By Fernando Luna Vera
Ph.D. Candidate, Chemistry Department, VCU
Science Museum of Virginia Volunteer
On August 16th 2008 millions of people witnessed one of the greatest achievements in the history of the Olympics games, Michael Phelps winning his eighth gold medal in Beijing. Beyond the incontestable talent of Phelps, this amazing feat was in part possible by the technology he wore. The fabric utilized in his swimsuit emulates a shark’s skin, which minimizes the drag of one’s body in the water. Using this technology, Phelps was able to swim like a shark as if he were hunting his prey.
Bio-inspiration applied to technology development is called Biomimetics. This discipline has shown us that by learning and unraveling nature’s tricks human life can be transformed in many ways, one ruse at a time. One example is the phenomenon of high water repellency (super-hydrophobicity) born by some plant leaves and especially distinguishable in the species Nelumbo lutea better known as the “Lotus,” from where the effect takes its name: The Lotus effect.
When water does not stick to a surface, like on Lotus leaves, the droplets adopt a spherical shape, so it looks like a marble sitting on top of a table. In contrast, when on a surface that is water-friendly (hydrophilic), droplets can spread out and appear flat like a Frisbee. A collateral effect of this is that the sphere-like droplets can roll along the surface instead of slide down on it. This fact creates one of the most appealing features of the Lotus: it can self-clean its own leaves.
This effect is due to water’s high surface tension. Molecules on the droplet’s surface eagerly want to adhere to anything that deter them from being exposed to the water/air interface. Because the Lotus offers a waxy, hydrophobic substrate, the water does not spread out on the leaf. Water has to adopt the shape that allows the smallest air-exposed area: a sphere. Now, since the surface molecules still want to get rid of the overwhelming job of being at edge of the droplet, when the drop rolls down the surface all the tiny particles like soot or pollen found on its path get picked up by it. The water surface then becomes covered with a layer of dirt. This is the way self-cleaning occurs and in the end the leaf and water droplet are happy.
So therefore, self-cleaning is possible when water does not stick to the substrate and retains a spherical configuration. But how does the Lotus achieve this? Using highly sophisticated microscopes, a technique called Scanning Electron Microscopy (SEM), researchers have shown that the Lotus’ surface is not smooth at all and in addition to its waxy layer it possesses a well organized structure. It is widely recognized now by scientists that nearly all super-hydrophobic and self-cleaning leaves consist of an intrinsic hierarchical structure where pillars of micrometric size (just millionths of a meter) arrange to form a forest of columns made of water repellent wax. If Lotus’ surface were waxy, but not smooth, it would repel water; water would not acquire a well-defined sphere-like shape and therefore would not pick up the dirt particles.
This knowledge from the Lotus plant has provided the secret to developing many products available today which produce well structured surfaces (Lotus-like). Some self-cleaning paints, which are used in exterior wall coatings (i.e. Lotusan ®) will self-clean a surface when it rains. Can you image your house having a fresh look just after a summer storm? There are water-repellent fabrics that never get dirty (i.e. Nanotex ®) and are used to manufacture outdoors furniture and clothes that very seldom need to get laundered. General Motors is currently working on developing a Lotus-like metal surface to be incorporated within aircraft-technology that avoids the formation of ice, simply because water won’t stick to this kind of surface. These technologies are not just providing comfort; they help in reducing water consumption too, which in the end turns into environmental gain. The human imagination is unlimited and hopefully new technological applications for the lotus effect are still to come. By the way….have you seen a gecko climbing a wall lately?
References and links:
Bhushan, B., Jung C. Y., Koch K., Phil. Trans. R. Soc. A 2009 367, 1631-1672