Lotus effect

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The lotus effect in material science is the observed self-cleaning property found with lotus plants. In some Eastern cultures, the lotus plant is a symbol of purity. Although lotuses prefer to grow in muddy rivers and lakes, the leaves and flowers remain clean.

Botanists who have studied lotus leaves have found that they have a natural cleaning mechanism. This cleaning mechanism was discovered by Professor Wilhelm Barthlott, the director of the Nees-Institue for Biodiversity in Bonn, Germany.

Their microscopic structure and surface chemistry mean that the leaves never get wet. Rather, water droplets roll off a leaf's surface like mercury, taking mud, tiny insects, and contaminants with them. This is known as superhydrophobicity, or more commonly, the lotus effect. Water droplets on taro leaves exhibit similar behavior.

Some nanotechnologists are developing methods to make 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 by treatment of the surface with a fluorochemical or silicone treatment. It is also possible to achieve such effects by using combinations of polyethylene glycol with glucose and sucrose. Clothing that repels water has already been developed and marketed by brands such as Gap and Dockers; It uses a fabric with the copyright name Nano-Care. Water repelling glass panels have also been brought onto the market for use on the roofs of conservatories.

Sto Lotusan is a water repellent paint that is commonly thought of as utilizing the lotus effect to repel water. This paint containts titanium dioxide that produces an effect opposite to the lotus effect called superhydrophilicity where water is attracted to the paint and washes debris away in sheets of water rather than droplets.

In one method ^[1] an aluminium surface is made superhydrophobic by immersing it in sodium hydroxide for several hours followed by spin coating a layer of perfluorononane to a thickness of 2 nanometers. This procedure increases the water contact angle from 67° to 168°, an effect that can be explained by Cassie's law. Electron microscopy shows that the aluminium surface resembles that of a lotus surface with a porous micro structure containing trapped air.

See also

• Biomimicry

References

• Barthlott, W. & C. Neinhuis, 1997: The purity of sacred lotus or escape from contamination in biological surfaces, Planta 202: 1-8.
• ^  Stable Biomimetic Super-Hydrophobic Engineering Materials Zhiguang Guo, Feng Zhou, Jingcheng Hao, and Weimin Liu J. Am. Chem. Soc.; 2005; 127(45) pp 15670 - 15671; (Communication) DOI: 10.1021/ja0547836 Abstract Electron microscopy

• Is the lotus leaf superhydrophobic?; Cheng, Y T, Rodak, D E; Appl. Phys. Lett.; 2005; 86 (14) pp 144101

• Water condensation on a super-hydrophobic spike surface Narhe, R. D., Beysens, D. A. Europhys. Lett.; 2006; 75 (1) pp 98-104

• Mimicking nature: Physical basis and artificial synthesis of the Lotus effect; Lai, S.C.S. [2]

• Forbes, Peter (4th Estate, London 2005) 'The Gecko's Foot - Bio Inspiration: Engineered from Nature' ISBN 0-00-717990-1 in H/B