Bubble pressure
Dynamic surface tension
With conventional surface tension measuring methods, such as the Wilhelmy plate method, the static surface tension is obtained, i.e. a statement about a completely formed surface which is in dynamic equilibrium.
Surfactant solutions require a much longer time than water and other liquids to achieve this dynamic equilibrium. This is because of the molecular construction of the surfactants: they consist of a hydrophilic (water-attracting) “head” and a hydrophobic (water-repelling) “tail”. As a result of this construction the surfactant molecules accumulate at the surface; the “tail” projects from the surface and causes a reduction in the surface tension.
This is shown in the following illustration:
- Fig. 1: Orientation of surfactant molecules
- An animated version
Interfaces are produced extremely quickly in processes such as foaming, cleaning, printing or coating. In such processes it is not just the equilibrium value of the interfacial tension that is the decisive influence, but also the kinetics of the interface formation. The molecular mobility of the surfactants used assumes a considerable influencing factor on the formation of the surface tension.
The following illustration is a schematic representation of the surface tension s of two surfactant solutions as a function of the surface age. As can be seen, at the start the surface tension falls rapidly and only approaches an equilibrium value after some time has passed.
- Fig.2: Time dependence of the dynamic surface tension