Anodic alumina oxide surfaces prepared by dual hard and mild anodization at subzero temperature: Surface microscopic characterization and influence on wettability

Multistage anodizations allow the preparation of new generations of ordered anodic alumina oxide porous structures with large interpore distances, yet the influence of second anodization and pore widening times on pore dimensions and wettability of the obtained structures has not yet been fully studied. The main objective of this study was to characterize the obtained pores (dimensions and morphology), for different anodizations and pore widening times, and to investigate their respective influence on surface wettability. Anodic alumina oxide ordered porous surfaces were prepared using a dual hard and mild anodization process with oxalic and phosphoric acids at subzero temperature, with or without pore widening. Statistical analyses were realized on pore measurements from samples produced with a large range of anodization durations (7 different tested times). The obtained surfaces possessed regular ordered pores of 99 nm and 307 nm for respectively median pore diameter and inter-pore distance, before pore widening, and did not conform to expected theoretical values for either usual lone mild or hard anodization conditions. The slow growth of pore depth (median of 11 nm/min) observed during second anodization could be explained by the subzero electrolyte temperature used. Pore widening effectively increased pore diameter, but had statistically a bigger impact on diameters for shorter second anodization time than for longer ones. Also, pore widening significantly decreased pore depth as well as the oxide barrier thickness. Additionally, porosity increased from 9.57% to 14.8% after 3 hours of pore widening, but did not significantly affect surface wettability, which was however increased when compared to flat control surfaces. The obtained results showed that a dual anodization with different electrolytes at a subzero temperature had a significant impact on pores dimensions. A discussion on the respective effects of surface chemistry and surface roughness in the wetting evolution was proposed. Previous article in issue