New insights into the role of surrounding medium temperature on the under-liquid wetting of solid surfaces.

The wetting of a solid surface by a liquid droplet under a liquid medium at elevated temperatures depends not only on the solid-drop and drop-medium interfacial tensions (IFTs) but also on the temperature dependency of the IFT of the surrounding medium. Previous studies have shown either a decreasing or nearly invariant trend of wettability with an increase in temperature. However, much of the research up to now has only focused on the evaluation of solid wettability in air or vapor, and no model has been proposed to predict the variation of solid wettability at high temperatures under a liquid medium. Here, we developed a theoretical framework and a novel experimental approach to evaluate the high-temperature solid-liquid-liquid wettability. We investigated the wettability of different polymeric and nonpolymeric surfaces, namely, glass, silicon wafer, poly(methyl methacrylate) (PMMA), and polytetrafluoroethylene (PTFE), for a wide range of polar and nonpolar probe droplets under water (as a liquid medium) at temperatures up to 90 degrees C. The experimental results revealed that the nonpolymeric highly polar solid surfaces, that is, glass and silicon wafer, showed a sharp increase in their contact angle with the probe droplets at elevated temperatures. Between the two polymeric surfaces, PMMA showed a decreasing trend of the contact angle over the variation of temperatures, while in the case of PTFE, no specific trend was observed. The predictions of our theoretical model were in good agreement with the experimental observations with less than +/- 25% deviation.