Wetting and evaporation of alkali metal sodium droplets on 304 stainless steel

The wetting and evaporation of sodium droplets on 304 stainless steel are closely related to the performance of high-temperature heat pipes, but related studies are scarce and imperfect. The aim of this study is to carry out an experimental study on them with a visualized experimental set-up. Firstly, the contact angle (θ, °) of sodium droplets under different pressure (P, Pa) and temperature (T, °C) conditions are measured and modeled. The θ tends to decrease with increasing T and P. Among all the conditions, the largest θ is 110.75° (P = 400 Pa, T = 200°C), and the smallest θ is 36.9° (P = 1000 Pa, T = 400°C). By fitting the data, an experimental correlation equation between θ, T and P is derived with a R² of 0.92. Secondly, the evaporation patterns of sodium droplets is investigated and revealed. In general, the sodium droplets showed mixed evaporation patterns under lower pressure, while constant contact diameters patterns under higher pressure. Eventually, the chemical composition on sodium droplets is analyzed by XPS. The results show that it consists of organic matter, as well as chemical products of sodium and stainless steel that migrate to the surface of the droplet (singlet C, Na₂O etc.). In addition, C-O organic impurities and C monomers are found precipitated near the droplet contact line. The results of the study can provide models and data for the design and performance analysis of high-temperature heat pipe, and provide valuable references for the engineering design of future high-temperature heat pipe functional materials.