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.

It is this last property that drew Barthlott’s attention. In the 1970s he became excited by the possibilities of the scanning electron microscope, which had become commercially available in 1965 and offered vivid images down to the nanometer realm. At that scale of magnification, specks of dirt can ruin the picture, and so the samples have to be cleaned. But Barthlott noticed that some plants never seemed to need washing, and the prince of these was the lotus.

Barthlott realized that the effect is caused by the combination of two features of the leaf surface: its waxiness and the microscopic bumps (a few microns in size) that cover it. He knew from basic physics that the waxiness alone should make the leaves hydrophobic, or water-hating. On such a material, drops of water sit up high to minimize their area of contact with the material. Water on a more hydrophilic, or water-loving, substance spreads across it to maximize the contact area. For a hydrophilic surface, the contact angle (where the droplet’s surface meets the material) is less than 30 degrees; a hydrophobic surface has a contact angle greater than 90 degrees.

In addition, he understood that the innumerable bumps take things a step further and cause the lotus surface to be superhydrophobic—the contact angle exceeds 150 degrees, and water on it forms nearly spherical droplets with very little surface contact that roll across it as easily as ball bearings would. The water sits on top of the bumps like a person lying on a bed of nails. Air trapped between the water and the leaf surface in the spaces around the bumps increases the contact angle, an effect that is described by the Cassie-Baxter equation, named after A.B.D. Cassie and S. Baxter, who first developed it in the 1940s.

Dirt, Barthlott saw, similarly touches only the peaks of the lotus leaf’s bumps. Raindrops easily wet the dirt and roll it off the leaf. This discovery that microscopic bumps enhance cleanliness is wonderfully paradoxical. I learned at my mother’s apron that “nooks and crannies harbor dirt”—capturing the conventional folk wisdom that if you want to keep things clean, keep them smooth. But contemplation of the lotus showed that this homily is not entirely true.

First and foremost a botanist, Barthlott initially did not see commercial possibilities in his observation of how the minuscule bumps keep lotus leaves spotless. In the 1980s, though, he realized that if rough, waxy surfaces could be synthesized, an artificial lotus effect could have many applications. He later patented the idea of constructing surfaces with microscopic raised areas to make them self-cleaning and registered Lotus Effect as a trademark.