Nanoscience in Nature

The High Technology of Butterfly Wings

What comes to mind when you hear the word “nanoscience”? Do you think of DNA or microscopic robots? Do you imagine groups of lab-coat clad scientists buzzing around some large and unnecessarily complex looking piece of equipment?

Or do you picture the electronics that power and control the smart phone or laptop with which you are reading this? Each of these are certainly examples of nanoscience (depending on the instrument your imaginary scientists are fiddling with). However, for the first installment of this project, I want to open with a brief discussion of some prominent examples of nanoscience in nature. For now, picture a butterfly. Among other impressive accomplishments, Mother Nature was Earth’s first nanoscientist. Perfecting her craft over millions of years of careful refinement, she has created nanoscale masterpieces which inspire and guide us today. In an impressive feat of nanoscale-engineering, butterfly wings are textured to include an array of micro and nanosized ridges and protrusions (see middle image on right).1,2

These nanoscale ridges make the wings of the butterfly strongly repulsive to water, and the orientation of the ridges results in an unusual behavior where water droplets will roll from the body of the butterfly out to the ends of the wings, but not in the reverse direction.1,2 This is beneficial for two reasons. First, the wings are self-cleaning; as water rolls off, if carries away dirt, dust, and other contamination with it. Second, this superhydrophobic (super water repelling) behavior prevents the butterfly’s wings from sticking together when wet via surface tension. Imagine dipping your finger in a glass of water, then slowly removing it. Just at the point when your finger rises above the surface of the water, you’ll notice that you “pull” some of the water with you for a short distance. The force required to break the surface tension of the water in this way may be trivial for a human, but for a butterfly, a drop of water between the wings might as well be glue.

Surface to Volume Ratio

The basis of this behavior is the incredibly high surface area that nanoscale structures exhibit. To properly convey this let’s use a simple example. Imagine a marble, about 1 cm in diameter. The volume of the marble is 0.524 cm3 and the surface area is 1.05 cm2. Now, image a pile of sand with the same volume of that marble. This pile will contain about 1000 grains of sand, and the total surface area of all of the sand will be 10.47 cm2. Continuing with this example, if we had a pile of nanoparticles with the same volume of the marble, we’d end up with a total surface area of 104800 cm2, or about 10–15% the area of a standard US apartment. Based on this example, hopefully it’s clear that as feature sizes get smaller and smaller, the amount of area increases dramatically even as the volume stays constant. This is one of the great advantages of working on the nanoscale. In the case of the butterfly wings, this means that there is a lot of surface area on the wing capable of interacting with a droplet of water. Butterfly wings are formed of chitin, a natural polymer (chain of molecules) that also makes up the exoskeletons of crabs, lobsters, shrimps, and many other insects, as well as the beaks of squids and octopuses. This material is naturally water repelling, taken to the extreme on the wings of the butterfly, where the interaction area between a droplet and the chitin can be enormous.3 This sort of nanoscale-based behavior can also be seen in lotus leaves, long seen as a symbol of purity due to their hydrophobicity and self-cleaning nature.

Why does this matter for us?

References

1. T. Wagner, C. Neinhuis, and W. Barthlott, Wettability and Contaminability of Insect Wings as a Function of Their Surface Sculptures, Acta Zoologica 77, 213–225 (1996).

2. Y. Zheng, X. Gao, and L. Jiang, Directional Adhesion of Superhydrophobic Butterfly Wings, Soft Matter 3, 178–182 (2007).

3. P.K. Dutta, J. Dutta, and V.S. Tripathi, J. Sci. Ind. Res. 63, 20–31 (2004).

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