ASTM D971 Interfacial Tension of Oil against Water by the Ring Method
The surface tension of an oil provides a relative indication of its capillary action, its ability to spread, and its ability to penetrate small spaces. The tendency to form small droplets and to puddle on smooth surfaces can also be indicated. In mineral oils, such as electrical insulating oils, small amounts of polar contaminants may change the properties and effectiveness of the oil. Interfacial tension can be used to indicate the level of these contaminants – In general, higher values indicate lower levels of contaminants. This test determines the interfacial tension of oils or liquids and may be used on new oils as a quality control tool and for service oils as an indication of the degree of deterioration.
Using a tensiometer, as per the method, water, the platinum ring, and the sample oil are carefully placed in the apparatus so as to avoid mixing of the layers. The ring is slowly raised until it passes between the water and oil layers. The interfacial surface tension of the sample is determined and reported in mN/m or dynes/cm as required.
Surface tension may be measured simply as the oil surface against air. The apparatus and procedure are similar. The value is referred to as “surface tension” rather than “interfacial surface tension”.