Surface Technology

The Miracle of the Lotus

The Lotus Effect

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The Lotus Effect refers to the very high water repellency (superhydrophobicity) exhibited by the leaves of the lotus flower. Dirt particles are picked up by water droplets due to a complex micro- and nano-scopic architecture of the surface, which minimizes adhesion.

Although the self-cleaning phenomenon of the lotus was known in Asia for more than 2000 years, its mechanism was explained only in the early 1970s after the introduction of the scanning electron microscope.

 

 

 

Functional principle

Due to their high surface tension, water droplets tend to minimize their surface 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 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 is related to 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.

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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.

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.

Nanotechnology

Nanotechnologists have developed treatments, coatings, paints, roof tiles, fabrics and other surfaces that can stay dry and clean themselves in the same way as the lotus leaf. This can usually be achieved using special fluoro-chemical or silicone treatments on structured surfaces or with compositions containing micro-scale particulates.

As self cleaning of superhydrophobic microscopic to nanoscopic surfaces is based on a purely physio-chemical effect it can be transferred onto technical surfaces on a biomimetic basis. Our EZ Armor™ System is an example of the products with superhydrophobic self-cleaning properties.

Further applications have been marketed, such as self-cleaning glasses installed in the sensors of traffic control units on German autobahns. Lotus effect superhydrophobic coatings applied to microwave antennas can significantly reduce rain fade and the buildup of ice and snow.

Some “Easy to clean” products in ads often mistakenly claim the self-cleaning process of the lotus-effect. While the technology is based from the Lotus Effect, occasional cleaning and proper maintenance are still required.

Source – Adapted from Wikipedia “Lotus Effect”