Alloying In with small amount of Ti (0.1-0.5at %) reduces dramatically the equilibrium wetting angle Θ formed by liquid In on CaF2. No evidence of formation of new phases at the solid-liquid interface (SLI) was found. In this system, which may be considered as non-reactive (NR), the drop spreading rate, V has to be controlled by the viscous flow. However, actual value of V is several orders of magnitude below the expected for the viscous flow controlled regime. A similar behaviour was reported also for other metal/ceramic and metal/metal systems.
We performed both the thermodynamic and kinetic analysis of the NR system and validate the theoretical results by comparison with experimental data for the CaF2/In-Ti system. It was demonstrated that the observed wetting improvement reflects the reduction in the SLI energy, γSL, as a result of Ti adsorption from the liquid, and that the experimental spreading kinetics can be explained in the framework of the kink model, applied usually to crystal growth and dislocation climb.
The important elements in our approach are the following:
i/ DFT based ab-initio calculations of Ti binding energy, (ΔETi) , with the SLI. It was found that along with the direct Ti interaction with fluorine adsorption centres of CaF2 also a strong lateral attraction between Ti adatoms exists. This interaction is responsible for the increase in ΔETi from 0.20 eV to 1.16 eV with the increasing surface coverage of SLI with Ti.
ii/ Application of the Shishkovsky adsorption isotherm for calculating the interfacial energy reduction ΔγSL at SLI as a function of Ti concentration.
iii/ It was established that the classical adsorption theory with the binding energy ΔETi = 1.16 eV given by DFT calculations and quite reasonable density ГMAX =(7±0.7)·1018 m-2 of the adsorption sites at the SLI interface provides quantitative explanation of all available experimental data on the concentration and temperature dependence of the interfacial energy.
iv/ It was assumed that the thermal fluctuation aided kink formation on the ledges of the dense adsorption layer, as well as further lateral displacement of the kinks, occur due to Ti diffusion in the liquid towards the adsorption sites ahead of the triple line. This mechanism with the binding energies given by DFT calculations allows explaining relatively slow linear spreading.