Soap film thinning

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. 

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]. 

As a general physical example one may consider a flexible membrane like a thin rubber sheet or a soap film stretched over an irregular support, such as a distorted ring. 

Initially devised to measure interactions in single soap films (air/water/air) [8], the TFB technique has been progressively improved and its application has been broadened to emulsion films (oil/water/oil) [ 12] and asymmetric films (air/water/oil or air/water/solid) [13,14]. In a classical setup, a thin porous glass disk is fused on the side to a capillary tube and a small hole is drilled in the center of the disk. The liquid solution fills the disk, part of the capillary, and a thin horizontal film is formed across the hole. The disk is enclosed in a hermetically sealed box, with the capillary tube exposed to a constant reference pressure Pr. Under the effect of the pressure difference AP between the box and the reference, the... 

The thermodynamics of thin liquid films and adhesion is well documented in the literature [15,17,100-103]. The first theoretical approaches were developed mainly for thin soap films. Most of the results of the thermodynamics of soap... 

J.A. De Feijter and A. Vrij Contact Angles in Thin Films, n. Contact Angle Measurements in Newton Black Soap Films. J. Colloid Interface Sci. 64, 269... 

Slightly thicker soap films (ca. 150 nm) sometimes look golden. In the thinning process, the different colors get cut off. Thus, if blue color gets cut off, the film appears amber to magenta. 

If a soap film is sufficiently thin, its equilibrium thickness is the result of the double-layer repulsion, given by Equation (82), and van der Waals attraction, given by... 

Various other physical processes lead in their mathematical description to equations of the same form as Flq (2). especially in its steady-state form, Such processes include the conduction of electricity in a conductor, or the shape of a thin membrane stretched over a curved boundary. This situation has led to the development of analogies (electric analogy, soap film analogy) to heal conduction processes, which are useful because they often offer the advantages of simpler experimentation. 

Fig. 7.11. Soap-film flow meter. Soap solution is poured into the rubber bulb so that the solution level is just below the level of the sidearm. After attaching the flexible lubing to the flowing gas source, the rubber bulb is gently squeezed. This allows the flowing gas to percolate through the soap solution, picking up a thin film of soap which will travel up the burel. By noting how long it takes for the film to travel between two points on the buret, the flow may be determined.

In the first part of the paper, the interaction energy of thin soap films was calculated, assuming planar, parallel... 

Thickness is one of the main parameters of a foam film. The most widely employed technique for its determination is interferometry. It is based on the comparison between the intensities of the light falling on the thin film and that reflected from it. This technique permits to evaluate the thickness of equilibrium as well as thinning films. It has been used by Perrin [48] and Wells [49] with soap films. The intensity of the reflected light was measured with an interferometer comparing the two parts of the visual field (in a microscope). Thus the film thickness was determined with an accuracy of about 0.5 nm. 

The black spots on soap films, which are not more than 10 to 20 molecules thick, can remain for weeks in equilibrium with the thicker, coloured parts of the film,4 and hence it is assumed that they have the same vapour pressure as the normal liquid, and that Thomson s formula can be applied for a radius of curvature of 200 x 10 cm. or less. Bakker<5 gave reasons for supposing that the surface tension is independent of the radius of curvature of the capillary layer, although he recognised that in very thin films it has abnormal values, and he calculated that the maximum ascent of a liquid occurs in a tube of 2 5 m[jL radius. Woodland and Mack found no change of surface tension in a tube of 6 7 [I radius. 

Let the investigated system consist of a soap film stretched on two rings made from thin wire (Fig. 3). 

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