Advancing and Receding Contact Angles of Water on a Hydrophobic Surface: High-Temperature and High-Pressure Insights

The wetting behavior of water droplets under high-temperature and high-pressure conditions is expected to differ from that under room conditions, yet the dynamic surface wettability under such extreme conditions remains unclear for hydrophobic surfaces. In this work, measurements of advancing and receding contact angles (ACA and RCA) of water on PTFE surfaces were conducted at various temperatures and pressures, up to 300 °C and 16 MPa, respectively. For the first time, the data of dynamic contact angles on PTFE surfaces over 160 °C were reported. The temperature dependence of ACA and RCA exhibits a weak negative correlation below 200 °C. The contact angle hysteresis increases sharply above 260 °C, which is attributed to the kinetic hysteresis induced by the high-temperature softening of PTFE, leading to the formation of a wetting ridge that enhances contact line pinning. The pressure dependence of ACA and RCA demonstrates an apparent positive correlation due to the enhancement of nitrogen adsorption on the PTFE surface. However, the increase patterns of ACA and RCA are completely different, which is potentially attributed to the varying sensitivities to pinning effects. Additionally, the higher sensitivity to pinning effects of RCA is supposed to affect the competitive adsorption of nitrogen and water vapor on the PTFE surface, leading to the temperature-pressure coupling effect. Based on the temperature dependence of the contact angles, the surface tension and surface entropy of PTFE at different pressures were estimated. These findings offer a novel perspective for understanding and predicting surface wettability under extreme conditions.