Reichelt Chemietechnik Magazine
Rain hits the window, beads up, runs off and taking the dirt with it. Façades, solar panels, paints, technical textiles: self-cleaning surfaces all copy one model, the leaf of the lotus flower. Here is how the lotus effect works, where biomimetics puts it to use and why nanocoatings remain controversial.
What is the lotus effect?
The lotus effect describes how extremely water-repellent surfaces clean themselves. Water does not spread across them as a film but beads into almost perfectly spherical droplets. These roll off as soon as the surface tilts. Dust, dirt and soot particles adhere more strongly to the droplet than to the surface beneath. The droplet carries them away. An ordinary shower of rain cleans the surface, with no cleaning agents and no mechanical action.
The effect takes its name from the lotus flower, an aquatic plant whose range runs from India to China. It also grows in the east of North America. A rhizome anchors it in shallow, muddy waters, and its flowers and leaves rise only slightly above the water. In this warm, humid environment, bacteria, fungi and suspended solids reach the leaves constantly. Self-cleaning is the plant’s answer. Every rainfall carries off deposits that would otherwise damage or shade the leaf.
The lotus flower is only the best-known example. The lotus effect also occurs in European plants such as common reed and nasturtium, as well as in broccoli, cauliflower and other brassicas. In these species, however, it is usually far less pronounced.
Who discovered the lotus effect?
The German botanist and biomimetics researcher Wilhelm Barthlott (b. 1946) was the first to describe the principle scientifically. Towards the end of the 1970s he investigated interfacial phenomena in plants at the University of Heidelberg. He worked with high-resolution scanning electron microscopy (SEM). With it he explained how lotus leaves clean themselves. He also coined the term lotus effect.
As a bio-engineer, Barthlott also worked on translating biological phenomena into technical solutions. His findings on the lotus effect laid the groundwork for giving ordinary surfaces self-cleaning properties. Today, such functional coatings are found on glass façades, greenhouses, photovoltaic modules, car paint and textiles.
How does the lotus effect work?
The leaf surfaces of the lotus flower carry what are known as papillae. These protrusions of the epidermis stand 10 to 20 micrometres high and sit 10 to 15 micrometres apart. They give the leaf a bumpy, studded texture.
The lotus plant also produces waxes in the form of nanocrystalline lipids. Their hydrophobic properties reduce the adhesion of water to the leaf surface. Together, structure and wax keep the area of contact with water extremely small.
When a water droplet lands on an ordinary, more or less hydrophilic surface, it wets a large contact area. The contact angle between the droplet and the surface measures less than 90°, so the droplet lies flat. If the surface is hydrophobic, or water-repellent, the droplet sits up on the lotus leaf instead. In this case the contact angle is far greater than 90°. The contact between droplet and surface is therefore extremely small.
Three factors produce the lotus effect: the surface tension of water, the leaf’s structure and its naturally hydrophobic wax coating. Water is a dipole and forms hydrogen bonds. This gives the water molecules strong cohesion. That cohesion drives water to minimise its surface tension, and with it its surface area. In a gravity-free space, a contact area of zero would produce a perfectly spherical droplet. After all, for a given volume the sphere has the smallest surface area of any body.
The lotus leaf is superhydrophobic. Here the contact angle reaches up to 170°. Only around 0.6 % of the droplet surface remains in contact with the plant surface. The water droplets therefore adopt an almost spherical shape.
The extremely small contact area also means low adhesion, so the droplets barely stick. Water rolls off the leaf at the slightest incline, carrying dust and dirt particles with it. For materials science this was an important insight, and the lotus effect has been influential ever since.
Applications far beyond biology
Dirt-repellent material surfaces that clean themselves in the rain were long a technical aspiration. The appeal was economic, since such surfaces promised considerable savings. The wish list ran from the glass façades of modern buildings to greenhouses and conservatories. Vehicles and their windscreens were on it too. For a long time, none of it was technically feasible. At home, too, windows that never need cleaning were a long-standing wish.
Applications modelled on the lotus effect have since become reality. Modern photovoltaic systems benefit as well, because soiling quickly costs them output.
Particles between one and 100 nanometres in size, known as nanoparticles, make the technical lotus effect possible. Applied to smooth surfaces such as panes of glass, they form nanostructures modelled on nature. Manufacturers either fix such functional coatings in place during production or apply them afterwards as a water-repellent formulation. The material of choice is often nanocrystalline titanium dioxide (TiO2), which sees wide use well beyond surface technology. As a non-yellowing white pigment it goes into writing and printing paper, into paints and into plastics. It also serves as a brightener in cosmetics and pharmaceuticals, as a mineral UV absorber in sunscreens and as the food colouring E 171.
How safe are nanocoatings? Risks to people and the environment
The physical properties of nanomaterials are impressive. They let engineers copy effects such as the surface protection of the lotus leaf. Artificial nanostructures can reduce the wettability of material surfaces or eliminate it entirely. Water then no longer wets the surface but drips off it. Dirt particles find no firm hold, and the water carries them away.
Even so, treating surfaces with nanoparticles remains controversial. Nanomaterials released into the environment end up affecting us too.
Hydrophobised functional surfaces shed nanoparticles mainly through mechanical abrasion, but also through leaching. Surfaces coated after manufacture are naturally more affected. Materials with the nanostructures firmly integrated into the surface hold up better. This becomes obvious with the commercially available nano-sealant kits used to make car windows water-repellent. The water- and dirt-repellent coatings they produce do not last. They are, however, a potential source of nanoparticles in the environment.
Released nanoparticles can enter the body through the lungs or the food chain. Once inside, they can cause lasting harm. Nanoparticles in the airways are the main concern. Researchers suspect they raise the risk of lung cancer substantially, on a scale discussed as comparable to asbestos. Studies also point to tissue changes and inflammatory reactions in the gastrointestinal tract caused by nanoparticles. They can impede blood circulation in the capillaries and lead to serious circulatory disorders that medication can barely address.
Imitating nature with nanobiotechnology confronts researchers with a dual task. They must study and assess not only the positive effects but also the harmful ones. Any careless or negligent handling of nanomaterials can become a danger to people and the environment.
Image sources: Professor Dr. Wilhelm Barthlott | © Issempa – commons.wikimedia.org Computer graphic of lotus leaf | © William Thielicke / Willa~commonswiki – de.wikipedia.org Self-cleaning glass | © René F. Appenzeller / Appi-TV – de.wikipedia.org Lotus effect | © H. Zell / Llez – de.wikipedia.org
