Key Concepts

• Microscopic bumps on a lotus leaf transform its waxy surface into an extremely water repellent, or superhydrophobic, material. Raindrops roll easily across such a surface, removing any dirt.
• Researchers have developed synthetic self-cleaning materials, some of which are based on this “lotus effect,” whereas others employ the opposite property—superhydro­philicity—as well as catalytic chemical reactions.
• Future products may combine the two water affinity properties or use substances that can be switched back and forth to control the flow of liquids through microfluidic components.

 Wilhelm Barthlott of the University of Bonn in Germany, discoverer and developer of the “lotus effect,” has a vision of a self-cleaning Manhattan, where a little rain washes the windows and walls of skyscrapers as clean as the immaculate lotus. Elsewhere, he sees tents and marquees using new textiles that stay equally spotless with no intervention from a human cleaner. He is not the only one with his sights set on a future populated with objects that rarely if ever need washing: in Japan, technologists are developing self-deodorizing and disinfectant surfaces for bathrooms and hospitals. Michael Rubner and Robert Cohen of the Massachusetts Institute of Technology envisage similar technologies keeping bathroom mirrors unfogged and controlling microfluidic “labs on a chip” (in which fluids move through microscopic pathways). Already with us are shirts, blouses, skirts and trousers that shrug off ketchup, mustard, red wine and coffee. A revolution in self-cleaning surfaces is under way.

The story of self-cleaning materials begins in nature with the sacred lotus (Nelumbo nucifera), a radiantly graceful aquatic perennial that has played an enormous role in the religions and cultures of India, Myanmar, China and Japan. The lotus is venerated because of its exceptional purity. It grows in muddy water, but its leaves, when they emerge, stand meters above the water and are seemingly never dirty. Drops of water on a lotus leaf have an unearthly sparkle, and rainwater washes dirt from that leaf more readily than from any other plant.

   
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