Surface Tension
The origin of the surface tension is the fact that the molecules close to the surface are statistically further apart compared to the molecules in the bulk. Measuring the surface tension of solutions allows the determination of adsorption of components.
This expansion of the liquid at the surface requires energy and hence the energy is higher at the surface than in the bulk liquid. Thus the liquid tends to contact into a spherical drop for which the surface is the smallest for a given volume. This is also the explanation for the fact that liquids with strong cohesive forces, e.g. water, have high surface tension compared to liquids where there only is a small energy required to expand the system, e.g. alkanes.
Du Noüy ring tensiometers
Here a platinum ring is forced through the surface and the resistance to this movement is measured, as it is proportional to the surface tension.
Pendant drop
In the pendant drop the shape of the drop is determined through image analysis. The more elongated the drop the lower is the surface tension. This measurement principle is also valid for determining the interfacial tension between two insoluble solutions.
Dynamic surface tension
The dynamic surface tension is the change of the surface tension as it approaches equilibrium. The reason for a system not to reach equilibrium instantaneously is most often due to diffusion of molecules from the bulk to the surface or rearrangement of macromolecules at the surface. The measuring principle is most often by measuring the pressure required to form a bubble.