The elastic ring method

An experimental set-up for the study of surface relaxation processes in soluble adsorption layers after transient surface area disturbances was developed by Loglio et al. (1986, 1988, 1991a, b). The main feature of the apparatus is the elastic circular ring, which confines the surface area in the sample vessel and substitute for the traditional barrier and trough (Fig. 6.6). 

Perspective view of sample vessel, area-confining ring and measuring plate according to Loglio etal. 1988 

By changing the shape of the ring, which is immersed into the solution, small area changes of the solution surface can be applied. The surface tension response after such deformations are registered via force measurements using a Wilhelmy plate. 

The software driven apparatus allows different types of area changes step and ramp type, square pulse and trapezoidal as well as sinusoidal area deformations. The construction ensures that area changes are almost isotropic. Area changes used in transient and harmonic relaxation experiments are of the order of 1 to 5%. The surface tension response measured via the Wilhelmy balance has an accuracy of better than 0.1 mN/m. 

---

Harmonic and transient relaxation experiments for dodecyl dimethyl phosphine oxide solutions were performed with the elastic ring method by Loglio [240]. This methods allows oscillation experiments in the frequency range from about 0.5 to 0.001 Hz and is suitable for comparatively slow relaxing systems. Slow oscillation experiments can be performed much easier now with the pendent drop apparatus [186]. Both techniques are also able to perform transient relaxation experiments. The two types of experiments have a characteristic frequency defined in the same way by Eq. (4.110). 

Oscillation experiments with the elastic ring method in the frequency range from about 0.5 to 0.001 Hz for dodecyl dimethyl phosphine oxide solution were first conducted by Loglio [68]. 

---