Understanding the effects of pressure on the contact angle of water on a silicon surface in nitrogen gas environment: Contrasts between low- and high-temperature regimes

[Display omitted] Pressure dependence of contact angle is expected to be influenced by temperature. Nevertheless, the correlation of water contact angle with pressure is rarely investigated at high temperatures (over 100 ℃). In this work, measurements of the contact angle and interfacial tension of water in N2 atmosphere were conducted at various pressures and temperatures (up to 17 MPa and 300 ℃). The experimental observations were elucidated based on the theory of surface thermodynamics. It was shown that the water-N2 interfacial tension linearly decreases with increasing the pressure, and that the pressure coefficient declines as temperature rises. The pressure dependence of the water contact angle was found to be different for the low- and high-temperature regimes: the water contact angle increases below 100 ℃, whereas an inverse variation occurs over 100 ℃. According to the theoretical analysis, the pressure dependence of both the water interfacial tension and contact angle is attributed to N2 adsorption on the surfaces of water and silicon. The variations in the water contact angle with pressure, including both the sign and magnitude, are actually the consequence of the changes of water-N2 and Si-N2 interfacial tensions manipulated by pressure and temperature.