July 27, 2009: Researchers from Georgia Tech have been examining small green beetles whose properties may help divine interesting facts about optics and liquid crystals.
The work, published in the July 24 issue of Science, focuses on the iridescent metallic green beetle Chrysina gloriosa, whose exoskeleton offers a unique structure to create striking colors — specifically, it incorporates a liquid crystalline material (composed of ~10μm hexagonal cells, pentagons, and heptagons), with surface structures that self-assemble into polygonal shapes, in relation to the shell’s curvature.
A closer look, from the abstract:
In bright field microscopy, each cell contains a bright yellow core, placed in a greenish cell with yellowish border, but the core disappears in dark field. With use of confocal microscopy, we observe that these cells consist of nearly concentric nested arcs that lie on the surface of a shallow cone. We infer that the patterns are structurally and optically analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. These textures provide the basis for the morphogenesis as well as key insights for emulating the intricate optical response of the exoskeleton of scarab beetles.
“When we looked at the beetle’s surface, we found tiles in the shapes mostly of hexagons, pentagons and heptagons,” patterns that likely arise “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,” said Mohan Srinivasarao, professor in the GaTech’s School of Polymer, Textile and Fiber Engineering, in a statement.
Examples of the jeweled beetle used in this study. In the second photo, the background is a close-up image of the insect’s light-reflecting structures. (Source: Georgia Tech)
The unique helical structure that reflects light of two specific colors, and of only left circular polarization, they noted. Solidified structures produced from a cholesteric liquid crystal and its defects on the beetle’s shell reflect bright green light (530nm wavelength) mixed with yellow light (580nm wavelength). The researchers speculate the specific colors and polarization may help ward off predators or help attract mates. Future work will study other insects with complex color-creating optical structures. “There are hundreds of thousands of species, and the way they generate color is just stunning — especially since it is all done with water-based systems, mostly based on the biopolymer chitin,” noted Mohan Srinivasarao. “This is self-assembly at several levels, and we need to learn a lot more to duplicate what these insects do.”
A slideshow of the Georgia Tech beetle research, put together by the National Science Foundation (NSF) which helped fund the work, can be viewed at: http://www.nsf.gov/news/newsmedia/beetles/.