Contact angle hysteresis due to roughness in four metal/sapphire systems

Reversible advancing and receding contact angles of four metallic liquids (Hg, Sn, Al and Al-Sn4,76at%) on sapphire were measured by the immersion-emersion tensiometric technique. Alumina cylinders with a mean random surface roughness from a few nanometres to about two micrometres are used. For the non-wetting liquids (Hg, Sn), composite wetting is observed even with roughness as low as 0.5mm. For the wetting liquids (Al and Al-alloy), a residual hysteresis of the contact angle was found at vanishing roughness, a fact that was ascribed to a slight attack of alumina by liquid aluminium, inducing surface anisotropy. Results are described by a single equation which relates the difference between the cosine of advancing and receding contact angles to the surface roughness parameters of the solid.