Nanostructure regularity in white beetle scales for stability and strong optical scattering [Invited

Abstract

Cyphochilus white beetle scales exhibit exceptionally strong light scattering power that originates from their regular random fibrillar network nanostructure. The structure is believed to be formed by late-stage spinodal decomposition in a lipid membrane system. However, the structure is characterized by nonconstant mean curvatures and appreciable anisotropy, which are not expected from late-stage spinodal decomposition, so that the surface free energy is not minimized. Nevertheless, a high degree of regularity represented by the relatively uniform fibril dimensions and smooth fibril surfaces in the structure may result from a process similar to spinodal decomposition. In this study, we investigate the role of regularity in the Cyphochilus white beetle scale structure in realizing strong light scattering. Irregularity is computationally introduced into the structure in a systematic fashion such that its anisotropy is preserved and its surface area is kept constant. Calculations show that optical scattering power decreases as irregularity increases with a high sensitivity. This effect happens because, remarkably, irregularity on a scale much smaller than the wavelength destroys anisotropy in optical diffusion. Thus, the result shows that the in vivo process in Cyphochilus white beetle scales utilizes structural regularity and anisotropy to achieve strong light scattering at a tolerable surface free energy. In typical fabrication of random media, irregularity and multiple length scales typically increase surface area, so that durability of the nanostructures may be negatively affected. Our study indicates that regularity in anisotropic random nanostructures can achieve strong light scattering with a moderate surface free energy.