Temperature dependence of contact angles of water on a stainless steel surface at elevated temperatures and pressures: In situ characterization and thermodynamic analysis.

Phase change heat transfer (e.g., boiling of water) on surfaces can be enhanced by tuning the surface wettability, which is often quantified by the contact angle and is expected to be influenced by temperature and pressure. However, the temperature (and pressure) dependence of contact angles of water on metallic surfaces remain unclear. In this study, an in situ characterization of the contact angles of water on 304 stainless steel surfaces at temperatures from room temperature to 250 °C and at pressures up to 15 MPa was performed using the sessile drop method. It was shown that three distinct regimes can be identified on the contact angle-temperature curves. A slightly-decreasing trend of the contact angles with temperature was observed below 120 °C, followed by a steeper linear decrease at higher temperatures. A further rise of the decreasing rate with temperature was observed above 210 °C. In contrast to temperature, the pressure was shown to have little effects on the contact angles. Based on the theory of surface thermodynamics, the effects of temperature (and pressure) on the contact angles were analyzed in terms of the interfacial tensions. An empirical correlation was developed to predict the contact angles as a function of temperature.