Thinning of Soap Films

After drainage some films may become very thin, and they hardly reflect light. They are called black films and can exist in two states the common black film has a thickness of the order of 30 nm the second (Newton s black film) is only 4.5 nm thick. In thin films the two surface layers are also separated by some interstitial fluid, but the strong interaction between the two surface layers determines the film thickness, which will remain uniform even if placed vertically. The black films are in metastable states, they can burst, but cannot undergo further drainage, and they also cannot become thicker later. To describe the thinning process leading to the black films, a number of models have been developed.  

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In the latter half of the nineteenth century Josiah Willard Gibbs, who is well known for his theoretical contributions to the study of statistical mechanics and thermodynamics, observed the draining and thinning of soap films. Some of these observations are reported in his paper entitled Equilibrium of Heterogeneous Substances. -... 

As a point of interest, it is possible to form very thin films or membranes in water, that is, to have the water-film-water system. Thus a solution of lipid can be stretched on an underwater wire frame and, on thinning, the film goes through a succession of interference colors and may end up as a black film of 60-90 A thickness [109]. The situation is reminiscent of soap films in air (see Section XIV-9) it also represents a potentially important modeling of biological membranes. A theoretical model has been discussed by Good [110]. 

Evaporation will thin the soap film by removing the water from the soap film. The decrease in the amount of water present will reduce the surface tension, by increasing the surfactant molecules, and cause the film to stretch (Fig. 2.4(d)). This can be easily observed by passing dry air from a jet over the surface of the soap film. 

Soap bubbles are spherical in shape and consist of a thin shell of water with surfactant molecules at each surface. A bubble encloses some air, or gas, at a pressure that is greater than the external pressure. Clusters of soap bubbles are produced by spherical bubbles that coalesce. Figure 4.16 shows two bubbles that have coalesced. The major part of each bubble consists of a spherical shell of soap film and they are separated by a spherical cap of soap film. In general, clusters of soap bubbles consist of a conglomeration of spherical surfaces and spherical caps of soap film. 

Our laminar stream, when air was injected, had, according to what precedes, a continuous part from 15 to 20 centimetres, under a load of 20 centimetres but this stream had, at the opening, a diameter of 3 centimetres if the annular slit had had only the diameter of the ordinary liquid streams, 6mm for example, it follows from the laws of the transformation of cylinders ( 384) that, under the same load, the continuous part would have been only 3 to 4 centimetres and if the layer, instead of being thick, had had the thinness of the films which constitute soap bubbles, this continuous part would have been much shorter stiU. 

In the case of solids, there is no doubt that a lateral tension (which may be anisotropic) can exist between molecules on the surface and can be related to actual stretching or compression of the surface region. This is possible because of the immobility of solid surfaces. Similarly, with thin soap films, whose thickness can be as little as 100 A, stretching or extension of the film may involve a corresponding variation in intermolecular distances and an actual tension between molecules. 

The rupture process of a soap film is of some interest. In the case of a film spanning a frame, as in Fig. XIV-15, it is known that rupture tends to originate at the margin, as shown in the classic studies of Mysels [207, 211]. Rupture away from a border may occur spontaneously but is usually studied by using a spark [212] as a trigger (a-radia-tion will also initiate rupture [213]). An aureole or ridge of accumulated material may be seen on the rim of the growing hole [212, 214] (see also Refs. 215, 216). Theoretical analysis has been in the form of nucleation [217, 218] or thin-film instability [219]. 

See K. J. Mysels, K. Shinoda, and S. Frankel, Soap Films, Studies of Their Thinning and a Bibliography, Pergamon, New York, 1959. 

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