Light scattering by soap films

Abstract

A theory is constructed describing the scattering from a liquid film (e.g., a soap film) of a light beam polarized normal to the plane of incidence. This scattering is due to the small irregular corrugations caused by thermal motion. The interference of the reflected incident beam with its multiple reflections (and refractions) and similarly the interference produced from the scattered beam have been taken into account. The corrugations of the two surfaces may be correlated depending on the forces holding the film in equilibrium. Where weak forces are operative, the corrugations of the two surfaces are not correlated. An estimation of the correlation between the corrugations of the two film surfaces was made from current theories of film stability in which the principal forces are attributed to an electrostatic repulsion between soap ions on the two surfaces of the film and van der Waals' attraction among the molecules in the film.

Preliminary measurements on soap films formed from ionic and nonionic soap solutions with glycerol are reported. It was found that the dependence of the light scattering on wavelength and angle of observation was in agreement with theory. Light scattering from very thin (black) films gave data about interaction which could be interpreted in terms of electrical forces and van der Waals' forces and led to values of the surface potential ψ0 = 40 mv. and a van der Waals' constant A = 1.3 × 10−12 erg. However, the value of the reciprocal Debye-Hückel thickness, κ, found in this manner was three times larger than could be expected from the concentration of the electrolytes present in the soap solution.