Friday, February 28, 2014

Lotus Leaves Inspire Self Cleaning Superhydrophobic Surfaces

In some Asian cultures, the lotus plant is considered a symbol of purity. The large fan-like leaves of the lotus plant stay dry and pristine despite the muddy ponds and lakes the plant calls home. How does his work and what does it mean to us?

The lotus leaf is classified as a super-hydrophobic surface. Hydrphobicity can be classified by the contact angle of the water with the surface it is resting on. If the water has an angle >90o then it is classified as a hydrophobic surface. For something to be super-hydrophobic, the contact angle must be >1500.  When water rests on a surface like this, only about 2-3% makes contact with the material.

The secret to the lotus plants amazing super-hydrophobicity is due to the micro and nanostructure of the leaves. The surface of the leaf is covered in papilla, which are small bumps or ridges about 10-20 microns in height and 10-15 microns in width. These papilla are also covered in a thin layer of wax. When water comes into contact with these wax covered bumps, it beads up and displays something called the lotus effect.

This phenomena occurs when water rests on the surface of the leaf (or another super-hydrophobic surface) and creates small spheres. The small gaps between each individual papilla keep a small layer of air trapped between water and the surface of the leaf. This creates a low surface energy material and causes the surface tension of water to create droplets instead of spreading out across the material.

Not only does the lotus effect mean that the leaf stays dry, it also means it is self cleaning. When dirt is deposited on the leaves, all it takes is some water to run over the surface to clear all the dirt particles. As the water moves across the leaf, it makes contact with and picks up the particles. It then just rolls off the plant, carrying the dirt along with it.

How can we use this? A great number of super-hydrophobic coatings have already been created. Some are used to coat everyday objects such as cloth or walls. Others are attempting to coat medical appliances in order to decrease the number of bacterial colonies on equipment or medical implants. One major application would be a surface coating for solar panels. The self cleaning properties of the lotus effect would allow rain to wash away any buildup of dust on the panels and keep them efficient without any labor.

The main problem with the coating is that it relies on microstructure to create this super-hydrophobic effect, not by using fluorine (a major component in Teflon) atoms to create a kind of hydrophobic sealant. This means that the coating is vulnerable to mechanical wear and has limited applications. Next post, we will explore a similar functional technology which attempts to overcome this shortcoming.

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