What are the effects of surface tension?
In practical applications, determining the surface tension allows us to see how liquids wet a solid, how they mix with another liquid, and how they behave against a gas. Generally, the higher the surface tension, the higher the interactions within the phase. This means that liquids with higher surface tension are less likely to mix with another phase.
An example: the surface tension of water (against air) is 72.8 mN/m at 20 °C, and is thus relatively high. Surface tension is the reason why a skin
forms on a water surface. This is why, for example, a paper clip floats on the water surface and a water strider can walk on the water. Oil, on the other hand, has a surface tension of only about 35 mN/m - this is why oil spreads easily on a surface or wets it more easily. This property is exploited, for example, in so-called penetrating oils.
Practical applications for the determination of the surface tension of liquids
In chemistry and materials science, the measurement of surface tension gives important insights into the behaviour of liquids on surfaces. This is relevant, for example, in the development of coatings and paints. A low surface tension can cause liquids to spread easily on surfaces and form an even coating, while a high surface tension can lead to uneven wetting.
In the food industry, measuring surface tension is important to ensure the quality of food. If the surface tension is too high, liquids do not penetrate food structures well, resulting in less flavour and aroma absorption. The right surface tension is also important for the formation of emulsions and foams.
In environmental science, measuring surface tension is important for understanding the behaviour of liquids in the environment. This can be important, for example, in the study of oil spills in water bodies or in the development of environmentally friendly cleaning agents.
The measurement of surface tension is also of great importance in pharmacy. Surface tension can influence the adsorption and release of drugs.
Measuring methods for determining the surface tension
Interfacial and surface tensions can be measured using a force-based tensiometer. Common methods for measuring surface and interfacial tensions are the Wilhelmy plate method and the Du-Noüy ring method.
The surface and interfacial tension of liquids can also be determined using an optical contact angle meter. Here, a drop hanging at the end of the dispensing needle is measured. This method is called the pendant drop method.
If very low interfacial tensions between two liquids are to be measured, a spinning drop tensiometer is the measuring instrument of choice. The spinning drop method is based on the optical contour evaluation of a rotating drop.