Anisotropically microstructured and micro/nanostructured polypropylene surfaces

Anisotropically microstructured and hierarchically micro/nanostructured surfaces were fabricated on polypropylene by injection moulding. Microstructured mould inserts were obtained by structuring electropolished aluminium foils with a micro-working robot, and hierarchically structured mould inserts by anodizing the microstructured aluminium foils. On both types of inserts, the microstructures were anisotropic, consisting of alternating smooth and microstructured zones. Anisotropy, and other properties of microstructures, can be controlled by adjusting the parameters of the micro-working robot. The mould inserts were used to prepare micro- and hierarchically structured polypropylene discs by injection moulding. Replication accuracy at both structure levels can be controlled through the moulding conditions. The behaviour of water on the structures was characterized by measuring the contact and sliding angles parallel and perpendicular to the microstructured zones. Surfaces with microstructures alone were highly hydrophobic, where water droplets adopted the Wenzel state and had clearly different parallel and perpendicular contact angles. Surfaces with dual structures had contact angles near 170° and sliding angles near 0°, and again the angles in parallel and perpendicular directions differed. Superhydrophobic, anisotropic Cassie-Baxter state was achieved.