Liquid–liquid phase separation morphologies in ultra-white beetle scales and a synthetic equivalent

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Abstract

Cyphochilus beetle scales are amongst the brightest structural whites in nature, being highly opacifying whilst extremely thin. However, the formation mechanism for the voided intra-scale structure is unknown. Here we report 3D x-ray nanotomography data for the voided chitin networks of intact white scales of Cyphochilus and Lepidiota stigma. Chitin-filling fractions are found to be 31 ± 2% for Cyphochilus and 34 ± 1% for Lepidiota stigma, indicating previous measurements overestimated their density. Optical simulations using finite-difference time domain for the chitin morphologies and simulated Cahn-Hilliard spinodal structures show excellent agreement. Reflectance curves spanning filling fraction of 5-95% for simulated spinodal structures, pinpoint optimal whiteness for 25% chitin filling. We make a simulacrum from a polymer undergoing a strong solvent quench, resulting in highly reflective (~94%) white films. In-situ X-ray scattering confirms the nanostructure is formed through spinodal decomposition phase separation. We conclude that the ultra-white beetle scale nanostructure is made via liquid–liquid phase separation.

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Burg, S. L., Washington, A., Coles, D. M., Bianco, A., McLoughlin, D., Mykhaylyk, O. O., … Parnell, A. J. (2019). Liquid–liquid phase separation morphologies in ultra-white beetle scales and a synthetic equivalent. Communications Chemistry, 2(1). https://doi.org/10.1038/s42004-019-0202-8

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