Self-cleaning materials

The spectacular ability of the lotus plant to repel dirt served as an inspiration for cleaning and self-disinfecting technologies that may also help control the flow of liquids in microscopic tubes of "labs on a chip"

By Peter Forbes

William Barthelot from the University of Bonn in Germany, the discoverer and developer of the "Lotus Effect", has a vision: a Manhattan that cleans itself. According to his vision, a little rain is enough to wash the windows and walls of the skyscrapers and make them as clean as the lotus plant. Elsewhere, he contracts tents and awnings made of new types of fabric that remain fresh without the intervention of a cleaning human hand. He is not the only person who foresees a future of objects that almost do not require washing: technologists in Japan are developing surfaces for bathrooms and hospitals that themselves emit bad odors and disinfect their surfaces. Michael Rubner and Robert Cohen of the Massachusetts Institute of Technology (MIT) envision similar technologies that will keep bathroom mirrors clear of fogging and others that will allow control of microscopic flow in "labs on a chip." (Tiny chemical arrays through which liquids flow through microscopic passages.) Already today there are tank tops, shirts, skirts and pants that repel ketchup, mustard, red wine and coffee stains. The revolution of self-cleaning surfaces has thus begun.

The story of self-cleaning materials begins in nature with the sacred lotus plant (Nelumbo nucifera), a graceful perennial aquatic plant that played a decisive role in the development of the religions and cultures of India, Burma, China and Japan. The lotus is revered for its extraordinary purity. It grows in muddy water, but its leaves, sticking out and floating above the water, always look clean. The water drops on a lotus leaf have a shine that seems out of this world, and the rainwater washes away dirt from the leaves more efficiently than it does on any other plant.

It was this cleanliness feature that caught Barthelot's attention. In the 70s, he marveled at the possibilities inherent in the scanning electron microscope, which went on the market in 1965, and produced sharp images down to the nanometer scale. At such a magnification, dust grains may destroy the image, so it is necessary to clean the samples. But Barthelot noticed that there were plants that almost never needed to be washed, and the prince of these plants was the lotus.

Barthelot realized that this feature is a combined result of two characteristics of the leaf's surface: the wax that covers them and the microscopic bumps (several microns in size) scattered over them. His basic knowledge of physics confirmed that the wax alone makes the leaves hydrophobic or "water hating". Drops of water placed on such a material stand high above the surface to reduce the contact area as much as possible. A drop of water placed on a hydrophilic, or "water-loving" material, is applied to it to expand the contact area as much as possible. On a hydrophilic surface the contact angle (at the point of contact between the surface area of ​​the drop and the surface) is less than 30 degrees. For a drop on a hydrophobic material that has a contact angle greater than 90 degrees.

And yes, he realized that the countless bumps increase the effect even more and make the surface of the lotus super-hydrophobic: the contact angle of the water is higher than 150 degrees and the drops that form on the surface therefore take on an almost spherical shape, and due to the minimal contact area they roll easily like bearing balls. The drops of water rest on the tops of the bumps like a man lying on a bed of nails. The air trapped between the water and the surface of the leaf, in the spaces between the bumps, increases the contact angle, an effect that is described mathematically using the Cassie-Baxter equation, named after A. B. D. Cassie and S. Baxter who first developed it in the 40s the 20th.

Even dirt, Barthelot noticed, only touches the heads of the bumps on the surface of the lotus leaf. The raindrops wet the dirt easily and roll it off the leaf. This discovery, that microscopic bumps increase cleanliness, is an astonishingly paradoxical discovery. My mother used to say that "holes and cracks accumulate dirt", and thus summed up the popular popular opinion that if you want to keep things clean you have to make sure they are smooth. But a closer look at the lotus leaves shows that this logic is not entirely correct.

Barthelot, who was first and foremost a botanist, did not initially see the commercial possibilities inherent in the observation that tiny nodules kept the leaves free from mildew. But in the 80s he realized that if they could make rough waxy surfaces, the artificial lotus effect would have many applications. Then he patented the idea of ​​producing surfaces with raised microscopic areas to make them self-cleaning and trademarked the name "Lotus Effect".

Designing a super-hydrophobic surface on the surface of objects using the lotus effect was not easy - a hydrophobic material by nature repels other materials, but in order to use such a material, which repels everything, it must be stuck to objects. And yet, in the early 90s, Barthelot created the "honey spoon". The spoon is coated with a homemade silicone surface with a microscopic roughness that allows the honey to slide over it and leave no residue. The product finally convinced several major chemical companies that the technique was practical, and their powerful research arms soon found additional ways to exploit the effect. The leading application so far is StoLotusan, a paint for the facades of buildings that the German multinational company Sto put on the market in 1999 and was a resounding success. The name "Lotus effect" is now a common colloquialism in German households. In October 2007, the German business journal Wirtschaftswoche listed the lotus effect as one of the 50 most important German discoveries of recent years.

End of restaurant disasters

If you ask people to fill in the missing word in the phrase "...cleaning themselves", most will choose the word "clothes". We don't often clean the front of our houses, but we always wash our clothes. After a hesitant start, self-cleaning fabrics are popping up in every corner. And it all started with Nano-Care.

It was the inventor and entrepreneur David Soan who developed this fabric coating, which is currently produced by his company Nano-Tex. Think of the peach plum. Now imagine the peach under a stream of tap water and visualize the Nano-Care effect. The "flume" in this coating is made of tiny bristles attached to cotton fibers. The bristles are so small, less than a thousandth the height of the lotus buds, that the cotton fiber looks like a tree trunk compared to them.

Nano-Tex's rival is the Swiss company Schuler Textile, which calls its technology NanoSphere. In this method, the clothing fibers are covered with nanoscopic particles of silica (nitrogen dioxide) or plastic, which give the fiber the bumpy, lotus-like roughness.

Many untested claims are made to support nanotechnology products. Standards institutes are therefore starting to set stricter tests for self-cleaning clothes based on innovations in this field. The German Hohenstein Research Institute offers inspections and certifications to commercial and industrial entities all over the world. This institute determined in October 2005 that NanoSphere fabrics are the first self-cleaning fabrics that passed a full series of tests, including water repellency and maintaining performance after normal washing cycles and other weathering tests. In the tests I conducted myself, samples of NanoSphere fabrics demonstrated an impressive ability to shake off greasy tomato sauce, coffee and red wine, which are some of the worst stains.

Easy-clean clothing is becoming common, but the largest market (in terms of financial expenditure) for coatings based on the lotus effect is probably the market for awnings, tents and sails. No one really wants to clean these big outbuildings.

Super wetting

The study of the lotus effect began with an attempt to understand the self-cleaning capacity of one type of surface - a waxy surface covered with microscopic or even nanoscopic structures. This research has now expanded into a new science dealing with wettability, self-cleaning and disinfection. Researchers began to realize that there are many ways to create superhydrophobic surfaces, and that the inversion of this property - superhydrophilicity - could also be interesting. The main player in the field of superhydrophilicity is the mineral titanium dioxide or titania.

Titania's journey to the status of a star in materials science began forty years ago with a feature that has nothing to do with wettability. In 1967, Akira Fujishima, then a research student at the University of Tokyo, discovered that when titania is exposed to ultraviolet light, the material is able to break down water molecules into oxygen and hydrogen. Decomposing water using light, or photolysis, has long been a kind of scientific "holy grail". If the process is efficient enough, it will produce cheap enough hydrogen that can be used as a carbon-free substitute for fossil fuels. Fujishima and other researchers worked hard on the idea, but eventually realized that the chance of obtaining a commercial exploitation was slim.

The studies revealed that thin layers of titania (with a thickness of nanometers to microns) worked more efficiently than larger particles. Then, in 1990, after Fujishima joined Kazuhito Hashimoto from the University of Tokyo and Toshiya Watanabe from the TOTO company that manufactures sanitary equipment, they discovered that ultraviolet light projected onto nanometer-thick titania layers activates them and allows them to accelerate chemical reactions (photocatalytic effect) of decomposition Organic substances, including the cell wall components of bacteria, to carbon dioxide and water.

Titania exhibits photocatalytic activity because it is a semi-conducting material: a moderate amount of energy excites an electron from the full energy levels of the "valence band" in the mineral, and raises it across the gap between the bands (which contains forbidden energy levels) into the "conduction band", the void, where Can the electrons flow and conduct an electric current. In Titania, this is done using a photon of ultraviolet light with a wavelength of about 388 nm. The excitation creates two mobile charge carriers: the electron that jumped to the conduction band and the "hole" that remains in the valence band, which behaves as a particle with a positive electric charge. When these two charges are free, they can act on water and oxygen molecules, which are on the surface of the titania, and create superoxide radical ions (superoxides - O2-) and hydroxyl radicals (OH), both of which are very active chemical species capable of breaking down organic substances into carbon dioxide oxygen and water.

In the mid-90s, the three Japanese researchers made a crucial discovery about Titania. They prepared a thin layer of the material from an aqueous suspension of titania particles and calcined it at a temperature of 500 degrees Celsius. After the scientists exposed the resulting transparent coating to ultraviolet light, it showed exceptional complete wettability: a contact angle of zero degrees, for both oil and water. The ultraviolet light removes some of the oxygen atoms from the surface of the titania and creates nanometric patches to which hydroxyl groups are adsorbed. These patches show super-hydrophilicity. The other areas are responsible for the high affinity for the oil. This property remains intact for several days after exposure to ultraviolet radiation, but the material slowly returns to its previous state when left in the dark.

Although the properties of titania are completely opposite to those of the lotus leaf, it turns out that super-hydrophilicity also gives this material a good capacity for self-cleaning: the water tends to spread over the entire surface, and the layer of flowing water carries the dirt further. The surface also does not fog, because the condensing water vapor is spread to the sides and does not accumulate in thousands of tiny droplets that make up the fog. In addition, the photocatalytic activity of the titania adds to the objects coated with it also a disinfecting and deodorizing capacity by killing bacteria and breaking down organic substances.

The titania coating industry is now developing. The TOTO company, for example, produces a variety of self-cleaning photocatalytic products such as ceramic tiles for outdoor use, and it grants licenses for the technology worldwide.

The nanoscale titania layers are transparent, so glass coating was an expected and obvious step. The first product to hit the market was Active-Glass, from the Pilkington company, the largest glass manufacturer in Great Britain. Glass is usually produced at temperatures higher than 1,600 degrees Celsius on a substrate of molten tin. To produce the active-glass coating, vapors of titanium tetrachloride are flowed over the surface of the glass in one of the later cooling stages. The steam deposits a titania layer as thin as 20 nanometers. Active-Glass is becoming common in the UK in greenhouse roofs and vehicle side mirrors.

Unfortunately, regular glasses block the ultraviolet light waves that activate titania, so nano-layers of this material are less useful indoors. The answer to this is "doping", seeding atoms of other materials in the titania layer, just as is done for silicon (silicon) and other semiconductors in the electronics industry. This doping action reduces the band gap in the material, meaning that the longer wavelengths of household lighting can stimulate the photocatalytic activity. Shinri Sato from Hokkaido University in Japan accidentally discovered in 1985 the advantages of doping titania with nitrogen atoms. Silver atoms can also be used for this purpose. But only in recent years have these approaches been translated into commercial processes.

It is thought that titania that has been drugged will find many uses in kitchens and bathrooms due to its disinfecting and deodorizing properties. Titania is also used in self-cleaning fabrics because it adds to them the ability to eliminate odor. Several methods have been developed to attach it to fabric, including direct chemical bonding.

A convergence of opposites

The materials created, inspired by the lotus leaves and the thin layers of titania, seem to stand in two opposite poles that are usually hard to find in the everyday world, where, as the English poet Philip Larkin wrote, "nothing can be made that is new or perfectly clean". For a long time, the methods and materials were completely different, and the study of the superhydrophobic effect was conducted completely separately from the study of photocatalytic superhydrophilicity. But recently there has been an unusual merger: researchers are currently working on combining the two effects and applying them to very similar materials. Researchers are even looking for ways to make a single chemical structure go from super-hydrophobic to super-hydrophilic and back.

The first to hint at this merger in 2000 were titania research pioneers Fujishima, Tanba and Hashimoto. They wanted to use titania to extend the life of lotus effect surfaces. At first glance, this approach seems destined for failure: it was expected that the photocatalytic activity of the titania would attack the hydrophobic waxy coating of the lotus surfaces and destroy the effect. Indeed, such destruction was indeed caused by high concentrations of titania. But the team found that adding a tiny amount of titania greatly prolongs the activity of the lotus effect without greatly altering the obtuse contact angle needed for strong water repulsion.

In 2003, Rubner and Cohen's lab at MIT discovered that a tiny change in the structure of the surface could determine which property was fixed to it: superhydrophilicity or superhydrophobicity. During a visit to China that year, Rubner recalled, he was very excited by superhydrophobic structures mentioned at the conference. Returning to the lab, he instructed several members of his research group to attempt to create such structures. His laboratory developed a method of producing thin membranes, layer by layer, from polyelectrolyte compounds. When ordinary electrolytic substances, such as table salt or sulfuric acid, are dissolved in water, they separate into positive and negative ions. Polyelectrolytes are organic polymers that, unlike most ordinary plastic materials, carry an electrical charge, positive or negative. Rubner and Cohen alternately stack layers of the poly(allylamine hydrochloride) polymer, which is charged with a positive electrical charge, and of silica particles, which are charged with a negative electrical charge. (In a previous study they used a coating of silica particles to mimic the rough hydrophobic surface of the lotus.)

They finally coated this multi-layered structure with crucible (which is a hydrophobic material), but as they did so they noticed something interesting: before they applied the crucible coating, the layer cake was actually superhydrophilic. The silica layers in Rubner and Cohen's experiments created dense surfaces of nanometer pores that act like a sponge, or a candle wick, which immediately sucks every drop of water from the surface - a phenomenon known as nano-wicks. The silica-polymer layers developed by the researchers never fog up, not even when held over steaming water. If the pores are filled, the water starts to flow over the rim. When the humid conditions end, the water in the nano-wicks gradually evaporates.

Glass consists mainly of silica, so the multi-layer coating is very suitable for application on glazing. The super-hydrophilic coating is not only transparent and prevents fogging but also prevents reflections. Rubner's team is collaborating with industry to make the discovery commercial. Possible applications include bathroom mirrors that never fog up and car windshields that never need to have hot air blown over them on cold winter mornings. Unlike Titania, Rubner's surfaces work equally effectively in light and darkness.

Sophisticated beetles

Millions of years before scientists combined the lotus effect and superwetting in technological applications, a small beetle in the Namib desert in southern Africa was busy applying these two effects for another purpose: collecting water for its own survival.

The Namib Desert is a very hostile place. Daytime temperatures can reach 50 degrees Celsius and rain is extremely rare. Almost the only source of moisture is the thick morning mists that are usually carried by the cool wind. The Stenocara beetle has developed a way to collect the water in these fogs: it swims in front of the foggy wind, its head down and its back raised. The water condenses on the back and drips down into the mouth. The scientific explanation of the beetle's method of collecting water served as an inspiration for ideas for collecting water in barren areas.

As so often happens, the beetle's water-gathering mechanism was discovered by a researcher looking for something completely different. In 2001, zoologist Andrew R. Parker, then at the University of Oxford, came across a photograph showing beetles eating grasshoppers in the Namib desert. The grasshopper, which was carried there by the strong winds of the area, died immediately when it fell on the hot sand. In contrast, the beetles who fed on the gift they received, literally from heaven, were clearly comfortable in the desert. Parker hypothesized that their surfaces reflect heat in a sophisticated way.

Indeed, Stenocra beetles reflect heat, but when Parker examined their backs he immediately suspected that he had some kind of adaptation of the lotus effect that enables the process of collecting water in the morning. Most of the dorsal surface of the Stenocra beetle is covered with a bumpy, waxy, superhydrophobic surface. But the heads of the bumps are free of wax and hydrophilic. These hydrophilic dots capture water from the fog, forming small droplets, which quickly grow to dimensions that allow gravity and the superhydrophobicity of the environment to dislodge them. In laboratory experiments on glass plates, Parker discovered that such an arrangement of areas is twice as effective as a smooth and uniform surface, whether it is hydrophobic or hydrophilic.

Parker has patented a program to mimic the water collection of the beetles, and QinetiQ, a contractor for the British Ministry of Defense, is developing it to harvest fog in parked areas. Others are also trying to imitate the Stenocra beetle. In 2006, Rubner and Cohen's team created super-hydrophilic dots of silica on top of a super-hydrophobic multilayer surface. This design is more effective than the beetles whose spots on their backs are only hydrophilic.

The new science of superwetting makes it possible to control the flow of liquids on a microscopic and nanoscopic scale as demonstrated by the artificial Stenocra surfaces. This may be used in much more sophisticated applications than keeping surfaces clean. Says Rubner: "Once you understand that designed surfaces can be super-hydrophobic or super-hydrophilic depending on the surface chemistry, many possibilities open up." A particularly interesting use could be for switchable surfaces, such that their wettability can be changed in precise locations.

Such branding can be achieved in many ways: ultraviolet light, electricity, temperature, solvent or acidity. In 2006, a team led by Kilwon Cho from Pohang University of Science and Technology in South Korea succeeded in achieving complete branding by attaching molecules of an azobenzene-based compound to a superhydrophobic multi-layer surface of silica-polyelectrolyte coated with zinc. The new surface is also superhydrophobic, but under the influence of ultraviolet light the azobenzene compound changes its structure and makes the surface superhydrophilic. Light in the visible field restores the situation to its original state.

Such control will be able to find important uses in the field of microflow, such as in the control of macroscopic arrays used today to scan drugs and in other biochemical tests. For example, hydrophilic passages can be closed or opened by switching parts of them and making them hydrophobic or hydrophilic respectively.

Dry under water

One of the pleasant surprises of the 21st century is that Lotus Glow penetrates previously unknown cracks and crevices, well beyond self-cleaning applications.

Barthelot, who saw the possibilities inherent in a single drop of water on a lotus leaf, now sees almost limitless vistas. But he warns that those who want to translate nature into technology may encounter a lot of skepticism, as he says. "Trust what your eyes see and not the textbooks, and if you manage to repeatedly confirm your observations, publish them," he advises. "But be very patient and expect many rejections of the article."

Not surprisingly, he is an ardent follower of biodiversity. He is sure that many other plants and animals may show useful properties, including probably species unknown to science and on the verge of extinction. His current research deals with underwater superhydrophobicity. After examining how aquatic plants, such as pistachio ("water lettuce") and the floating salvinia fern, trap air on the surface of their leaves, Barthelot created fabrics that remain dry for several days underwater. He hopes to create a swimsuit that doesn't get wet. But the big prize will be if he manages to reduce the drag of ship hulls. The Lotus may repel dirt, but it attracts an impressive array of patents.

key concepts

Microscopic bumps covering the lotus leaves give its waxy surface an extremely high ability to repel water and make it a super-hydrophobic material. The raindrops roll easily on such a surface and remove any dirt.

Researchers have developed artificial materials with self-cleaning capacity, some of them based on this "lotus effect". Other materials show the opposite property: the surface of these materials also act as chemical catalysts.

Future products will have materials that combine the two contradictory properties: super-hydrophobic and super-hydrophilic or materials that can change their affinity for water, back and forth, to control the flow of liquids in microscopic components.

The inspiration

The lotus effect:

The amazing ability of the lotus leaf to stay clean inspired the development of self-cleaning materials.

The base

The physics of the lotus:

The self-cleaning effect of the lotus is due to the fact that the surface of the leaf is extremely hydrophobic (water repellent). The hydrophobicity or hydrophilicity (attraction to water) of the material determines the contact angle between the material and the surface of the water.

The lessons of the lotus

stay clean:

Commercial companies have created fabrics that are able to repel water and food stains because they are super-hydrophobic like a lotus leaf. The effect is created by changing the cotton fibers of the material. In one of the methods, tiny particles create bumps on the surface of the fibers with a diameter of several hundred nanometers. Many other products, such as paints for the facade of the house or tiles for the roof, are covered with a coating with a microscopic or nanoscopic roughness that gives them the lotus effect.

opposite approach

Titania cleaning herself:

Thin layers of titania show a completely opposite property from the lotus leaves: super-hydrophilicity. However, they also repel dirt and have antiseptic activity against bacteria.

What does the water do:

On top of a super-hydrophilic surface, the water drops are spread and cover the surface with a uniform coating. The water easily dislodges the dirt and carries it in its flow. Superhydrophilicity also prevents fogging of the surface because the water does not collect into the countless tiny droplets that make up the fog.

The chemistry

Ultraviolet light (like that found in sunlight) excites electrons and "holes" (the "absence" of an electron that behaves as a positively charged particle) in titania. The electrons bind to oxygen molecules and form superoxide radicals charged with a negative charge, while the "holes" combine with hydroxide ions found in water and form neutral hydroxyl radicals. These highly active forms kill bacteria and break down organic matter on the surface. Ultraviolet light also changes the structure of the titania layer and makes it super-hydrophilic, a property that allows water to wash away dirt from the surface of the layer.

Multi-layered technology

Anti-fog coating:

Massachusetts Institute of Technology (MIT) researchers have developed a multi-layer super-hydrophilic anti-fog and anti-reflection coating.

Layers of nanoscale silica particles (which have hydroxyl groups attached to their surface) and polymer alternately form a super-hydrophilic coating that can be placed on glass and other materials. The surface of the coating is dotted with nanometer pores, and highly hydrophilic hydroxyl groups help the pores absorb water and immediately remove it from the surface.

beyond self-cleansing

Brandable surfaces:

By switching the hydrophobicity at a precise location on the surface, the scientists hope to control the flow of liquids through networks of microscopic channels in "microflow chips".

Combined effects

Water collection:

Scientists inspired by a desert beetle are developing devices that will utilize a combination of the lotus effect and superhydrophilicity to harvest water from the air in remote and remote areas.

The Stenocra beetle collects water from the morning mists carried by the wind in the Namib desert in Africa. The beetle crouches with its back lifted up against the wind. Most of the beetle's back is superhydrophobic due to the half-millimeter bumps and macroscopic roughness of its waxy surface. But the water droplets still collect across tiny hydrophilic regions at the apex of the bump. The drops roll down into the beetle's mouth.

About the author

Peter Forbes is a science reporter living in London. His book "The Foot of the Gecko" (published by V. V. Norton) from 2006 reviews a variety of technologies that imitate biology (biomematics) or were inspired by it. He was also the editor of The Century Review: The Penguin Book of 20th Century Poetry (Penguin, 2000).

And more on the subject

• The lotus effect. Hans Christian von Baeyer in The Sciences, Vol. 40, no. 1, pages 12-15; January/February 2000.
• water capture by a desert beetle. Andrew R. Parker and Chris R. Lawrence in Nature, Vol. 414, pages 33-34; November 1, 2001.
• self-cleaning surfaces – Virtual Realities. Ralf Blossey in Nature Materials, Vol. 2, no. 5, pages 301-306; May 2003.
• The gecko's foot. Peter Forbes. WW Norton, 2006.
• Patterned Superhydrophobic Surfaces: Toward a Synthetic Mimic of the Namib Desert Beetle. Lei Zhai et al. in Nano Letters, Vol. 6, no. 6, pages 1213-1217; June 2006.
• The dream of staying clean: Lotus and biomimetic surfaces. Andreas Solga, Zdenek Cerman, Boris F. Striffler, Manuel Spaeth and Wilhelm Barthlott in Bioinspiration & Biomimetics, Vol. 2, no. 4, pages S126-