The surface of a lotus leaf is ‘ultrahydrophobic’; when water is dripped onto its surface it does not wet the leaf at all, but rolls off:
You can try this yourself with dried lotus leaves, purchased from a Chinese supermarket. Not only are the leaves extremely water-repellent, they are also ‘self-cleaning’, due to the surface structure of the leaves. The lotus leaf has a series of protrusions that are roughly 10 μm (1.0 x 10– 5 m) high covering its upper surface, these can be seen in the microscope images shown below, by N. J. Rogers Simpson:
This ‘rough’ surface structure with microscale bumps has a second smaller structure as each protrusion is itself covered in bumps of a hydrophobic, waxy material that are roughly 100 nm (1 x 10-7 m) in height. This means that water droplets sit lightly on the tips of hydrophobic protrusions as if on a bed of nails. This combined structure traps a layer of air in between the surface of the leaf and the water droplet. Hence, the water is not allowed to wet the surface and is easily displaced. This ultrahydrophobicity gives rise to the self-cleaning process on the surface because as the droplets travel along the leaves, they pick up any dirt or other matter they encounter along the way. This process keeps the lotus leaves dry, clean and free of pathogens such as bacteria and fungi.
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