Anisotropic diffusion in Cyphochilus white beetle scales

Abstract

Cyphochilus white beetles possess an exceptional ability to scatter visible light from their scales, which have anisotropic nanofibrillar network structures. We discover a striking effect that diffusely incident light on the beetle scales is preferentially channeled sideways and scattered backward on the average after traversing a vertical distance corresponding to only two scattering events. For normally incident light, the scattering is only slightly forward, resulting in close proximity between two optical characteristic lengths: scattering mean free path (1.78 µm) and effective transport mean free path (1.85 µm). We have used effective transport mean free path to properly describe the scattering strength in anisotropic random media. For improved description of light scattering in beetle scales, we apply complete solutions to the anisotropic diffusion equation, where the solution approach made use of optical boundary layer theory. Our method enables highly accurate determination of mean free paths and extrapolation length in the white beetle scales.