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Title: Biological structural whites and synthetic routes to mimic these structures
Author: Burg, Stephanie
ISNI:       0000 0004 7428 2362
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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Nature has many examples of structural whites, the most striking of which are the scales of the Cyphochilus and the Lepidiota stigma (L. stigma) beetles. Previous work suggested that the reason for their superior reflectance was the highly anisotropic network of random chitin filaments contained within the scales which had a filling fraction of ~45%-70% [1, 2]. The work in this thesis will show that the beetle scales distort upon sectioning and in order to fully understand the microstructure within these scales, the structure must be characterised using intact scales. This was accomplished using 3-D x-ray nano-tomography on single scales from both species which found the filling fractions of both scales to be ~ 30 % and that the Cyphochilus scales were anisotropic, but the Lepidiota stigma scales were actually isotropic. The 3-D data revealed that the scale structures were highly voided continuous networks reminiscent of structures that arise from spinodal decomposition. Simulated reflectance patterns from finite difference time domain modelling of the beetle structures and simulated spinodal structures from Cahn-Hilliard theory showed excellent agreement between the two. This suggested that it could be possible to create a highly reflective synthetic mimic of the beetle scale structure via the spinodal phase separation of a polymer solution. This hypothesis was tested through the fabrication of porous films of cellulose acetate, which were confirmed to phase separate via spinodal decomposition using ultra small angle X-ray scattering to observe the dynamics of the drying films. Using spin-echo small-angle neutron scattering it was shown that the films were isotropic and had a filling fraction of about ~22 %. Micro-reflectance results for cellulose acetate films and the beetle scales showed that the films achieved a higher reflectance for a comparable thickness, verifying that it is possible to improve upon nature and achieve highly reflective structural whites through the spinodal decomposition of a commercial polymer system.
Supervisor: Jones, Richard A. L. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available