The effective surface age in adsorption kinetics experiments

The determination of the effective surface age is the key for comparison of results obtained by different experimental techniques. If for example the drop volume technique is used in its classical version, which is based on continuously growing drops, dynamic surface tensions are obtained as a function of drop formation time. It was shown in the previous chapter, that the process of adsorption at the surface of a growing drop is overlapped by a radial flow inside the drop, which changes the diffusion profile. In addition, the drop area increases and 

On the contrary, the so-called quasi-static drop volume version is based on drops having almost constant volume and surface area. Thus, the result of these experiments is a dynamic surface tension as a function of adsorption time. In Fig. 5.25 a comparison of both types of experimental data is shown plotted as surface tension vs. I / 

If the classical and quasi-static results are compared, different times are necessary to reach one and the same surface tension value. The ratio of the two values amounts to about 3, in good agreement with the theory described above (cf section 4.5.). 

Similar situations exist for other methods. If quantitative theories are applied to data interpretation, all peculiarities of a method have to be considered. However, this does not allow a direct comparison of experimental results from different methods, because the time scales are different and depend on the specific experimental conditions. A solution to this problem is the determination of the effective surface age, which then allows a direct comparison between experimental data independent of their origin. 

The diagram in Fig. 5.26 shows schematically the ratio between the experimental time t, which is the life time of a drop or bubble in the respective experiment, and the effective adsorption time Tg. It becomes clear that each experimental method works under specific conditions and, therefore, different relations between a specific experimental time and the effective adsorption time or surface age exist. While the effective age igina maximum bubble pressure experiment 

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