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Beetles Offer Blueprint For Light-Reflecting Materials

July 24, 2009

“Jewel beetles” are well known for their striking metallic green external skeletons that appear to change colors according to the angle of view.

Now, thanks to new research by scientists at the Georgia Institute of Technology in Atlanta, the beetles may also offer a blueprint for materials that reflect light rather than absorbing it to produce colors.

The scientists discovered that jewel beetles (Chrysina gloriosa) change their color because of the light-reflecting properties of the cells that make up their external skeletons, not because of the unique, light-absorbing properties in their pigment.  

The findings could be significant for companies such as car manufacturers that look to manufacture reflective light paints for automobiles, the researchers said.

“The Chrysina gloriosa beetle reflects a green light,” said Mohan Srinivasarao with Georgia Tech’s School of Polymer, Textile & Fiber Engineering, and the study’s lead researcher.

“This has been known for about 100 years, but we’ve determined the colors we see result from the beetle’s physical structure rather than its biology.”

Srinivasarao had previously studied cholesteric liquid crystals, which are used in hi-tech reflective displays.

“The two systems were remarkably similar,” he said during an interview BBC News.

“When I first looked at the beetle’s exoskeleton (with the microscope) I thought, ‘I’ve seen this somewhere before’.”

When light hits an opaque surface, the surface will scatter, absorb or reflect that light to produce colors.  In the case of the jewel beetle’s external skeleton, its five, six and seven sided cells spontaneously arrange themselves to reflect light at certain wavelengths that produce green, yellow and red colors.

Srinivasarao hypothesizes that beetle cells form similar to a certain kind of liquid crystal called a ‘cholesteric’ liquid crystal, whose free surface has cone-like structures and that has a helical arrangement of molecules.

The research suggests the cells derive from spontaneous arrangement of glucose-like particles known as chitin molecules, which form as cones like those in a cholesteric liquid crystal.  When these cones solidify, they preserve their structures and produce colors as light hits them from different angles.

The beetle’s structure also forms helices similar to a cholesteric liquid crystal in that its straight cells sit on or are used to form the curved structure of its external skeleton. Research shows that when the pitch of the helix of cholesteric liquid crystals is close to the wavelength of visible light, they reflect light with specific wavelengths, leading to brilliant metallic colors.

“It’s stunning how similar the two things are,” said Srinivasarao.

Miniature optical devices and photonics such as those imagined for microlasers and implantable medical sensors could benefit from the finding. Scientists already are analyzing ways to commercialize and apply materials that have properties similar to jewel beetles.

Indeed, researchers in New Zealand are studying beetles to produce a thin, solid mineral, magnesium oxide, which can be ground into flakes and potentially used as a currency security measure.

Automakers might also be able to use the materials to paint cars that change color depending on a person’s angle of view.   Additional applications might include decorative paints and use on any surface requiring light reflection without light absorption.

Srinivasarao says he doesn’t yet completely understand all the optical properties and behavior of light as it interacts with the beetle’s external skeleton.   Additional work needs to be done, he says, but it is something he and his team are pursuing.

The study was published in the July 24 issue of the journal Science.

Image 1: Scientists at the Georgia Institute of Technology in Atlanta recently discovered jewel beetles, Chrysina gloriosa, change color because of the light-reflecting properties of the cells that make up their external skeletons, not because of unique, light-absorbing properties in their pigment. Credit: Georgia Tech, Gary W. Meek

Image 2: Research suggests jewel beetle cells come from spontaneous arrangement of glucose-like particles called chitin molecules that form as cones. When these cones solidify, they preserve their structures and produce different colors as light hits them from different angles. Credit: Zina Deretsky, National Science Foundation

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Beetles Offer Blueprint For Light-Reflecting Materials