Controlling of water wettability by structural and chemical modification of porous anodic alumina (PAA): Towards super-hydrophobic surfaces

• Porous anodic alumina membranes of various geometrical parameters were fabricated.
• Water wettability of the PAA surface was analyzed.
• Water contact angle of pristine PAAs was correlated with the PAAs porosity.
• The largest WCA was obtained for the PAAs characterized by the porosity of ~ 50%.
• After silanization surface of some PAAs has become superhydrophobic.

Porous anodic alumina (PAA) membranes with increasing pore diameters (Dp) and various interpore distances (Dc) were fabricated and the water contact angles (WCAs) on the PAAs' surface were determined. No correlation was observed between the WCAs and Dp unless the Dc was taken into account. The WCAs were dependent on porosity (P); according to this dependence, there is a critical value of P beyond which the WCAs drop substantially. The critical P was ~ 50% and the maximal WCA obtained for this P value was 102°. No effect of pore arrangement on the water wettability was observed — the PAAs characterized by poor hexagonal pore ordering (the PAA NO) did not show any deviations from the observed trends. After modifying the PAAs surfaces by lauric acid and perfluorooctyltrichlorosilane (FOTS), the samples became more hydrophobic. For the FOTS-modified PAAs with the highest Dp (> 230 nm) and Dc (> 260 nm), the surfaces became superhydrophobic. The results obtained in this work can be used for controlling the PAAs' surface topography and chemistry to attain desired surface properties, ranging from hydrophilicity to ultrahydrophobicity.

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