Soap films surface fluctuations

Let us now turn our attention to the effect of a thin monolayer of soap molecules present on both film surfaces. In the first place it was found (see Section III.B) by Bouchiat and Langevin that density and orientation fluctuations of the soap molecules (in the case of a single interface) make only a negligible contribution to the scattering process, except possibly at a (two-dimensional) critical point of the monolayer, or by addition of a few fluorescent soap molecules. Thus we are left with the optical effect of the monolayers on the interference of the light waves. It turns out (see Section VI) that this interference effect can be taken into account by redefining the (optical) film thickness. 

The first theory for film fluctuations resembles Cahn s theory of spinodal decomposition of unstable bulk systems. A very simple mechanism was adopted for the liquid flow. It was assumed that because of the presence of the soap monolayers, the film surfaces were stagnant (see Section 11) and that the film liquid was pumped back and forth through a slab with thickness h according to Reynolds s law ... 

Intensity fluctuation spectroscopy was used in our laboratory to study the dynamic behavior of surface ripples on thin liquid films. Both squeezing and bending modes were examined. To our knowledge one other group of researchers has obtained dynamic light-scattering data from thin soap films but as far as we know, nothing has been published in the official literature. Also some experiments were reported on lipid bilayers in water. ... 

An absence of the Gibbs-Marangoni effect is the main reason why pure liquids do not foam. It is also interesting, in this respect, to observe that foams from moderately concentrated solutions of soaps, detergents, etc., tend to be less stable than those formed from more dilute solutions. With the more concentrated solutions, the increase in surface tension which results from local thinning is more rapidly nullified by diffusion of surfactant from the bulk solution. The opposition to fluctuations in film thickness by corresponding fluctuations in surface tension is, therefore, less effective. 

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