Lotus effect self-cleaning leaves

Settled in 1997 by Wilhelm Barthlott, Christoph Neinhuis (University of Bonn).

Across many ancient cultures the lotus flower symbolized purity in part because its leaves never appear muddy despite growing out of murky water, but the physical mechanism behind this remained folklore rather than science for millennia. In 1997 Barthlott and Neinhuis published electron-microscope studies across many plant species showing that self-cleaning, superhydrophobic leaves like the lotus owe their property to a hierarchical micro- and nanostructure of wax-covered papillae, which drastically reduce the real contact area between the leaf and water droplets or dirt particles, causing both to roll off together. This paper, now credited with launching the field of biomimetic superhydrophobic surfaces (the effect is now often called the 'Barthlott effect'), triggered thousands of follow-up studies and commercial self-cleaning coatings and fabrics.

A flower long used as a religious symbol of purity turns out to owe its cleanliness to a precisely engineered nanoscale roughness, not smoothness.

Explanations that circulated

• Lotus leaves stay clean simply because they are smooth and water beads off a flat surface — the opposite is true: the self-cleaning ('lotus effect') depends on micro- and nano-scale roughness (wax-covered papillae) that minimizes contact area with water and dirt, not smoothness; a smooth waxy leaf would actually be less self-cleaning.

References

• Purity of the sacred lotus, or escape from contamination in biological surfaces, W. Barthlott, C. Neinhuis, Planta, 1997