Have you ever wondered how raindrops simply bead-up and slide off from taro leaves? Have you ever thought the reason behind the leaf’s waterproof ability? The cuticle or waxy layer on the epidermis of the leaf caused the extreme water-shedding preventing it from getting wet.
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This phenomenon is called the “lotus effect” referring to the same water repelling characteristic exhibited by the lotus leaves. This is also present in other insects and birds, notice how butterfly wings don’t get drenched from the rain and how water gently rolls off a duck’s back.
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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 and also on the wings of certain insects.
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. This hierarchical double structure is formed out of a characteristic epidermis (its outermost layer called the cuticle) and the covering waxes. The waxes are hydrophobic and form the second layer of the double structure. This system regenerates. This bio-chemical property is responsible for the functioning of the water repellency of the surface.
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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. Therefore, the hydrophobicity of a surface is no protection against graffiti.
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The cuticle minimizes transpiration from the leaf. The cuticle is hydrophobic so water beads up and rolls off just like water does on a freshly waxed car. 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.
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