Correlating hydrophobicity to surface chemistry of microstructured aluminium surfaces

Highly hydrophobic aluminium surfaces fabricated by chemical etching are investigated by Contact Angle Goniometry, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy in order to correlate the wettability of our samples with their morphology at the sub-micrometer scale and with the chemical composition of the very first surface layers. We find that the etched aluminium surfaces have binary structures with nanoscale block-like convexities and hollows, which provide more space for air trapping. We also demonstrate that both hierarchical micro/nanostructures and surface composition endow these surfaces with excellent hydrophobic properties. XPS analysis shows indeed that the contact angle anti-correlates with the amount of metallic aluminium present at the surface, but also confirms the essential role of the adsorption of airborne carbon compounds. The hydrophobic behaviour depends therefore on the combined effects of surface morphology and surface chemistry.