Techniques for Studying Foam Films

One of the most important characteristics of foam films is the contact angle 0 appearing at the contact of the film with the bulk phase (solution) from which it is formed. This could be obtained by a topographic technique (which is suitable for small contact angles) that is based on determination of the radii of the interference Newton rings when the film is observed in a reflected monochromatic light. 

Another technique for studying foam films is to use a-particle irradiation, which can destroy the film. Depending on the intensity of the a-source, the film either ruptures instantaneously or lives for a much shorter time than required for its spontaneous mpture. The life time of a black film subjected to irradiation is considered as a parameter characterising the destructive effect of a-particles. 

Deryaguin and Titijevskaya [37] measured the isotherms of disjoining pressure of microscopic foam films (common thin films) in a narrow range of pressures. At equilibrium, the capillary pressure in the flat horizontal foam film is equal to the disjoining pressure (ti) in it. 

Several other techniques have been applied to measure foam films, e.g. ellipsom-etry, FT-IR spectroscopy. X-ray reflection and measurement of gas permeability through the film. These techniques are described in detail in the text by Exerowa and Kruglyakov [4] to which the reader is referred. 

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A third technique for studying foam films is the fluorescence recovery after photobleaching (FRAP). This techniques was applied by Clarke et al. [36] for lateral diffusion in foam films, and involves irreversible photobleaching by intense laser light of fluorophore molecules in the sample. The time of redistribution of probe molecules (which are assumed to be randomly distributed within the constitutive membrane lipids in the film) is monitored. The lateral diffusion coefficient, D, is calculated from the rate of recovery of fluorescence in the bleaching region due to the entry of unbleaching fluoroprobes of adjacent parts of the membranes. 

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