Other dynamic surface and interfacial tension methods

Other Dynamic Surface and Interfacial Tension Methods 

The following so-called dynamic capillary method was developed by Van Hunsel Joos (1987b) and complements the area of application with respect to other methods. This method allows measurements from 50 ms up about 1 s, similar to the inclined plate and growing drop techniques described above, and can be used at liquid/liquid and liquid/gas interfaces without modification. The principle of the experiment is schematically given in Fig. 5.23. Two fluids are contained in a tube of diameter R. The interface (or surface in case of studies at the water/air interface) is located in such a way that its interfacial tension can be measured by the capillary rise of the lower liquid in a narrow capillary c, which connects the both fluids. The height of the capillary rise h is determined via a cathetometer Cat. 

When the liquid flows through the tube T with a constant flow rate the interface moves downwards with the velocity v. Under the assumption that the liquid film adheres to the wall of the capillary new surface is created by the receding liquid meniscus. Assuming a hemispherical meniscus, the dilation rate is given by 

Depending on the flow rate v a stationary state is reached (Van Voorst Vader et al. 1964) and the surface or interfacial tension in function of the dilation rate results 

Although many other experimental set-ups were developed to study the dynamics of adsorption, mainly via surface and interfacial tensions, of solutions of surface active compounds and polymers, they cannot all be described in detail here. More are given in textbooks of surface chemistry, e.g. by Adamson (1990) or Edwards et al. (1991). The last original technique, briefly discussed in this chapter, is the overflowing cylinder method used for example by Bergink-Martens et al. (1990). 

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