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Title: Structural colour in fruits
Author: Middleton, Roxanne
ISNI:       0000 0004 7961 7977
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Structural colour arises from the constructive interference of light with a material structured on a lengthscale corresponding to optical wavelengths. This phenomenon is responsible for the appearance of many of the brightest colours in nature and recently the existence of structural colouration in plants has been demonstrated across multiple species. This thesis extends our understanding of the effect specifically in fruit epidermal tissues, it uses the physical principles underlying structural colour to understand biological development and it extracts design ideas from biological tissues to enhance the optical response of biomimetic materials. In more detail, this thesis reports the optical characteristics and architecture of structural colour in several fruits of the genera Pollia and Viburnum. Previous work showed that the external cells of Pollia condensata fruits have extremely thick cell walls which act as photonic crystals to reflect circular polarised light. Here, the spectral and morphological characteristics of photonic cell walls in this and three other Pollia species are reported and it is shown that the unusual right handed circular polarisation reflection is apparent in only P. condensata. It is also shown that the occurrence of right circular polarisation is associated with longer wavelength reflection. This thesis extends this analysis by demonstrating the use of structural colour to observe the development of thickened cell walls in Pollia condensata and Pollia japonica using optical microscopy. It is shown that during development, cell wall material is built up gradually and with a fixed structural periodicity in both species, and that significant cell wall growth occurs in the earliest stages of the long fruit maturation period. In the other genus investigated, structural colour analysis is extended to the fruits of Viburnum tinus and Viburnum davidii and found to arise from layers of globular vesicles in the specialised cell wall. Inspired by these studies, a novel templating technique for reflective self-assembled cellulose nanocrystal films is described, which mimics the morphology of Pollia cells by successfully introducing a curvature in the photonic multilayer whilst maintaining its optical response. Enhancement of the angular independence of light reflected from this curved surface is demonstrated.
Supervisor: Vignolini, Silvia Sponsor: EPSRC ; University of Cambridge ; Cambridge Philosophical Society
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Pollia ; Pollia condensata ; Pollia japonica ; structural colour ; cellulose ; cell wall ; cell development ; multilayer ; helicoidal ; circular polarisation ; optical microscopy ; biomimetics ; Cellulose Nanocrystals ; CNC ; templating ; coloration ; Viburnum ; V. tinus ; V. davidii ; Fruit coloration