Slippery and Sticky Superoleophobicities on Hierarchical Aluminum Surfaces Fabricated by Electrochemical Etching and Anodizing Methods

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Slippery and sticky superoleophobic aluminum surfaces were fabricated by electrochemical etching and anodizing methods. Collective structures of sub-micrometer-scale cubic-shaped aluminum were formed by electrochemical etching of high-purity aluminum in HCl. The etched specimens were then anodized in H4P2O7 to form anodic alumina nanofibers (AANFs) on the cubic aluminum surface, resulting in a hierarchical aluminum-alumina structure. As the water/oil-repellent self-assembled monolayers were formed on their surfaces, the superoleophobicity of dodecane was more than 150° in the advancing contact angle exhibited on the anodizing surfaces. Conversely, the receding contact angle changed drastically with anodizing time because of the different nanomorphology of AANFs; short anodizing caused slippery superoleophobicity with high contact angle values, whereas long anodizing resulted in sticky superoleophobicity with 0°. We demonstrated the slipping behavior of dodecane droplets on superoleophobic aluminum surfaces with completely opposite sliding properties. The corrosion resistant property of the superoleophobic aluminum surface was also investigated by the measurements of potentiodynamic curve in a 3.5 wt% sodium chloride solution.