Kenilworth Park and Aquatic Gardens and Lotus Leaves

This past Monday I finally got around to visiting Kenilworth Park and Aquatic Gardens National Park in Washington, DC.  It was a lovely visit, considering how hot it was, and I came away with some fun photographs.  I thought I'd share some of my favorite images as well as talk a little bit about the properties of the lotus, which is one seriously cool plant.


Kenilworth Aquatic Gardens-July 19, 2010 - Images by Elijah Goodwin

Besides its cultural significance, the sacred lotus (Nelumbo nucifera) has some very interesting biological properties as well.  First off, it was recently discovered that the flower is capable of thermogenesis, producing its own heat and regulating its own temperature (much like a mammal does) despite the outside temperature dropping lower.  It is likely that the flower does this to attract insect pollinators (which are ectothermic or cold-blooded and would be attracted to the heat when the outside temperature drops).  Secondly, the leaves of the lotus are superhydrophobic to water droplets.  In other words, water does not stick at all to the leaves and just rolls off leaving the leaf dry.  This is known as the lotus effect.



So, how/why does this superhydrophobicity occur (warning nerd alert here)?  Well initially, when this property was first noticed, people thought that the lotus leaves must be super smooth, giving the water nothing to stick to.  However, when the scientists got the leaf under an electron microscope, it turned out to be quite the opposite; lotus leaves are very rough, in a finely structured way.  It turns out that the surface of the leaf is covered with micro-scale bumps which are then covered with nanoscale hair-like tubes.  These structures are coated with a hydrophobic (literally "water hating") wax.  Thus the droplet sits on top of these fine structures instead of settling down amongst them.  This also means a very small portion of the drop is actually touching the leaf (2-3% of the droplet-covered surface).  Because of this, the cohesion (attraction of the water molecules for each other) forces are much greater than the adhesion (attraction of the water molecules for the leaf surface) forces and the water remains in a spherical drop (almost like it would when travelling through the air).  If the leaf is tilted, the drop will literally roll right off the surface.  In fact the way the water behaves on the surface of the leaf looks and moves a lot like quicksilver (liquid mercury) if you every had a chance to observe or play with it before it was determined to be a hazardous material.  The other cool thing about this is that the same properties cause dirt and other detritus not to adhere to the leaf very well either.  When a water droplet rolls over the dirt, its adherence to the water is much stronger than its adherence to the leaf, so the dirt rolls off with the water (see the image below).  Thus lotus leaves are said to be "self-cleaning".  Through a process known as biomimicry, industry has copied the structure of the lotus leaf and applied it to all sorts of products, including early on, a paint known under the brand name Lotusan which both repels water and is self-cleaning. 

Learn more about the lotus effect and other examples of biomimicry at asknature.org.