Emergence of colour by tunable surface wrinkling in one and multi-dimensions

Abstract

Naturally occurring surface patterns often exhibit micro and nano-scale topography with various functionalities, including displaying optical and photonic effects. One such natural topography that contributes to structural colour is the presence of wrinkles. This thesis explores the controlled wrinkling of bi-layered materials as a powerful patterning method to create bioinspired topographies, and examines their optical properties. The primary method employed in this work involves the use of plasma oxidation of polydimethylsiloxane, combined with mechanical strain, to create wrinkles with varying topographies, ranging from nano to micronscales, by controlling the plasma and strain conditions, as well as the superposition of various generations of wrinkles with prescribed relative angle of orientation. This work firstly investigates the formation of one-dimensional uniaxial wrinkles acting as tunable sinusoidal phase gratings, and quantitatively models the diffractive behaviour of the wrinkled surfaces as a function of strain. Under white light, these wrinkles exhibit iridescent, structural colour on the surfaces that depends on the observation angles and incident light spectrum, and we explored the concept of optical and colour directionality by creating gradient and isotropic wrinkles. In addition to surface diffraction in reflection, we found that the wrinkled materials could act as transmission gratings, leading to multi-faceted structural colour through diffraction combined by total internal reflection. Finally, we investigated the potential and limitations of sequential two-dimensional wrinkling and its structural colour properties. The results of this study provide promising directions for using structural coloured wrinkles in applications such as sensors, displays, and packaging.