At the University of California San Diego, a team of physicists used x-rays to discover two physical attributes that make butterfly wings so bright and colorful.
“Over millions of years, butterflies have evolved sophisticated cellular mechanisms to grow brightly colored structures, normally for the purpose of camouflage as well as mating,” says UC San Diego associate professor Oleg Shpyrko. “It’s been known for a century that the wings of these beautiful creatures contain what are called photonic crystals, which can reflect light of only a particular color.”
But still, it was a mystery how the wings are assembled in a way that make them so bright and colorful. In an effort to answer that question, Shpyrko and Andrej Singer, a postdoctoral researcher in his laboratory, went to the Advanced Photon Source at the Argonne National Laboratory in Illinois, which produces laser-like x-rays.
By combining these laser-like x-rays with an advanced imaging technique called “ptychography,” the physicists developed a new microscopy method to visualize the internal nanostructure of the tiny “scales” that make up the butterfly wing without the need to cut them apart.
The researchers revealed that these tiny wing structures consist of “highly oriented” photonic crystals.
“This explains why the scales appear to have a single color. We also found through careful study of the high-resolution micrographs tiny crystal irregularities that may enhance light-scattering properties, making the butterfly wings appear brighter,” says Singer.
According to the researchers, these crystal dislocations or defects occur 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. In photonic crystals, defects can enhance light-scattering properties through an effect called light localization.”
“In the evolution of butterfly wings,” he adds, “it appears nature learned how to engineer these defects on purpose.”
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