Liquid alkali metals and alloys on their basis are advanced materials in nuclear and thermonuclear power engineering, as well as in other fields of novel technologies. Study of surface tension (ST) of liquid alkali metals and their alloys is of great importance for a development of theory and decision of practical problems in physics of surface phenomena, as well as in wetting and spreading processes, and constructing the heat – transfer pipes and capillary pumps in nuclear space power engineering. It is known that, to a large degree, ST depends on the surface state of the investigated metals and alloys. Therefore, determining ST under the same conditions, especially for highly active alkali metals and their alloys, becomes a very important issue.
Analysis of literature data showed that, the Na-Cs alloys ST remain least of the all studied compared with other alkali metals systems. An all – soldered device which allows determining the ST of the binary liquid metallic systems over the entire concentration range at just a single load of the pure starting components and under identical high static vacuum conditions was used.
The temperature and concentration dependences of high – purity Na, Cs and their alloys ST were measured by a large drop method. The overall error of the ST measurements was about 1%. It is shown that, Cs reveals a high surface activity in Na-Cs liquid alloys amounting to 3000 mN/m at. fraction. The obtained results are confirmed by available values of the activity coefficients of Cs in Na-Cs alloys and agree to basic criteria for surface activity of components in the binary metallic systems.