Anodizing aluminum in a concentrated pyrophosphoric acid solution (H₄P₂O₇) leads to the formation of numerous anodic alumina nanofibers measuring less than 10 nm in diameter. As the alumina nanofibers are modified with self-assembled monolayer (SAM), the nanofiber-covered aluminum surface exhibits a superhydrophobic behavior with a water contact angle measuring more than 150°. In the present investigation, we demonstrate the fabrication of slippery and sticky superhydrophobic aluminum surfaces covered by the anodic alumina nanofibers. The effect of the nanostructure of the anodic oxide on the adhesion interaction between the water droplet and the surface was investigated by dynamic contact angle measurements.

High-purity aluminum specimens (99.99 wt%, 400 µm thick) were ultrasonically degreased in ethanol for 10 min. The specimens were electropolished in a 78 vol% CH₃COOH/22 vol% 70%-HClO₄ solution (T = 280 K) at a constant voltage of 28 V for 1 min. The electropolished specimens were immersed in a concentrated pyrophosphoric acid solution (74.0 wt%, T = 293 K), and then were anodized at a constant voltage of 50 V using a PC-connected direct-current power supply. After anodizing, the specimens were immersed in a 0.5 mM 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (FOPA) / ethanol solution (T = 313 K) for 24 h to form SAM on the anodic alumina nanofibers. The surface nanomorphology of the anodized specimens was examined by field emission scanning electron microscopy (FE-SEM). The water contact angles on the anodized specimens were measured by an optical contact angle meter. The distilled water droplets were formed on the surface using an auto dispenser and a microsyringe with a flow rate of 2 µL/s, and the advancing and receding contact angles were measured by a charge-coupled device (CCD) camera (frame rate: 10 fps).

Figure 1 shows the changes in the advancing and receding contact angles with the anodizing time. Here, the anodic oxide was modified with the FOPA-SAM after anodizing. The advancing contact angle rapidly increased to superhydrophobicity more than 160° by the initial stage of anodizing, and the contact angles were maintained at similar values via the long-term anodizing. In contrast, the receding contact angle was drastically changed with the anodizing time: the contact angle rapidly decreased to approximately 0° in the initial stage of anodizing, rapidly increased to 150° in the middle stage, and then gradually decreased to 0° in the final stage. This complicated behavior was due to the nanostructural surface changes via pyrophosphoric acid anodizing.

Figure 2 shows the side views of the 20 µL water droplet on the superhydrophobic aluminum specimens fabricated by pyrophosphoric acid anodizing for a) 4 min and b) 20 min and subsequent FOPA-SAM modification. The water droplet was pinned on the surface anodized for 4 min when it was turned upside down (i.e. a sticky surface). Conversely, the droplet easily rolled off with a small sliding angle on the surface anodized for 20 min (i.e. a slippery surface).