Liquid\u2013liquid phase separation morphologies in ultra-white beetle scales and a synthetic equivalent

Stephanie L Burg,Andrew J Parnell,Simon J Martin,Steven R Parnell,Antonino Bianco,Mike Croucher,Robert M Dalgliesh,Rajeev Dattani,Julie Villanova,Cvetelin Vasilev,Nigel Clarke,Sylvain Prévost ,Richard Jones ,A L Washington ,David M Coles,Andrew J C Dennison,J Patrick A Fairclough,Daragh Mcloughlin,Chris Hill ,Peter Vukusic ,Mark Hutchings,Scott Doak,Oleksandr O Mykhaylyk,Anthony J Ryan,William R Furnass ,Andrew R Parker

doi:10.1038/s42004-019-0202-8

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.

Highlights

Cyphochilus beetle scales are amongst the brightest structural whites in nature, being highly opacifying whilst extremely thin
The intrascale nanostructure is a highly continuous and interconnected air-voided network with a characteristic length scale, reminiscent of morphologies observed for phase separating polymer blends[28], which have undergone phase separation via spinodal decomposition[29,30,31]
In the case of the anisotropic network within the Cyphochilus, the internal α-chitin optical structure is highly interconnected and prone to distortion on sectioning, meaning that anisotropy must be imparted to the optical structure before it fully solidifies

Summary

Introduction

Cyphochilus beetle scales are amongst the brightest structural whites in nature, being highly opacifying whilst extremely thin. The 3D photonic structures observed to date are ordered crystal grain structures that are highly faceted and strongly break up and segment the internal scale volume These structures are proposed to be patterned using the physics of hydrophobic–hydrophilic interactions[8] to control morphology (gyroid, cubic, hexagonal, sphere, lamellar etc.) akin to the structural diversity seen in photonic block copolymers[10,11,12]. Pseudo-photonic materials, which have a degree of disorder can be optically modelled using Bragg type reflecting structures, with a distribution of length scales in the layers and their spacings[16] While such models may capture the optical properties well, they do not inform us as to the process by which these optically active nanomaterials form. It has been reported previously that blue structural colour in bird feathers and beetles is probably a result of biopolymer[15,17,18] phase separation[19] via spinodal decomposition with drying[8] and chitin crystallisation[20,21] being potential arrest mechanisms of phase separation[19] within arthropod scales

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Results

Conclusion