The lotus (Nelumbo nucifera) is India’s national flower and is usually pink or white. It is a symbol of purity in Buddhism and Hinduism, as its leaves are self-cleaning. This property is due to the leaves being superhydrophobic. Although superhydrophobicity may sound complicated, it simply means that a surface is extremely difficult to wet. It is also known by a much simpler name: the lotus effect.
We can measure wettability of a surface by measuring the angle formed between a water droplet and the surface. If the droplet tends to f latten on the surface, with a contact angle less than 90°, the surface is wettable (the material is hydrophilic) and the droplet tends to stick to the surface, even if it is tilted. However, superhydrophobic surfaces have a water contact angle of over 150° and a tilting angle of less than 10° will allow water droplets to roll off (see Figure 1). The droplets collect dirt particles as they roll away, which is what makes lotus leaves self-cleaning.
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