Adapted from Peter Forbes article in Scientific American
The sacred lotus (Nelumbo nucifera) is a radiantly graceful aquatic perennial and has played an enormous role in the religions and cultures of India, Mynamar, China and Japan. Yogiis use this symbol almost everyday. The lotus is venerated because of its exceptional purity.It grows in muddy water,but its leaves,when they emerge, stand metres 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
Engineering a superhydrophobic surface on an object by using the lotus effect was not easy—the nature of a hydrophobic material is to repel, but this stuff that repels everything has to be made to stick to the object itself. Nevertheless, by the early 1990s Barthlott had created the “honey spoon”: a spoon with a homemade microscopically rough silicone surface that allows honey to roll off, leaving none behind. This product finally convinced some large chemical companies that the technique was viable, and their research muscle was soon finding more ways to exploit the effect. The leading application so far is StoLotusan facade paint for buildings, introduced in 1999 by the German multinational Sto AG and a huge success. “Lotus Effect” is now a household name in Germany; last October the journal Wirtschaftswoche named it as one of the 50 most significant German inventions of recent years.
Because many untested claims have been made to support nanotechnology products, standards institutions are beginning to set stringent tests for self-cleaning clothing that are based on these innovations. In October 2005 the German Hohenstein Research Institute, which offers tests and certifications to trade and industry around the world, announced that NanoSphere textiles were the first of such fabrics to pass a whole range of tests, including those examining water repellency and the ability of the fabric to maintain its performance after ordinary wash cycles and other wear and tear. In a test of my own, samples of NanoSphere showed an impressive ability to shrug off oily tomato sauces, coffee and red wine stains—some of the worst of the usual suspects.
Easy-clean clothes are becoming widely available, but buyers of marquees, awnings and sails are expected to constitute the biggest market (in terms of money spent) for lotus effect finishes. No one really wants to have to clean these large outside structures.
The exploration of the lotus effect began as an attempt to understand the self-cleaning powers of one type of surface—waxy ones with microscopic or even nanoscale structures. This research has now broadened into an entire new science of wettability, self-cleaning and disinfection. Researchers realized that there might be many ways to make superhydrophobic surfaces and that superhydrophobicity’s reverse—superhydrophilicity—might also be interesting. The leading player in superhydrophilicity is the mineral titanium dioxide, or titania.
Titania’s journey to stardom began more than four decades ago with a property that has nothing to do with wettability. In 1967 Akira Fujishima, then a graduate student at the University of Tokyo, discovered that when exposed to ultraviolet light, titania could split water into hydrogen and oxygen. The splitting of water powered by light, or photolysis, has long been something of a holy grail because if it could be made to work efficiently, it could generate hydrogen cheaply enough to make that gas a viable, carbon-free substitute for fossil fuels. Fujishima and other researchers pursued the idea vigorously, but eventually they realized that achieving a commercial yield was a very distant prospect.
The studies did reveal that thin films of titania (in the range of nanometers to microns thick) work more efficiently than do larger particles. And, in 1990, after Fujishima teamed up with Kazuhito Hashimoto of the University of Tokyo and Toshiya Watanabe of the sanitary equipment manufacturer TOTO, he and his colleagues discovered that nanoscale thin films of titania activated by ultraviolet light have a photocatalytic effect, breaking down organic compounds—including those in the cell walls of bacteria—to carbon dioxide and water.
Titania is photocatalytic because it is a semiconductor, meaning that a moderate amount of energy is needed to lift an electron from the mineral’s so-called valence band of filled energy levels across what is known as a band gap (composed of forbidden energy levels) into the empty “conduction band,” where electrons can flow and carry a current. In titania’s case, a photon of ultraviolet light with a wavelength of about 388 nanometers can do the trick, and in the process it produces two mobile charges: the electron that it hoists to the conduction band as well as the hole that is left behind in the valence band, which behaves much like a positively charged particle. While these two charges are on the loose, they can interact with water and oxygen at the surface of the titania, producing superoxide radical anions (O2–) and hydroxyl radicals (OH)—highly reactive chemical species that can then convert organic compounds to carbon dioxide and water.
In the mid-1990s the three Japanese researchers made another crucial discovery about titania when they prepared a thin film from an aqueous suspension of titania particles and annealed it at 500 degrees Celsius. After the scientists exposed the resulting transparent coating to ultraviolet light, it had the extraordinary property of complete wettability—a contact angle of zero degrees—for both oil and water. The ultraviolet light had removed some of the oxygen atoms from the surface of the titania, resulting in a patchwork of nanoscale domains where hydroxyl groups became adsorbed, which produced the superhydrophilicity. The areas not in those domains were responsible for the great affinity for oil. The effect remained for several days after the ultraviolet exposure, but the titania slowly reverted to its original state the longer it was kept in the dark.
Although it is the very opposite of the lotus leaf’s repulsion of water, titania’s superhydrophilicity turns out also to be good for self-cleaning: the water tends to spread across the whole surface, forming a sheet that can carry away dirt as it flows. The surface also resists fogging, because condensing water spreads out instead of becoming the thousands of tiny droplets that constitute a fog. The photocatalytic action of titania adds deodorizing and disinfection to the self-cleaning ability of coated items by breaking down organics and killing bacteria.
The titania-coating industry is now burgeoning. TOTO, for instance, produces a range of photocatalytic self-cleaning products, such as outdoor ceramic tiles, and it licenses the technology worldwide.
Because nanocoatings of titania are transparent, treated window glass was an obvious development. In 2001 Activ Glass, developed by Pilkington, the largest glass manufacturer in the U.K., became the first to hit the market. In general, glass is formed at about 1,600 degrees C on a bed of molten tin. To make Activ Glass, titanium tetrachloride vapor is passed over the glass at a later cooling stage, depositing a layer of titania finer than 20 nanometers thick. Activ Glass is fast becoming the glass of choice for conservatory roofs and vehicles’ side mirrors in the U.K.
Unfortunately, ordinary window glass blocks the ultraviolet wavelengths that drive titania’s photocatalytic activity, so titania nanolayers are less useful indoors than out. The answer is to “dope” the titania with other substances, just as silicon and other semiconductors are doped for electronics. Doping can decrease the material’s band gap, which means that the somewhat longer wavelengths of indoor lighting can activate photocatalysis. In 1985 Shinri Sato of Hokkaido University in Japan serendipitously discovered the benefit of doping titania with nitrogen. Silver can also be used to dope the titania. Only in recent years, however, have these approaches been translated into commercial processes.
The antibacterial and deodorizing properties of doped titania are expected to have wide applications in kitchens and bathrooms. Titania is also being used in self-cleaning textiles and offers the advantage of removing odors. Various techniques have been devised to attach it to fabrics, including via direct chemical bonds.
…. Staying Dry Underwater
It is one of the pleasant surprises of the 21st century that the radiance of the lotus is penetrating into previously unknown nooks and crannies, as well as beyond self-cleaning applications.
Barthlott, who saw the potential in a drop of water on a lotus leaf, now sees almost limitless vistas. But he warns those who want to translate from nature to technology that they are likely to encounter great skepticism, as he did. “Do trust your own eyes and not the textbooks, and if your observation is repeatedly confirmed, publish it,” he advises. “But take a deep breath—expect rejections of your manuscript.”
He is, not surprisingly, a passionate advocate for biodiversity, pointing out that many other plants and animals may have useful properties—possibly including species unknown to science and in danger of extinction. His current research involves superhydrophobicity underwater. After studying how plants such as the water lettuce Pistia and the floating fern Salvinia trap air on their leaf surfaces, Barthlott created fabrics that stay dry underwater for four days. An unwettable swimsuit is in prospect. The big prize would be to reduce the drag on ships’ hulls. The lotus collects no dirt, but it is garnering an impressive string of patents.
Note: This story was originally published with the title, “Self-Cleaning Materials”.