Fabrication of Sticky and Slippery Superhydrophobic Aluminum Surfaces Covered with Nanostructured Anodic Oxide

Abstract

Anodizing aluminum in a concentrated pyrophosphoric acid solution leads to the formation of numerous anodic alumina nanofibers, and this nanofiber-covered surface shows superhydrophilicity measuring less than 10° in water contact angle. In contrast, as the alumina surface is modified with a self-assembled monolayer (SAM), the surface exhibits superhydrophobicity measuring more than 150°. Herein, we demonstrate the fabrication of sticky and slippery superhydrophobic aluminum surfaces by optimizing the nanostructure of anodic alumina nanofibers. Commercially available 5N aluminum and 3004 aluminum alloy were anodized at a constant voltage of 75 V for up to 60 min for the formation of anodic alumina nanofibers on the surface. The specimens, then, were immersed in a 1.5 mM 3,3,4,4,5,5,6,6,7,7,8,8,8,-tridecafluorooctylphosphonic acid (FOPA)/ethanol solution at 313 K for 48 h to form SAMs on the surface of the anodic oxide. The sliding behavior of the water droplet formed on the anodized surface was examined by advancing and receding contact angle investigations. Numerous anodic alumina nanofibers and nanofiber-tangled intermetallic particles were formed on the aluminum surface by anodizing in pyrophosphoric acid. As the nanofiber-covered aluminum alloys were modified with fluorinated phosphonic acid SAMs, the aluminum surface exhibits a superhydrophobic behavior with contact angles of more than 170°. The sliding behavior of the water droplet was drastically changed with the anodizing time, and the highly sticky and slippery aluminum surfaces could be fabricated by controlling the structure of alumina nanofibers (Figure 1). A superhydrophobic aluminum surface with different wettabilities including sticky and slippery properties was fabricated by the selective anodizing process. Figure 1