Micropatterning On Planar And Spherical Surfaces Based On Surface Wrinkling

It is well known that surface topographies play an important role in material properties. In recent years, fabrication of regular micropatterns on material surface has attracted lots of interest. Among various surface patterning methods, Surface wrinkling, as a novel and simple method to create ordered surface wrinkle morphologies, has become one of research focuses. In this thesis, fabrication of ordered micropatterns on material surface has been explored based on the surface wrinkling mechanism. According to the geometry of the wrinkled surfaces, the thesis is divided into two parts – wrinkling on planar surface and wrinkling on spherical surface.In the first part, ordered hierarchical stress-relief wrinkles with well-defined microstructures have been fabricated via a template-guided surface wrinkling. The novel stable structured template is the highly oriented sinusoidal wrinkle with the tunable wavelength and amplitude, which is produced by oxygen plasma(OP) treatment of a uniaxially strained polydimethylsiloxane(PDMS) sheet followed by strain release. When the wrinkled template is conformally contacted with a polystyrene(PS) film that is coated on the compliant PDMS foundation under heating, the confinement-induced wrinkling from the template takes place on the PS–PDMS substrate. The resulting aligned wrinkles with a style A mode and style M mode are controlled by the ratio of the applied template wavelength to the film intrinsic wrinkling wavelength, regardless of the geometric feature of the underlying substrate in our case. When the template-directed wrinkling is carried out two times on the same PS-PDMS substrate, inaccessible hierarchical complex wrinkles from the well-organized A-A, M-M and A-M are obtained by means of the smart combination of the same or different wavelengths of wrinkled templates. The involved unprecedented template-induced wrinkling on the as-formed wrinkles has been discussed briefly. During the whole patterning process, the applied templates and the as-prepared structures are thoroughly based on the surface wrinkling mechanism without any lithographic method involved.In addition, the fabrication process of the gradient wrinkles is investigated. The so-called gradient wrinkles refer to the wrinkles with gradient wavelength. It is well known that the intensity of ultraviolet(UV) radiation decays with the spreading distance. When the PDMS slab is exposed to ultraviolet ozone(UVO), the distance between UV lamp and PDMS surface is varied gradiently by intentionally loading a “toy brick”. As a result, the larger the distance between UV lamp and PDMS surface, the smaller the wavelength of wrinkles on the PDMS surface, vice versa. Subsequently, the influence of applied factors, such as the treatment way, treatment time and so on is systematically investigated. At last, taper wrinkles and butterfly-shape wrinkles are obtained by adjusting the loaded direction of the “toy brick”. The taper wrinkles mean that the wavelength of wrinkles increases or decreases monotonically along the direction parallel to the wrinkles. The butterfly-shaped wrinkles mean that the wavelength of wrinkles decreases firstly, and then, increases along the direction paralled to the wrinkles. The whole fabrication processes of the gradient wrinkles, the taper wrinkles and butterfly-shape wrinkles have not been reported.In the second part, three kind of spherical wrinkles are investigated: wrinkling on Pt/PDMS core-shell microspheres, wrinkling on Pt/PS core-shell microspheres, and wirnkling SiOx/PDMS core-shell microspheres.Firstly, smooth PDMS microspheres and monodisperse PS microspheres with different size are prepared by emulsion polymerization and dispersed polymerization, respectively. Pt/PDMS and Pt/PS core-shell microspheres are prepared by ion sputtering, respectively. And wrinkle pattern is induced by swollen. According to the analysis of the theoretical literature, the effect of experimental parameters, such as the radius of microsphere, the thickness of Pt shell and so on, is systematically investigated. Futhermore, the wrinkled PDMS-SiOx-Pt triple core-shell microspheres and the hierarchical PS microspheres with wrinkles and nano-bulge patterns are prepared by pre-treatment of oxygen plasma method, respectively. This study improves our understanding of the spherical wrinkling behaviors.Secondly, a simple low-cost yet robust method to realize spontaneously wrinkled morphologies on spherical surfaces is introduced here. It is based on surface chemical oxidation of aqueous-phase-synthesized PDMS microspheres in the mixed H2SO4/HNO3/H2 O solution. Consequently, curvature and overstress-sensitive wrinkles including dimples and labyrinth patterns are successfully induced on the resulting oxidized PDMS microspheres. A power-law dependence of the wrinkling wavelength on the microsphere radius exists. The effects of experimental parameters on these tunable spherical wrinkles have been systematically investigated, when the microspheres are pre-deposited on a substrate. These parameters include the radius and modulus of microspheres, the mixed acid solution composition, the oxidation duration, and the water washing post-treatment. Meanwhile, the complicated chemical oxidation process has also been well studied by in-situ optical observation via the microsphere system, which represents an intractable issue in a planar system. Furthermore, we realize surface wrinkled topographies on the whole microspheres at a large scale, when microspheres are directly dispersed in the mixed acid solution for surface oxidation. These results indicate that the introduced wet surface chemical oxidation has the great potential to apply to other complicated curved surfaces for large-scale generation of well-defined wrinkling patterns, which endow the solids with desired physical properties.