Regular Patterned Surfaces from Core-Shell Particles. Preparation and Characterization

Abstract

A simple route for fabrication of regularly patterned surfaces with specifically designed surface roughness and chemistry is reported using colloidal particles. The surface was built up from self-assembled submicrometer- and micrometer-sized monodisperse core-shell particles of different radius (0.1–10 μm) forming ordered arrays. In this way, an increase in the vertical roughness is achieved with increasing particle radius, but without changing the Wenzel roughness factor. The morphology of the ordered particle arrays was characterized using an optical imaging method (MicroGlider), scanning force (SFM) and scanning electron (SEM) microscopy. The organic shell was either prepared by covalent grafting of polymer brushes or by chemisorption of a silane with a long fluoroalkyl tail. From FTIR-ATR, diffuse reflection IR spectroscopy, and capillary penetration experiments, it was concluded that the grafted polymer completely covers the surface of the silica particles. The solid surface tension of the organic shell obtained from contact angle measurements on smooth surfaces decreased in the following order: polystyrene brush-PS (γ_sv = 28.9 mJ/m²) > copolymer of polystyrene and 2,3,4,5,6-pentafluoropolystyrene brush-FPS (γ_sv = 24.3 mJ/m²) > chemisorbed (tridecafluoro-1,1,2,2-tetrahydrooctyl) dimethylchlorosilane-FSI (γ_sv = 18.3 mJ/m²). Water contact angle measurements revealed an influence of the surface height roughness and the shell chemistry on the wettability. For all surfaces investigated, the contact angle hysteresis increased on the rough model surfaces compared to the smooth surfaces due to the increase of the advancing contact angle and the decrease of the receding angle. The lower the surface free energy of the shell chemistry, the smaller is the contact angle hysteresis on the closely packed surface arrays. Further the contact angles varied with increasing height roughness. A possible explanation for this behaviour is that the vertical roughness influences the curvature radius of the liquid in trapped air pockets at the solid-liquid interface as was already assumed in the literature for nanostructured metal surfaces and paraffin-coated steel balls.