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