example Tropaeolum (nasturtium), Opuntia (prickly pear), Alchemilla, cane, and on the wings of certain insects.
The phenomenon of superhydrophobicity was first studied by Dettre and Johnson in 1964 using rough hydrophobic surfaces. Their work developed a theoretical model 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.
Due to their high surface tension, water droplets tend to minimize their surface by trying to achieve a spherical shape. 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. This hierarchical double structure is formed out of a characteristic epidermis (its outermost layer called the cuticle) and the covering waxes. The epidermis of the lotus plant possesses papillae with 10 to 20 µm in height and 10 to 15 µm in width on which the so-called epicuticular waxes are imposed. These superimposed waxes are hydrophobic and form the second layer of the double structure.
The hydrophobicity of a surface can be measured by its contact angle. The higher the contact angle the higher the hydrophobicity of a surface. Surfaces with a contact angle < 90° are referred to as hydrophilic and those with an angle >90° as hydrophobic. Some plants show contact angles up to 160° and are called super-hydrophobic meaning that only 2–3% of a drop's surface is in contact. Plants with a double structured surface like the lotus can reach a contact angle of 170° whereas a droplet's actual contact area is only 0.6%. All this leads to a self-cleaning effect.
Dirt particles with an extremely reduced contact area are picked up by water droplets and are thus easily cleaned off the surface. If a water droplet rolls across such a contaminated surface the adhesion between the dirt particle, irrespective of its chemistry, and the droplet is higher than between the particle and the surface. As this self-cleaning effect is based on the high surface tension of water it does not work with organic solvents. Therefore, the hydrophobicity of a surface is no protection against graffiti.
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 a reduced photosynthesis.
When it was discovered that self-cleaning qualities come from the physical-chemical properties of superhydrophobic surfaces at the microscopic to nanoscopic scale, and does not result from any of the specific chemical properties of the surface of the leaf, that discovery opened up the possibility of using this effect in our own manmade surfaces, by mimicking nature in a general way rather than a specific one.
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. Super-hydrophobic coatings comprising Teflon microparticles have been used on medical diagnostic slides for over 30 years. It is possible to achieve such effects by using combinations of polyethylene glycol with glucose and sucrose (or any insoluble particulate) in conjunction with a hydrophobic substance.
Further applications have been marketed, such as self-cleaning glasses installed in the sensors of traffic control units on German autobahns developed by a cooperation partner (Ferro GmbH). Evonik AG has 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. "Easy to clean" products in ads are often mistaken in the name of the self-cleaning properties of hydrophobic or superhydrophobic surfaces. Patterned superhydrophobic surfaces also show promise for "lab-on-a-chip" microfluidic devices and can greatly improve surface-based bioanalysis.
Superhydrophobic or hydrophobic properties have been used in dew harvesting, or the funneling of water to a basin for use in irrigation. The Groasis Waterboxx has a lid with a microscopic pyramidal structure, based on the superhydrophobic properties that funnel condensation and rainwater into a basin for release to a growing plant's roots.
Although the self-cleaning phenomenon of the lotus was possibly known in Asia long before (reference to the lotus effect is found in the Bhagavad Gita) its mechanism was explained only in the early 1970s after the introduction of the scanning electron microscope. Studies were performed with leaves of Tropaeolum and lotus (Nelumbo). The Lotus Effect is a registered trademark of Sto AG (US Registration Nos. 3561156 and 2613850).
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