The lotus, a radiant and graceful aquatic plant with a magnificent ability to repel dirt, has been the inspiration behind a range of self-cleaning and dirt-repelling technologies. Now a registered trademark, the “Lotus Effect” promises a future filled with objects that rarely, if ever, need washing. Finally, this means owning bathroom mirrors that unfog itself and shirts that shrug off ketchup and coffee.

And the earth, We have spread it out like a couch; and how excellent We are in spreading it. (Adh-Dhariyah, 51:48)

The factory of the universe and the guesthouse of the earth are so pure and clean and so untainted and fresh that … if … the act of cleansing is not attributed to the Creator of the universe, then … [e]ither all the creatures would have a share in the universal act of cleansing… or there would have to be a consultative committee the size of the universe in order to decide and regulate all those acts together… This is impossible not just once, but hundreds and thousands of times over. (4)

Have you ever come across a “stinky forest”? How about a “messy desert”? These questions are primarily posed as rhetoric since various means of sanitation is ubiquitous in nature (Stinky forests and lakes actually exist only thanks to pollution—a modern problem introduced by human intervention via misconduct of technology) where Lotus Effect is only one of the mechanisms contributing to the steady “house-keeping” in nature.

In muddy waters, the lotus plant stands out with its ever-clean leaves making it a symbol of purity in Asian cultures. Moreover, lotus leaves keep dry under even the heaviest monsoon rain. Such seemingly ironical feats are accomplished by capitalizing on a principle named after the plant itself: The Lotus Effect.

The Lotus Effect

At first sight, the cleansing of the leaves by the rainfall seems utterly trivial. However, the cleaning of the lotus plant (Nelumbo nucifera) by the downpour is not something to be taken for granted. Curiously, when raindrops encounter the lotus leaf, they adopt an almost perfectly spherical shape, resembling ball bearings, and start rolling off the surface carrying away all the dirt (Figure 1). Easier said than done, the lotus plant always keeps pristine, even at the microscopic level.

In the early 1970s, soon after the electron microscope (which can yield vivid images of the ultra small details that are at length scales on the order of a billionth of a meter) became commercially available, German botanist Wilhelm Barthlott (of University of Bonn, Germany) started imaging plants using the newly discovered technology. Sample preparations for electron microscopy normally demanded tedious cleaning procedures since even a speckle of dust could ruin the portrayed landscape at such minuscule scales. To Barthlott’s surprise, some plants apparently were “self-cleaning”: They required very little (or sometimes none whatsoever) cleaning for detailed inspection with the microscope and the lotus plant was a prince of these (1). Further intrigued by the fact, Barthlott looked at the lotus leaves through the electron microscope to find out what renders the lotus plant remarkably competent to repel even the tiniest dirt.

Water’s love-hate affair

Before delving into the secrets for lotus plant’s sanitation, let’s look at the interaction of water with other materials. At the molecular level, the electrical charges are unevenly distributed across a water molecule (i.e. water is highly “polar”) which becomes entangled in a love-hate type of relationship with other materials: Some materials “love” water tending to maximize their interaction with it, whereas others “hate” it, trying to avoid their encounter with water as much as possible.1 To put things in perspective, we can immediately tell from everyday experience that oil is “water-hating” since oil and water do not mix, while sugar is “water-loving” because sugar can dissolve in water without much effort.

Macroscopically—that is, one can immediately realize by touching a lotus leaf—the lotus leaf surface feels waxy, and should therefore be water-hating. However, the waxiness is not enough to equip the lotus plant with its unusual capabilities to remain clean since the lotus leaf is not alone among plants in its leaves’ surface waxiness. Indeed, there is more to the lotus leaf’s curious surface properties than that, a property which was first recognized by Barthlott under the electron microscope.

Super water-hating surfaces

When Barthlott looked at the lotus leaf, besides its spotlessness in the microscopic sense, the leaf surface was decorated with numerous bumps a few micron sizes each (a micron is one millionth of a meter). Such bumpiness served to enhance the water-hating aspect of the surface making it “super water-hating,” corresponding to a contact angle that is close to 170 degrees. As a result, water encountering the lotus leaf surface rapidly beads up forming a nearly spherical shape (akin to the scenario when one drips water on a hot cooking pan) and drops roll off the surface even with an ever-so-slight inclination of the leaf. Apparently, a rolling water droplet is much more effective in picking up the surface dirt than one that is merely sliding, and all the surface debris is thus wiped off.

Although discovered first in the lotus plant, the super water-hating surfaces are serving a passive yet effective means of cleaning for animals like butterflies, dragonflies and other insects that are not able to clean all their body parts actively. For plants, preventing the coverage of their leaves by water (or other contaminants) is important to maximize the exposure to sunlight which would otherwise cause reduced photosynthesis. Another great biological relevance of surfaces of such nature for all these creatures is that it provides protection against the growth of pathogens by keeping the surface dry at all times.

Inspirations from biology for technology

There are ever-growing biologically inspired technologies, so-called biomimetics, and the lotus effect provides a nifty example. Unfolding the mystery behind lotus’ exceptional competence in self-cleaning, Barthlott patented the idea of artificially manufacturing microscopically-raised, water-repelling surfaces to mimic the lotus leaf. “Lotus Effect” is now a registered trademark which underlies commercial products such as self-cleaning windows and fabrics, as well as a dirt-repelling paint. Other applications that are waiting around the corner are: swimsuits that stay dry for days allowing prolonged underwater excursions, coatings on metals to avoid the deposits of marine bioorganisms which would enable up to 40% reduction on fuel consumption by decreasing friction (2). Metal coatings will also find applications to prevent ice formation on plane engines alleviating their wear-and-tear (3).

Apparently, lotus does not collect dirt, but only patents.

Note

1 Contact angle is a metric for the water propensity of a surface. Water drop displays a higher contact angle (hence lower contact area) on a water-hating surface when compared to a lower contact angle (hence higher contact area) on a water-loving one.

References

1. Forbes, Peter. August, 2008. “Self Cleaning Materials,” Scientific American.

2. http://www.basf.com/group/

corporate/en/innovations/events-presentations/nanotechnology/basf

3. “Water-Repelling Metals,” Prachi Patel, MIT Technology Review, 2008, http://www.technologyreview.com/energy/21530/

4. “The Thirtieth Gleam,” Bediuzzaman Said Nursi, Risale-i Nur Collection.

The innovations implemented in nature have the potential to improve the way we do everything, from desalinating water, gluing things together, to streamlining cars. Where there is a design problem, there is a solution for it in nature created by nature’s Designer. We can distinguish the levels in biology that technology can be modeled after as i) mimicking the natural methods in the manufacture of chemical compounds to create new ones, and ii) imitating mechanisms found in nature. There are a few examples of biomimetic materials that are already part of our daily lives. Velcro, for instance, is a brand name of a fabric that consists of hook and loop fasteners used to connect objects. It was invented by Georges de Mestral, a Swiss engineer/inventor. The idea came to him after he took a close look at the Burdock seeds which stuck to his clothes and his dog’s fur on their daily walk in the Alps. He closely examined the hook-and-loop system that the seeds used under a microscope, and realized that the same approach could be used to join other things together. Velcro is commonly used in many different areas, such as in the automotive industry, clothing, shoe making, and for bringing rigid or soft surfaces together. The lotus, which possesses tiny wax crystals on the surface of its leaves, remains pristine and white, even in the midst of swampy, contaminant-rich conditions. For some, the lotus plant is even a symbol of cleanliness.

The lotus effect in material science is defined as the observable self-cleaning property found in the lotus plant. The characteristics of the lotus brought about a new application of biomimetics to the self-purification of surfaces, such as paints and roof tiles that maintain a clean surface like the lotus, by creating a surface that is similar to that of the lotus plants.1 The figure shows that dirt particles are unable to adhere to the paint and simply flow away with the rain. Everybody knows about the vivid colors of butterflies. But where does this color come from? One would naturally think that butterflies must use pigments, as in the paint industry. Actually, there are two fundamental mechanisms by which color is produced on butterfly wings. One leads to what we call ordinary color, and the second leads to the spectacular iridescent color. The ordinary color is due to the presence of chemical pigments, which absorb certain wavelengths and transmit or reflect others. The iridescent color is produced not by pigmentation, but by the interference of light due to multiple reflections within the physical structure of the material. The parts of a butterfly wing are shown in the Figure 3 in the following order, from left to right: Wing > Scales > Veins > Ridges. The size and periodicity of arrangement of the features on the wings causes interference with the visible light, creating color. Using this concept, structures and physical mechanisms that produce a shining color, like that found on the wings of butterflies, have been reproduced in carbon by an international team based at Allied Signal in Morristown, N.J. These highly periodically patterned novel carbon materials possess unique and potentially useful properties. 2 Another striking example that inspires design principles is the box-fish. These are rigid-bodied marine fish that live predominantly in shallow-watered, highly energetic, tropical reef environments. They are remarkably stable and agile swimmers.

They are able to maintain smooth swimming trajectories with minimal pitching, rolling, or yawing, even in highly turbulent waters. Moreover, they are capable of swimming rapidly (> 6 body lengths s-1), can spin around with a minimal turning radius, and can maintain precise control of their position and orientation.3 What applications could these types of properties be used for? In fact, one of the leading car manufacturers produced a bionic concept car that is based on the contours of the boxfish carapace and takes advantage of its drag reduction benefits. Not only the shape, but also the organizational composition of living organisms is highly advanced.

Therefore, great efforts are made to study and understand the formation of the hierarchical structures of these creatures. The shell of the abalone, for instance, is known for being exceptionally strong. It is made of microscopic calcium carbonate tiles that are stacked like bricks. Between the layers of tiles is a sticky protein substance. Even though calcium carbonate is one of the softest materials in nature, when the abalone shell is struck, the tiles slide, instead of shattering and the protein stretches to absorb the energy of the blow. Material scientists at the University of California, San Diego are studying the tiled structure for insight into stronger ceramic products, such as body armor. Researchers at Princeton, working on a grant from NASA, are analyzing the remarkable strength of abalone shells to help make impact-resistant coatings for thermal tiles. There are numerous groups that are working towards a better understanding of the structure and the governing mechanisms involved in the assembly of natural composite systems that have amazing mechanical properties. In synthetic composite structures, the hardness of the material is proportional to the inorganic/mineral content. However, there are striking examples of design in nature in which almost negligible amounts of minerals are used in a specially tailored environment, and very high levels of hardness, comparable to human dentine, can be achieved. An interesting example is sea-worms. Although mainly consisting of soft tissue, these worms have very hard jaws that have an exceptionally low amount of inorganic consistency. The jaw material is of particular interest because of its hard, lightweight and abrasive-resistant properties due to some gradient elements. The chemical surrounds and forms of these elements are not clear enough to be able to identify or mimic the arrangement/structure. These jaws, in addition to their extraordinary mechanical properties, are very good examples of natural gradient materials that have a perfect interface between the hard and soft tissues. Although many high-tech analysis techniques have been devised to understand how such a composite could be formed, particularly in highly unfavorable salty sea or ocean water, and how they have such great mechanical strength, the findings are still incomplete.

The information gathered is like the scattered pieces of a puzzle; to finish the puzzle, the missing pieces must be found with new advancements in analytical tools. What about mussels then? “If we have Batman and Spider-Man, why don’t we have any mussel super heroes?” asks Professor Herbert Waite of the University of California, Santa Barbara. Mussels may not be the biggest or the flashiest creatures in the sea, but they do one thing exceedingly well. They make a glue that lets them anchor themselves firmly to a rock and remain there-drenched by water, buffeted by the ocean’s waves. “I don’t know any other adhesive that can do that,” says Waite.6 Not only the glue, but the threads they make to attach themselves to the rocks are very significant in terms of both their composition and complexity, according to Niels Holten, who is conducting research on these systems at the University of California, Santa Barbara. These threads can elongate and relax with extraordinary mechanical flexibility under great impacts from ocean waves. The Waite group research on these thread cuticles reveals a very important aspect of material science, the significance of which has only very recently been understood: interface engineering. These threads have a very low amount, ca. 1-2 wt%, of metal ions in a polymeric matrix holding together large polymeric chains, which is possibly what gives the structure its flexibility and extensibility. Man-made structures cannot compete with the mechanical performance of these threads, especially at such a low volume of metal ion ingredients.

The ultimate goal of ongoing research is to understand the formation principles of these features so that similar structures can be made, using the same set of principles in laboratory conditions. In fact, the perfections of designs that are implemented in nature turn out to be an enormous fountain of ideas. Jewel beetles, which lay their eggs in freshly charred trees, can detect fires from miles away; the defense industry is studying these beetles for clues to design new low-cost, military-grade infrared detectors. Meanwhile, one of the leading car manufacturers is tapping the locusts’ famed ability to fly in dense swarms without colliding for a possible key to anti-collision devices in cars. And the Defense Advanced Research Projects Agency is funding development of a robot that can climb vertical surfaces, using the same principle that geckos use to walk up walls and saunter upside down across ceilings. There are several examples that could be given on this matter, but, due to limited space, we can only briefly summarize some of them. However, our understanding of the mechanism in nature is very limited, and it is expected that better insight will be gained with the advancement of available analytical tools. The great diversity of product designs in nature is produced from only a few common components, whereas we use a great number of materials and components to achieve new designs. Such high control and hierarchy in design in nature can only be attributed to an artist or designer who hides the perfection of his creation in the details. It is up to us to find out, see, and appreciate these perfections. Material scientists, of course, have the duty of transferring the findings from nature for the service of humankind by turning them into applicable forms in our daily lives.

Notes

1. Lotusan Paints. (2002). Retrieved 12 Nov, 2003, from http://www.lotusan.de Translated by http:// www.google.com.
2. Anvar A. Zakhidov et. al., Science, 282, 897 (1998).
3. URL: http://www.gharib.caltech.edu/bioinspired_ design/index.html
4. http://www.daimlerchrysler.com/dccom/0-5-7154-1-503504-1-0-0-503518-0-0-135-7145-0-0- 0-0-0-0-1.html
5. http://en.wikipedia.org/wiki/Abalone#_note-0
6. Anne Underwood, “Nature’s Design Workshop,” Newsweek, U.S. Edition, September 26 (2005).