Influence of sub-10 nm anodic alumina nanowire morphology formed by two-step anodizing aluminum on water wettability and slipping behavior

Abstract The influence of the nanomorphology of alumina nanowires (ANWs) fabricated by a two-step anodizing process on the superhydrophobicity was investigated through advancing contact angle (ACA) and receding contact angle (RCA) measurements. Aluminum nanobowl specimens were anodized in pyrophosphoric acid to fabricate an ordered ANW structure with an average diameter of 7.1 nm, and the outermost surface of the ANWs was chemically bonded with fluorinated self-assembled monolayers. The growing ANWs bent immediately their own weight, and pyramidal ANW structures were formed as they joined the surrounding nanowires together. The ACA value increased with the number density of pyramidal ANW structures due to the reducing area fraction of ANWs, and an increased superhydrophobicity with a contact angle of approximately 165° was measured on the low-density pyramidal structure with a density of 8.1 × 1011 m−2. Additional anodizing led to complete nanowire bending; thus, the advancing contact angle decreased. The pyramidal nanowire structure exhibited a large slipping property with a contact angle hysteresis (CAH)   100°. Superhydrophobic surfaces with opposite water slipping properties were demonstrated by a water dropping experiment.