Towards Bio-Inspired Broadband Optical Scattering Systems
Thesis or dissertation
University of Exeter
This thesis presents a study of the mechanisms by which white coloured appearances are created in biological samples. The primary aim of this work is the identification and characterisation of white insect species species, namely, achieving a fundamental understanding of the designs by which the highly efficient broadband scatter is cre- ated. The subsequent aim of this work concerns the use of these design principles in bio–inspired applications for technological gain. Previously observed structures are de- scribed, relevant electromagnetic theory is summarised, a range of key sample species is investigated and bio–inspired broadband scatter principles are used to fabricate pro- totype paper products. A wide range of white butterflies was investigated with electron microscopy. Pieris rapae, the small cabbage white, was chosen for detailed characterisation. An array of ellipsoidal pterin beads enhances optical scatter from its wing scales. The colour quality of its wing appearance could be controlled by chemical extraction of the pterin beads. Detailed optical modelling revealed the contribution to optical scatter of the wing–scale superstructure. The disordered structure within the scales of the beetle Cyphochilus is compared with that of two further beetle species, Lepidiota stigma and Calothyrza margaritifera. The bright white colouration of each species is attributed to high efficiency, wavelength– independent optical scatter from irregular microstructures within their scales. Finite element method and Mie theory modelling, along with experimental data, indicate varying degrees of optimisation of the three species’ microstructures. Cyphochilus ex- hibits the structure with the highest degree of optimisation; its scale filling fraction, scattering centre size and spacing are shown to be highly optimised. The final section of this thesis is devoted to the application of bio–inspired designs to the paper industry. Enhanced optical performance, through the optimisation of the mineral particle coating layers which are used to improve the optics of paper products, through mimicry of these butterfly and beetle structures is the goal of this section of the work. Knowledge gained through the investigation of the biological samples was applied to a range of paper samples with experimental coating layers.
PhD in Physics