Wetting and Prewetting on Ceramic Surfaces

The stabilization of nanoscale surficial amorphous films (SAFs) for Bi2O3 on ZnO, VOx on TiO2, SiOx on Si, and several other oxide systems provides evidence for the existence of prewetting phenomena with analogies in water and other simple systems, as well as the stabilization of intergranular amorphous films in ceramics. Experimental results show that in the subeutectic regime, the equilibrium film thickness decreases monotonically with decreasing temperature until it vanishes at a dewetting (prewetting) temperature. With increasing temperatures, nanometer-thick SAFs persist into a solid-liquid coexistence regime, in equilibrium with partial-wetting drops, with a gradual decrease in the macroscopic contact angle upon heating. The presence of an attractive dispersion force can significantly delay or inhibit the (otherwise expected) occurrence of complete wetting at higher temperatures. The equilibrium thickness of SAFs is explained from a balance between several interfacial interactions, including dispersion forces, short-range forces of structural or chemical origins, volumetric free-energy terms, and electrostatic interactions. In a generalized Cahn critical-point wetting model, these SAFs are alternatively considered to be disordered multilayer adsorbates formed from coupled prewetting and premelting transitions.