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On the colour of wing scales in butterflies: Iridescence and preferred orientation of single gyroid photonic crystals
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry. The Australian National University, Australia.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2017 (English)In: Interface Focus, ISSN 2042-8898, E-ISSN 2042-8901, Vol. 7, no 4Article in journal (Refereed) Published
Abstract [en]

Lycaenid butterflies from the genera Callophrys, Cyanophrys and Thecla have evolved remarkable biophotonic gyroid nanostructures within their wing scales that have only recently been replicated by nanoscale additive manufacturing. These nanostructures selectively reflect parts of the visible spectrum to give their characteristic non-iridescent, matte-green appearance, despite a distinct blue-green-yellow iridescence predicted for individual crystals from theory. It has been hypothesized that the organism must achieve its uniform appearance by growing crystals with some restrictions on the possible distribution of orientations, yet preferential orientation observed in Callophrys rubi confirms that this distribution need not be uniform. By analysing scanning electron microscope and optical images of 912 crystals in three wing scales, we find no preference for their rotational alignment in the plane of the scales. However, crystal orientation normal to the scale was highly correlated to their colour at low (conical) angles of view and illumination. This correlation enabled the use of optical images, each containing up to 104-105 crystals, for concluding the preferential alignment seen along the k100l at the level of single scales, appears ubiquitous. By contrast, k110l orientations were found to occur at no greater rate than that expected by chance. Above a critical cone angle, all crystals reflected bright green light indicating the dominant light scattering is due to the predicted band gap along the k110l direction, independent of the domain orientation. Together with the natural variation in scale and wing shapes, we can readily understand the detailed mechanism of uniform colour production and iridescence suppression in these butterflies. It appears that the combination of preferential alignment normal to the wing scale, and uniform distribution within the plane is a near optimal solution for homogenizing the angular distribution of the k110l band gap relative to the wings. Finally, the distributions of orientations, shapes, sizes and degree of order of crystals within single scales provide useful insights for understanding the mechanisms at play in the formation of these biophotonic nanostructures.

Place, publisher, year, edition, pages
Royal Society , 2017. Vol. 7, no 4
Keyword [en]
Biophotonic, Butterfly, Gyroid, Nanostructure
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-210620DOI: 10.1098/rsfs.2016.0154ISI: 000403686000013Scopus ID: 2-s2.0-85020882274OAI: oai:DiVA.org:kth-210620DiVA: diva2:1118888
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research
Note

QC 20170703

Available from: 2017-07-03 Created: 2017-07-03 Last updated: 2017-07-11Bibliographically approved

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