Defects on Purpose Make Butterfly Wings Shine
Illustra Media’s film Metamorphosis: The Beauty and Design of Butterflies showcased many dazzling butterfly wings, but didn’t have time to discuss the secrets behind those colors. Scientists have long identified the source of the colors as not coming from pigments, but from light interference patterns at the nanoscale. A regularly repeating 3-D structure called a “photonic crystal” reinforces some colors and interferes with others. That’s how the dazzling blue of a Morpho butterfly is produced: crystals in the scales of the wings play tricks with light. Grind up the scales and you won’t get a pile of blue dust, just brown or gray material. It’s the highly-organized nanostructure that generates the brilliant blue of the living insect flapping in the sunlight.
But how does a butterfly grow those crystals? Something has to guide them into position as the wing develops in the chrysalis. Metamorphosis showed the wings taking shape but only touched on the complexity involved. Now, scientists at the University of California, San Diego, have shined some X-ray light onto the process. In a paper in the AAAS open-access journal Science Advances, they used X-ray diffraction to examine the properties of individual scales.
Many organisms in nature have evolved sophisticated cellular mechanisms to produce photonic nanostructures and, in recent years, diverse crystalline symmetries have been identified and related to macroscopic optical properties. However, because we know little about the distributions of domain sizes, the orientations of photonic crystals, and the nature of defects in these structures, we are unable to make the connection between the nanostructure and its development and functionality. [Emphasis added.]
As Illustra showed with electron micrographs, the wings are composed of scales that overlap like shingles on a roof. The scales, made of the protein chitin, begin as individual cells. The cells must manufacture chitin and place it in the right orientation. The scientists found that the crystals of chitin are strongly oriented normal to the plane of the scale (i.e., perpendicular). The crystals apparently grow from the edges of the cell toward the middle, where they meet at domain boundaries.
We report on nondestructive studies of the morphology of chitinous photonic crystals in butterfly wing scales. Using spatially and angularly resolved x-ray diffraction, we find that the domains are highly oriented with respect to the whole scale, indicating growth from scale boundaries. X-ray coherent diffractive imaging reveals two types of crystalline domain interfaces: abrupt changes between domains emerging from distinct nucleation sites and smooth transitions with edge dislocations presumably resulting from internal stresses during nanostructure development. Our study of the scale structure reveals new aspects of photonic crystal growth in butterfly wings and shows their similarity to block copolymer materials. It opens new avenues to exploration of fundamental processes underlying the growth of biological photonic nanostructures in a variety of species.
The domain boundaries, they found, are randomly oriented, yet each crystal is highly organized within each domain. “The photonic crystal is reminiscent of a thin polycrystalline film, with the domains being highly oriented in the direction normal to the scale boundary, which suggests a layer-by-layer crystal growth process starting at the cell membrane,” the paper says. The boundaries, furthermore, are “extremely sharp,” on the order of one unit cell. “The preferred orientation of the domains, normal to the scale plane, is quite remarkable and is consistent with crystal nucleation at the scale boundary and crystal growth outward.” This gives the researchers some ideas about how to manufacture artificial photonic crystals. Again, it’s remarkable as well as intriguing:
The discovery of the edge dislocations in the biological photonic crystals is particularly remarkable because artificially manufactured topological defects in photonic crystals lead to interesting optical properties such as Anderson localization of light. Two intriguing questions are whether nature-engineered defects for a particular purpose and whether mimicking similar growth conditions allows for controlled manufacturing of defects in artificial photonic structures.
“Nature-engineered defects for a particular purpose”? Can it be that defects have a function? Indeed they can. The press release from UCSD describes why scientists sometimes “engineer” defects to improve their strength and function:
These crystal dislocations or defects occur, the researchers say, when an otherwise perfectly periodic crystal lattice slips by one row of atoms. “Defects may have a negative connotation, but they are actually very useful in improving materials,” explains Singer. “For example, blacksmiths have learned over centuries how to purposefully induce defects into metals to make them stronger. ‘Defect engineering‘ is also a focus for many research teams and companies working in the semiconductor field. In photonic crystals, defects can enhance light-scattering properties through an effect called light localization.”
The bug is a feature. Defects consisting of random domain boundaries in the butterfly scale actually enhance the light-scattering properties. Post-doc Adrej Singer describes “tiny crystal irregularities that may enhance light-scattering properties, making the butterfly wings appear brighter.”
It looks like we will have to add “defect engineering” to our list of intelligent design sciences in action. How strange that the butterfly figured this out before mankind did:
“In the evolution of butterfly wings,” he adds, “it appears nature learned how to engineer these defects on purpose.”
We can dismiss the references to evolution. Nature does not learn to engineer things for a purpose. Organisms do not evolve sophisticated cellular mechanisms in order to produce optical properties. Minds do that. If our best optical engineers look to butterflies for inspiration to improve their manufacturing, what does that tell you? It implies that engineers do not employ unguided, aimless Darwinian processes of chance. They use intelligent design. By arguing from lesser to greater, we explain what goes on in the “black box” of the chrysalis, where “remarkable” and “sophisticated” engineering generates structure, function, and beauty.
Image credit: Illustra Media.