On The Temperature And Pressure Dependence Of The Contact Angles Of Water On Solid Surfaces At Elevated Temperatures And Pressures

The wetting behavior of water against solid surfaces is a common phenomenon in nature.In thermodynamics,the wetting of water,which is often characterized by the macroscopic contact angle at the gas/liquid/solid triphase interface,is a typical Gibbsian multiphase interfacial issue of equilibrium.An indepth understanding of the effects of the most basic thermodynamic parameters(i.e.,temperature and pressure)on water contact angle is not only beneficial to quantify the interfacial free energies between different phases,but also is critical to establish the balance between the interfacial energies in the involved system and to understand the mechanisms underlying the interfacial energy and mass transport phenomena.Additionally,the investigation on the temperature(or pressure)dependence of surface wettability of water at elevated temperatures and pressures(especially under near critical conditions)is also of great significance to the thermal design and safety optimization of the energy and power systems with water being the working fluid(e.g.,thermal and nuclear power systems based on steam power cycle).The measurement of the contact angles of water at elevated temperatures and pressures,which requires the test apparatus to meet visualization and hightemperature andpressure test conditions at the same time,is very difficult.Therefore,the water contact angle data for elevated temperatures and pressures are quite scarce in the literature,especially those in the near critical temperature regime over 300 ℃.Moreover,the quantification of some key surface thermodynamic parameters(such as interfacial tensions and surface entropy)is of significance to understand the mechanisms of surface wetting.However,it is very challenging to measure these interfacial parameters under elevated temperatures and pressures,thus leading the study on the corresponding wetting mechanisms to be stalled.Therefore,the present study performed an indepth study on the wetting behavior of various solid surfaces near the critical point of water,based on both experimental and surface thermodynamic analysis.On a basis of the sessile drop approach,a visualized test platform designed and constructed in the present work for measuring the hightemperature andpressure contact angles,of which the design temperature and pressure are 350 ℃ and 18 MPa,respectively,is far ahead of its international counterparts.In this work,the sealing method using soft graphite pads and locking bolts was applied to solve the gas leakage of the pressure vessel at elevated temperatures and pressures.Additionally,a back pressure valve system was used to lower the pressure instability in the test area,so as to reduce the impact of measurement errors and uneven distribution of temperature.By reasonably adjusting the positions of the heaters and the windows of the pressure vessel,the fogging on the windows under high temperatures is significantly reduced,thereby improving the imaging quality of the drops.Moreover,the removable extension module of the pressure vessel can further match the dynamic drop volume approach to accurately measure hightemperature andpressure dynamic contact angles.Via using this experimental apparatus,the present work first investigated the effects of temperature and pressure on the contact angles of water on solid surfaces,then studied the wetting behavior of water under the synergistic effects of temperature,pressure and surface properties(i.e.,surface material and roughness),and finally elucidated the corresponding physical mechanisms underlying the wetting phenomena on a basis of the theory of surface thermodynamics and adsorption.The conclusions are shown as follows:(1)The piecewiselinearly negative temperature dependence of contact angle of water can be classified into three regimes,namely the lowtemperature(< 120 ℃),mediumtemperature(120 ℃–240 ℃),and hightemperature(up to 300 ℃)regimes.A slightlydecreasing or nearlyinvariant trend of water contact angles on both nonmetallic and metallic hydrophilic surfaces was reported for the lowtemperature regime.Nevertheless,a steeper linear decline in water contact angle was demonstrated at the temperatures above 120 °C.Additionally,the water contact angle either again becomes nearly temperatureindependent or slightly increases when the temperature is above 240 °C.However,for the hydrophobic surfaces,the water contact angles hardly change with ambient temperature due to the negligible gas adsorption on the solid surface.(2)Compared to the temperature effects,the pressure influence on water contact angle has shown to be less significant.The pressure dependence of water contact angle presents different at lowand hightemperature regimes: the water contact angle is positively correlated to pressure below 100 ℃,whereas an inverse variation occurs over 100 ℃.The aforementioned phenomena are mainly because of the competition between the variations of the solidgas and gasliquid interfacial tensions caused by increasing pressure and temperature respectively.At low temperatures,the rise in pressure promotes the adsorption of gas on solid surfaces,and then the decrease of the solidgas interfacial tension leads to the increase of the contact angle.However,when the ambient temperature was raised to a higher value,the adsorption of gas was weakened.At this monment,the change of the water contact angle with pressure is mainly controlled by the influence of the gasliquid interfacial tension.(3)In addition,the present work showed that surface material and temperature have synergistic effects on water contact angle.The variation of water contact angle with temperature(such as temperature coefficients and the critical values separating temperature regimes)is significantly dependent of the type of surface material.In short,the surface wettability of metallic materials is more sensitive to the changes of temperature than that of nonmetallic materials.Moreover,temperature also affects the relevancy of water contact angle versus surface material:increasing temperature was shown to decrease the influence of surface material on the contact angle,thus leading to the nearly consistent wettability of various solids at the temperatures over240 °C.Moreover,due to the internal flows caused by the strong evaporation of water droplets at high temperatures,it was observed that the gasliquid interface for water droplets deviates from the spherical shape under the hightemperature andpressure conditions.(4)Similar to the impact of surface material,the temperature and pressure dependence of water contact angles is also affected by surface roughness.Surface roughness has a relatively significant effect on water contact angle at low pressures,but the effects can be weakened by increasing pressure.Additionally,at the low temperatures less than 160 ℃,increasing surface roughness can boost the influence of environmental pressure on the contact angle of water.However,the wettability of the SS304 surfaces with various roughness shows similar correlations of pressure at the temperatures higher than 160 ℃.Similarly,the effects of surface roughness on the contact angles of water are mainly reflected in the lowtemperature regime(below120 ℃): the reduction of surface roughness is likely to increase the contact angle of water,but raising temperature diminishes the effects of roughness.In short,the influence of surface roughness mainly occurs under lowtemperature andpressure conditions.Besides,the pressure dependence of water contact angle is more easily affected by surface roughness,in comparison to that of temperature.In conclusion,via overcoming the difficulties of measuring the hightemperature andpressure contact angle,the relevant data over 300 ℃ have been filled in the present work.Based on the measured contact angle data,the mechanisms of temperature and pressure effects on water contact angle were unravelled via the theory of surface thermodynamics and a new method to estimate some key surface thermodynamic parameters(such as solid surface tension,surface entropy,adsorption/wetting heat,etc.)using the temperaturedependent contact angles of water was also proposed in this work.The results of this study will not only be beneficial to develop the thermodynamic theory of surface wetting phenomena,but also will be of practical value to explore the interfacial transport and phase change phenomena of water.