The mesmerizing wings of the tropical butterfly Morpho often shimmer blue, green and violet. The iridescence of these wings doesn’t result from pigments or dyes, but from structures that reflect and scatter light waves, making them interfere with each other to generate colors. Scientists have already taken lessons from these butterflies to create new kinds of displays, fabrics and cosmetics. Now researchers have discovered a new aspect of these structures, one having to do with how they differ from top to bottom, which they say could stimulate many technological applications, detail findings in the Proceedings of the National Academy of Sciences.
The iridescence of the scales on Morpho wings arises from vertical ridges on them, stacks of chitin separated by air gaps. After investigating the surfaces of these structures, analytical chemist Radislav Potyrailo at the GE Global Research Center in Niskayuna, N.Y., and his colleagues found the bottom of these ridges differed from their tops.
The tops of the ridges are polar — that is, the electric charges of the molecules within them are separated. In contrast, the bottoms of these ridges are less polar. The researchers confirmed the existence of this gradient by exposing the ridges to polarity-sensitive dyes as well as vapors of different polarity, each of which responded in expected ways to different parts of the ridges.
“We believe that this feature is not essential for butterfly survival, but rather is a byproduct of the process of butterfly wing scale development,” Potyrailo says.
Although this gradient may serve the butterflies no function, Potyrailo and his colleagues feel mimicking it could have many technological applications. For instance, the fact that vapors of different polarity stick onto different parts of the ridges could lead to sensors that can each detect the presence of a variety of vapors, rather than requiring a battery of sensors each devoted to a specific vapor.
“The next steps should be focused on detailed tests of responses of these structures to mixtures of vapors and seeing what the limitations of these natural sensors are,” Potyrailo says. “Next, nanofabrication of such bio-inspired structures should be involved, with additional tests to bring these sensors from pristine laboratory conditions into practical applications.”
Other applications might include security tags, self-cleaning surfaces and protective clothing. For instance, security tags might change color after exposure to specific compounds in the breath or vapors from food and beverages. Developing coatings that vary across their surfaces might also lead to self-cleaning surfaces that can repel several types of dirt, and protective clothing that can repel several types of chemicals, Potyrailo says.