Measurements of contact angles of water on Al-based intermetallic surfaces

A systematic study was performed of the contact angle of minute droplets of water deposited on various aluminium-based intermetallics in ambient conditions. The dataset was complemented by measurements of electronic partial densities of states and oxide layer thicknesses. We studied a broad variety of specimens, ranging from simple cubic, CsCl-type phases to quasicrystals and high-order approximants. The thickness of the oxide layer, that is always present in air, was varied between 2 and more than 10 nm. Within these limits, the contact angle varies considerably and, surprisingly, decreases with increasing thickness of the oxide dielectric. Furthermore, it is clearly the highest on samples that show the largest crystal complexity, i.e. quasicrystals. It follows that the reversible adhesion energy directly deduced from contact angle is essentially controlled by the (squared) density of states at Fermi energy in the bulk of the sample and the inverse of the (squared) thickness of the native oxide film at the surface of the material. We interpret these results in terms of electrostatic image forces developed in the conduction cloud by the dipoles of the water molecules.