Water penetration fronts in thin films deposited at oblique incidence

Abstract Water penetration spots in thin films deposited at oblique incidence are shown to be nearly elliptical in shape. Water spreads laterally at greatest speed in the direction normal to the plane of incidence of the evaporant molecules. The ratio of the maximum and minimum speeds increases with the angle of incidence of the vapour and, for magnesium fluoride, is about 1.5 for an angle of 60°. The primary cause of the anisotropic water penetration effect is believed to be self-shadowing which occurs during the deposition of thin films, leading to a columnar microstructure with intermediate voids which act as sinks for penetrating liquids.