Exploring Plastron Stability and Fluid Friction Reduction on Robust Micro-Textured Non-Wetting Surfaces

Non-wetting surfaces are characterized by the presence of stable pockets of vapor trapped within the asperities of the surface morphology. The utility of these surfaces in reducing skin friction in viscous laminar and turbulent flows is experimentally and theoretically investigated and is the main focus of this thesis. The development of a surface coating that reduces fluid drag can potentially lead to a number of practical applications such as fuel savings in marine vehicles and energy savings in pipe flows. First, a single-step, scalable spray coating technique to fabricate liquid-repellent micro-textured surfaces by depositing a blend of poly(methyl methacrylate) (PMMA) and the low surface energy molecule 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS, γsv ≈ 10 mN/m) is presented. The surface morphology of the polymer coating was studied systematically and varies from a corpuscular or spherical microstructure to a beads-on-string structure and finally to bundled fibers by controlling the solution concentration c and molecular weight M of the sprayed polymer solution. A scaling law is proposed that relates the minimum necessary concentration of a polymer required to produce fibers cspin to its molecular weight M and the quality of the solvent (through the excluded volume exponent ν) of the form cspin ∼ M−(ν+1). These POSS/PMMA coatings are shown to exhibit both superhydrophobic and superoleophobic properties due to the presence of a film of trapped air (or ‘plastron’) within the surface microtextures upon contact with a liquid. A combination of texture and surface chemistry is also important in the design various biomimetic interfaces. Many species of aquatic birds dive tens of meters into water to prey on fish while entraining a ‘plastron film’ within the complex microstructures of their feathers, which is often thought to confer water repellency to the bird. By combining electron microscopy with contact angle measurements