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It has only been one year since the material scientists around Prof. Erdmann Spiecker from the Centre for Nanoanalysis and Electron Microscopy (CENEM) at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) were granted funding for one of the world's best X-ray microscopes and they could already help unravelling an open question in butterfly research with fascinating 3D analyses. Their results have recently been published in the renowned scientific journal Science Advances.

Fascinating colours generated by photonic crystals

Who is not fascinated by the wonderful iridescent colours of butterfly wings? Those who want to find out more about this phenomenon will realise that often the colour is not generated by pigments, rather by periodic structures made of chitin, a structure-forming polysaccharide. These so-called photonic crystals give rise to structural colour by only reflecting specific wavelengths of the incoming solar spectrum. The resulting colour is not random, it serves as camouflage or signalling. But how do millions of these photonic crystals form within the tiny scales of butterfly wings? The opinions of scientists differ in this matter.

Time-frozen stages of crystal growth

In a cooperation with two leading experts in butterfly research, Dr. Bodo Wilts from University of Fribourg/Switzerland and Dr. Gerd Schröder-Turk from Murdoch University in Perth/Australia, as well as with Dr. Stephen Kelly from Zeiss-Xradia, the material scientists from Erlangen employed different high-resolution microscopy techniques to reveal the formation mechanism of these photonic crystal structures. The green butterfly Thecla opisena, which has its habitat in the Neotropics from Mexico to Venezuela, features separated photonic crystal domains that increase in size from the base to the tip of the wing scales. This is a so-far unique characteristic, distinct to other butterflies. The scientists interpret this finding by proposing photonic crystal growth was time-frozen at different stages of the metamorphosis. Hence, detailed microscopic analyses can be employed to obtain important insights into the formation processes of these crystals.

X-ray tomography resolves finest details

'High-resolution X-ray tomography provided essential findings for a deeper understanding of the formation mechanisms,' explains Prof. Dr. Erdmann Spiecker. The scientists assume that nascent chitin is extruded into a casting mould made of membranes. 'The unique capability of X-ray tomography to analyse the 3D structure of entire wing scales was used to clarify where the chitin was finally extruded from.' Dr. Benjamin Apeleo Zubiri, who analysed the 3D data in detail was amazed: 'The resolution of the reconstructed tomograms is so high, that we were able to clarify this question and, moreover, identify the chirality (handedness) of each individual photonic crystal.' Previously, this was only possible by using electron tomography, as the researchers from CENEM demonstrated in an earlier study. However, for the electron tomography investigation, the wing scales needed to be cut into little segments which caused serious disadvantages.

Modern materials science using X-ray tomography

Even though the material scientists have already been investigating these butterfly wing scales for several years, this is a rather exotic application at CENEM. 'We typically employ advanced microscopy and tomography techniques to enhance our knowledge of modern functional and energy materials and to optimise their properties for applications,' explains Prof. Erdmann Spiecker. 'The new X-ray microscope will also enable inimitable insights in such areas as the investigation of porous structures for catalytic applications or the search for tiny faults in turbine materials.' Photonic crystals are also relevant to modern materials science. These intriguing 3D structures with their unique optical properties may serve as prototypes for novel functional materials with applications in fields such as photovoltaics.

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