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ramé-hart instrument co. April 2013 Newsletter

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Nanobiomimicry: Five things we have learned from nature
It turns out that if you want to satisfy your curiosity, you can study how things are made and how they work. How It's Made is my boys' favorite show. However, if you want to be truly marveled, study how things in nature are made and how they work. Start with your own body - it's a wonderland of unanswered questions. Take your appendix, for example, even the experts aren't sure what it's for, how it works, and why it's there.

Biomimicry involves studying how things work in nature and then using the patterns, materials, systems, and answers that nature has found for solving problems to find solutions to our human problems. The "bio" comes from the Greek root meaning life; "mimicry," or to mimic, means to copy. Add "nano" to the front, which narrows the structures in question down to the nanoscopic scale (say from 1 to 100 nanometers) and you have "nanobiomimicry."

Our purpose is to discover five things that nanotechnologists have learned from Mother Nature. Here's our list:

1. Butterflies in Latin America don't need to take showers. It turns out that the wings of a Morpho butterfly which is found in Central and South America have a near superhydrophobic contact angle of 140°. They also have a very low contact angle hysteresis with a roll-off angle of only 3°.1 As a result, the wings of the Morpho are not only water-repellent but also self-cleaning. 

Further, the colors on the wings of a Morpho are the result, not of pigmentation, but of structural coloration. Light that passes through the wings are reflected at different wavelengths producing vibrant colors - kind of like a soap bubble. Nanotechnologists (including some at Qualcomm) are developing biomorphic mineralization processes which can be used to produce photonic microstructures for color displays that mimic the structural coloration of the Morpho. How's that for cool?

2. You don't have to wash your house if you can make it more like a Lotus leaf. As cool as the Morpho butterfly wing is, the Lotus leaf is even more intriguing - and more hydrophobic. Some leaves have a measured water contact angle as high as 170°. In my experience, it's difficult to achieve a water contact angle in excess of about 120° with a flat surface. However, if you add either nanostructures (like nanotubes) or a microstructure, the contact angle can be increased. In the case of the Lotus leaf, however, it has both microstructures and nanostructures. This double or hierarchical structure (as illustrated in the graphic below) further increases the contact angle and lowers the contact angle hysteresis. Dr. Wilhelm Barthlott, a botanist at the University of Bonn (Germany), discovered the secret behind the Lotus leaf thirty years ago and patented the discovery calling it the Lotus Effect.

The Lotus leaf uses a hierarchical structure.

As a result of the non-wetting surface, the Lotus leaf is the archetype for self-cleaning surfaces. Nanotechnologists have developed a number of commercial products that mimic the hierarchical structure of the Lotus plant. Among them is StoCoat Lotusan2 paint -- which is what you need to paint your house with to make it self cleaning. The video below illustrates the self-cleaning properties of a surface coated with Lotusan.

3. There are two ways to skin a cat. But if you're a bug, watch out for SLIPS. Researchers at the Aizenberg Biomineralization and Biomimetics Lab at Harvard are using a different role model for making slippery slopes slipperier. Rather than emulate the hierarchical structure of the Lotus leaf, they are copying the method used by pitcher plants.

A pitcher plant captures and consumes bugs by lubricating the rim (also called ribs) and the inside walls (called wings) of a tubular structure. The victims that fall in are unable to climb back out due to the extreme slipperiness of the tube, they end up at the bottom of the pitcher where they are consumed by acids and digested in time by the plant. The slippery surface is a combination of lubricating liquids and a porous surface structure.

The researchers at Harvard have developed materials and methods that emulate the walls of the Nepenthes pitcher plant. This technology has been named SLIPS (Slippery Liquid-Infused Porous Surfaces) and offers some compelling advantages over other non-wetting and self-cleaning methods. SLIPS can be tweaked to work in difficult environments such as high temperatures or pressure. They can be made to be self-healing. They can be made from low-cost materials. They are also oleophobic (oil-repelling). Since SLIPS can be made optically transparent they are potentially ideal for solar panels, sensors, lenses, and other optically sensitive applications.

4. There will be no more pounding on the bottom of the ketchup bottle. A technology similar to SLIPS known commercially as LiquiGlide also uses a lubricant to make surfaces slipperier. But that we already covered in our January Newsletter. Read about it here: http://www.ramehart.com/newsletters/2013-01_news.htm

5. Michael Phelps swims like a shark. Actually, it might be more accurate to say that he swims faster with the help of technology copied from sharkskin. The secret behind sharkskin are dermal denticles, small tooth-like scales that improve the flow of water and reduce friction. Speedo has designed a series of swimwear that uses their Fastskin technology - which is basically a manmade textile that copies the riblet topology of sharkskin in order to make fast swimmers faster.3 

Researchers that seek to tweak the parameters that define the topology used for sharkskin-inspired swimsuits are concerned with both contact and roll-off angles. A high contact angle reflects a low surface energy. The lower the surface energy the less propensity there is for the liquid to want to bond to it. Further, a low roll-off angle would indicate less friction between the solid (fabric) and water. So, the key to a good design is high contact angle and low roll-off angle.

1Collins, M. 2004. Design and nature II: comparing design in nature with science and engineering. Southampton: WIT.
2StoCoat Lotusan is a registered trademark of Sto Corp.
3Speedo and Speedo Fastskin are registered trademarks of Speedo International Limited.



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