title: Gradient topographies and functional gradient materials
Creators: name: Schedl, Andreas
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: gradient topographies
subject: gradient surfaces
subject: gradient materials
subject: wrinkling
subject: surface relief gratings
subject: photonic crystals
subject: 500
subject: 540
subject: 620

description: This thesis focuses on the preparation, characterization and application of gradient topographies and gradient functional materials. For that, polydimethylsiloxane elastomers are surface-modified in different manners by three methods controlled wrinkling, formation of surface relief gratings, and preparation of responsive photonic crystals. The motivation for the work arises from nature. Nature utilizes well-defined hierarchical structures and gradients to create functional materials with advanced surface topography and tailored mechanical properties. Examples for tailored surface topographies are colorful butterfly wings and shark skin with improved hydrodynamics. Examples for responsive functional surfaces are the impressive skin color variations of chameleons and the brilliant appearance of tropical fishes, both based on photonic crystals. Improved mechanical properties can be found for example in the sea mussel. The byssus reveals a stiffness gradient which allows an improved connection of the soft interior of the mussel to a hard surface of a rock. The natural concepts of these functional materials and surfaces can be transferred to technical systems. In this context, the introduction gives a brief overview about tailored materials and surfaces in nature and their corresponding functionalities. Furthermore, methods to artificially prepare surface topographies and functionalities are presented with respect to the state of the art. The first method is controlled wrinkling which is a comparatively simple technique to form surface patterns. The principle is based on a hard surface film which is attached to a soft elastomer substrate. Compression of such a composite material leads to the formation of wrinkled patterns. Hereby, the wavelength can be tuned via the Young’s moduli and Poisson ratios of film and substrate, respectively, as well as the thickness of the hard film. The second method is the inscription of surface relief gratings in azobenzene films utilizing a holographic technique. This technique allows the inscription of gratings with a sinusoidal wave pattern. The third approach utilizes photonic crystals. These materials reveal a photonic band gap and decouple a certain wavelength upon illumination with visible light. This cumulative thesis consists of five topics which result in five publications. An overview of the thesis as well as a summary of the major achievements is presented in a synopsis. The first and the second topic focus on the formation of gradient surfaces via controlled wrinkling. In the first topic, the formation of such gradient surfaces is based on a continuous gradient in the Young’s modulus of the polydimethylsiloxane elastomer substrate. Embedding of such a stiffness gradient material in a matrix allows the systematic formation and investigation of hierarchical line-defects at the interface. The investigations on these line-defects and the developed prevention strategy contribute essentially to the fundamental understanding of controlled wrinkling. The second topic covers the formation of gradient surfaces via controlled wrinkling by variation of the thickness of the hard film. For that, thin metal films with a gradient in film thickness were deposited on polydimethylsiloxane substrates using a combinatorial vapor deposition technique. Gold, chromium, and indium were investigated with respect to wrinkling behavior and surface morphology. The developed technique is a powerful approach to efficiently screen metal wrinkles with respect to their wavelengths on one substrate. The third and the fourth topic cover the preparation of functional materials with sinusoidal surfaces via surface relief gratings. These gratings were inscribed optically in azobenzene films with a holographic technique. The third topic demonstrates the non-destructive grating transfer to polydimethylsiloxane and two thermoplastic polymers. Replicas with surface relief gratings with varying grating height were used as confinement templates for the alignment of nanoparticles. This represents a very elegant and efficient technique to screen the assembly of colloidal nanoobjects. The fourth topic presents a novel technique to determine the refractive-index from transparent solids based on surface relief gratings. The principle is based on the angle-dependent investigation of the first and second order diffraction of monochromatic light on these gratings. The presented technique is a powerful alternative to refractive-index determination via an Abbe refractometer or spectral ellipsometry. The fifth topic focuses on the preparation of gradient photonic materials inspired by the chameleon skin. These materials consist of a one-dimensional photonic crystal which is attached on top of a polydimethylsiloxane elastomer substrate with a gradient in stiffness. Unstrained samples reveal a uniform reflectance over the entire surface of the photonic crystal. Upon strain, a blueshift with a gradient in reflectance occurs. The reflectance pattern can be controlled by tailoring the architecture of the polydimethylsiloxane substrate. This is demonstrated for rainbow-like and stripe-like reflectance patterns. The presented gradient photonic materials are models for the development of nature-inspired mechanochromic sensors.