Jewel Beetle with complex structures to create unique colors

30 Jul '09

5 min read

A small green beetle may have some interesting lessons to teach scientists about optics and liquid crystals – complex mechanisms the insect uses to create a shell so strikingly beautiful that for centuries it was used in jewelry.

In an article published in the July 24 issue of the journal Science, researchers provide a detailed analysis of how the jeweled beetle Chrysina gloriosa creates the striking colors using a unique helical structure that reflects light of two specific colors – and of only one polarization: left circular polarization. The reflecting structures used by the beetle consist predominately of three different polygonal shapes whose percentages vary with the curvature of the insect's shell.

“Iridescent beetles, butterflies, certain sea organisms and many birds derive their unique colors from the interaction of light with physical structures on their external surfaces,” said Mohan Srinivasarao, a professor in the School of Polymer, Textile and Fiber Engineering at the Georgia Institute of Technology. “Understanding how these structures give rise to the stunning colors we see in nature could benefit the quest for miniature optical devices and photonics.”

View a National Science Foundation presentation on beetle iridescence

With support from the National Science Foundation, Srinivasarao and colleagues Vivek Sharma, Matija Crne and Jung Ok Park used two different microscopy techniques to study the surface structures on the shell of the beetle. What they found confirmed earlier suggestions that the colors are produced from liquid crystalline material, which self-assembles into a complex arrangement of polygonal shapes each less than 10 microns in size.

“When we looked at the beetle's surface, we found tiles in the shapes mostly of hexagons, pentagons and heptagons,” Srinivasarao said. “These patterns arise, we think, because of the nature of the cholesteric liquid crystal and how the liquid crystal phase structures itself at the interface between air and fluid. We think these patterns result because the liquid crystal must have defects on the surface when exposed to air, and those defects create the patterns in the beetle's shell or exoskeleton.”

Because of simple geometric restrictions, the percentage of each shape depends on the curvature of that particular section of the shell. “This is really a pattern formation issue,” said Srinivasarao. “It is difficult to pack only hexagons onto a curved surface. On flat surfaces, there are fewer defects in the form of five- and seven-sided cells.”

In addition, the five- and seven-sided cells normally appear in pairs, an issue also dictated by the geometric difficulties of packing the shapes onto curved surfaces. The researchers found very similar structures in the ten different beetles purchased from an insect supply house.

Liquid crystalline materials are valuable industrially, used in displays for laptop computers, portable music players and other devices. They are also used in children's thermometers, where temperature affects the color of light reflected from the material, indicating whether or not a child has a fever.

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