接触角,宏观表面润湿性的定量测量,在理解液-气非均相相变现象中起着重要作用,例如,沸腾传热。对于理解基于蒸汽的发电中润湿性调节的沸腾传热,在高温下水的接触角特别重要。从更理论的角度来看,水的接触角的温度依赖性也是必不可少的几个关键表面热力学性质的估计,如固体表面张力,表面熵,以及浸没和吸附的热量。这里,全面回顾了在宽温度范围内测量水在各种固体上的接触角的历史努力,不限于金属表面,是presented。正如文献数据所表明的那样,水接触角的温度依赖性可分为三种状态:(a)低于饱和点的低温(即,在大气压下100°C),(b)介质温度高达~170°C,和(c)在加压条件下高达300°C的高温。据报道,在低温状态下,水在非金属和金属表面上的接触角呈略微下降或几乎不变的趋势。相比之下,在高于100°C的温度下,水接触角的线性急剧下降。几个金属表面上的少量实验数据表明,水的接触角再次变得几乎与温度无关,或者随着温度高于210°C而进一步降低。基于表面热力学分析,给出了对温度依赖性的理论理解,尽管这些实验观察的确切分子机制尚不清楚。因此,预测水接触角随温度变化的理论模型尚不完善。当水的临界点(374°C和22.1MPa)接近时,表面张力,因此接触角,应该变得越来越小。然而,由于在如此高的温度/压力下缺乏实验数据,这一理论预期尚未得到证实。最后,确定了未来的研究方向,包括对接近临界温度下的接触角的系统探索,表面氧化的影响,腐蚀,以及锅炉和反应堆运行过程中接触角的沉积,以及辐照对核反应堆应用中接触角的特殊影响。
Contact angle, a quantitative measure of macroscopic surface wettability, plays an important role in understanding liquid-vapor heterogeneous phase change phenomena, e.g., boiling heat transfer. The contact angles of water at elevated temperatures are of particular interest for understanding of wettability-regulated boiling heat transfer in steam-based power generation. From a more theoretical perspective, the temperature dependence of contact angle of water is also essential to estimation of several key surface thermodynamic properties, such as the solid surface tension, the surface entropy, and the heats of immersion and adsorption. Here, a comprehensive
review of historical efforts in measuring the contact angles of water over a wide temperature range on a variety of solids, not limited to metallic surfaces, is presented. As suggested by the literature data, the temperature dependence of contact angle of water may be classified into three regimes: (a) low temperatures below the saturation point (i.e., 100 °C at atmospheric pressure), (b) medium temperatures up to ~170 °C, and (c) high temperatures up to 300 °C at pressurized conditions. A slightly-decreasing or nearly-invariant trend of the contact angles of water on both non-metallic and metallic surfaces was reported for the low-temperature regime. In contrast, a steeper linear decline in water contact angle was demonstrated at temperatures above 100 °C. The few experimental data available on several metallic surfaces showed that the contact angle of water either again becomes nearly temperature-independent or further decreases with temperature above 210 °C. A theoretical understanding of the temperature dependence is given based on surface thermodynamic analysis, although the exact molecular mechanisms underlying these experimental observations remain unclear. Consequently, the theoretical model for predicting the variation of the contact angle of water with temperature is not well-developed. As the critical point of water (374 °C and 22.1 MPa) is approached, the surface tension, and hence the contact angle, should become vanishingly small. However, this theoretical expectation has not yet been verified due to the lack of experimental data at such high temperatures/pressures. Finally, future research directions are identified, including a systematic exploration of the contact angle at near-critical temperatures, the effects of surface oxidation, corrosion, and deposition on contact angle during operation of boilers and reactors, and the particular effect of irradiation on contact angle in nuclear reactor applications.