Adsorption effect in non-reaction wetting: In-Ti on CaF2

The experiments show that the alloying liquid In with only (0.1-0.5) at% Ti dramatically reduces the equilibrium contact angle Θ∞ formed by In on the surface of CaF2. The aim of this paper is to clarify whether this practically important and conceptually challenging effect can be explained solely by Ti adsorption at the F-terminated solid-liquid interface without resorting to any other Ti-induced effect. The combination of ab initio calculations and regular solution approximation was proposed for finding the binding energy, ΔETi of Ti adatom with the interface ''CaF2/liquid solutions In-Ti.'' With thus obtained ΔETi=1.16 eV, we calculated from the Shishkovsky isotherm the reduction in the solid-liquid interface energy, ΔγSL induced by Ti adsorption from liquid In with various Ti concentration, C. It was found that Δγ SL(C) dependence demonstrated close inverse correspondence with Θ∞(C) and that the theory fitted very well all available experimental data on the concentration and temperature dependence of ΔγSL. It was concluded that the Ti adsorption effect is large enough to account for the observed wetting improvement. The proposed multiscale modeling approach to the role of adsorption in wetting can be applied also to other nonreactive systems ''liquid metal-ceramics'' where the substrate determines the surface density of the adsorption sites for the active element. (orig.)