Superhydrophobic hierarchical surfaces fabricated by anodizing of oblique angle deposited Al–Nb alloy columnar films

A combined process of oblique angle magnetron sputtering and anodizing has been developed to tailor superhydrophobic surfaces with hierarchical morphology. Isolated submicron columns of single-phase Al–Nb alloys are deposited by magnetron sputtering at several oblique deposition angles on a scalloped substrate surface, with the gaps between columns increasing with an increase in the deposition angle from 70° to 110°. Then, the columnar films have been anodized in hot phosphate–glycerol electrolyte to form a nanoporous anodic oxide layer on each column. Such surfaces with submicron-/nano-porous structure have been coated with a fluoroalkyl phosphate layer to reduce the surface energy. The porous surface before coating is superhydrophilic with a contact angle for water is less than 10°, while after coating the contact angles are larger than 150°, being superhydrophobic. The beneficial effect of dual-scale porosity to enhance the water repellency is found from the comparison of the contact angles of the submicron columnar films with and without nanoporous oxide layers. The larger submicron gaps between columns are also preferable to increase the water repellency.