Architecture of soap films and the forces that stabilize them
Soap films are nothing more than thin liquid layers whose water-air interfaces have been stabilized by a surfactant. The picture below illustrates an ideal soap film:
In reality there is always excess surfactant remaining in solution. Surfactants have a dual character built-in within the same molecule: part of it -the head- is hydrophilic, whereas the other part - the tail- is hydrophobic. The surfactant molecules remaining within the water core need to shield their tails from water as much as possible. To do so, they self-assemble into quasi-spherical structures called micelles:
When you put a lot of soap into water, the density of micelles is high within the core of a soap film, and they must be packed tightly. This problem becomes interesting when the the film is thin. To appreciate this, recall what happens when you fill a bottle with marbles: adjacent to the bottle's walls, the marbles order neatly into hexagonal domains. This order, imposed by the wall, propagates inside to a certain extent. If two parallel walls happen to be close enough, all marbles will be found in one layer or another.
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| Thickness h vs time t of a film drawn from a solution with c=12 wt % |
The same happens with micelles within the confining walls of a soap film if there are many of them! This gives rise to the beautiful phenomenon of stratification: when you draw a frame out of a soapy solution spanning a soap film, the film rapidly thins smoothly due to evaporation, capillary and van der Waals forces. Eventually it becomes thin enough, and further thinning proceeds via "steps" which correspond to the removal of individual micellar layers. Our light scattering work studies this phenomenon and illustrates the behavior of fluids in confining geometries, the micelles playing the role of the fluid's molecules.
"Micellar stratification in soap films: a light scattering study"
O. Krichevsky and J. Stavans, Phys. Rev. Lett. 74, 2752 (1995).
"Confined fluid between two walls: the case of micelles inside a soap film"
O. Krichevsky and J. Stavans, Phys. Rev. E. 55, 7260 (1997).