Soap films colours

We have noted that the adhesion of the polar groups to water and to one another is much greater than the weak adhesion of hydrocarbon chains either to water or to one another. It is thus reasonable to assume, an anticipation verified by Perrin Ann. Rhys. X. 160, 1918), that soap films may be made up of composite surfaces each of which consists of two layers of orientated molecules of soap the outer surface of each side consisting of hydrocarbon chains and the polar groups held together with water as a sandwich between the orientated hydrocarbon chains. These elementary leaflets which will possess but little adhesion for one another may be built up to form thick films similar in structure to the crystalline fatty acids examined by Shearer (see p. 73). The leaflets may slip over one another with great ease, thus providing the play of interference colours noticed in soap films. The elementary leaflet has in fact been shown by Perrin and others to be two molecules in thickness. 

The equilibrium thickness of a (meta-)stable soap film will depend on the strength and range of the repulsive forces in the film. Electrostatic forces are long-range in water and hence give rise to thick (0.2 micron) films, which are highly coloured due to the interference of visible light... 

In this situation, the equilibrium thickness at any given height h is determined by the balance between the hydrostatic pressure in the liquid (hpg) and the repulsive pressure in the film, that is n = hpg. Cyril Isenberg gives many beautiful pictures of soap films of different geometries in his book The Science of Soap Films and Soap Bubbles (1992). Sir Isaac Newton published his observations of the colours of soap bubbles in Opticks (1730). This experimental set-up has been used to measure the interaction force between surfactant surfaces, as a function of separation distance or film thickness. These forces are important in stabilizing surfactant lamellar phases and in cell-cell interactions, as well as in colloidal interactions generally. 

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. 

The transitions from an unstable thick film to black films arc easily demonstrated by allowing a vertical soap film supported on a frame to drain (Figure 12.11). Initially the whole film shows interference colours, then a dark boundary, separated from the... 

Figure 12.11 Draining soap film showing interference colours from thicker films and silver and black bands from thin films. The two types of black film are not distinguishable.

Those who have seen the colour plates in this book and particularly those of us who have seen Dr. Isenberg s skilful and artistic manipulation of soap films, during one of his lectures, will now be tempted to return to our early adventures in the blowing of bubbles and to repeat the experiments in a more sophisticated and enlightened way. 

It was Joseph Plateau s experiments with soap films that provided mathematicians with renewed motivation to investigate the problems of minimum area surfaces. Some of these beautiful analogue solutions are examined in Chapter 4 with coloured plate illustrations. Although substantial efforts were made to obtain analytic solutions to these problems it was not until the 1930 s that significant progress was made by mathematicians such as Jesse Douglas, who obtained some general solutions, and Tibor Rado. " ... 

Table 2.1 is taken from Lawrence s book Soap Films. It contains a descriptive indication of the colours produced by the interference of white light. The white light is incident normally on a vertical soap film of refractive index jti = 1.41. The Table also contains the thickness of soap film associated with each colour of film. For an incident beam which has an angle of refraction 6, an additional factor of cos 6 is required, Eq. (2.26), in order to obtain the thickness of the film from its colour in Table 2.1. Thick soap films will produce overlap of different orders of interference for the different spectral colours. The film will consequently appear increasingly white as the correlation between the interference produced by different colours decreases to zero.

Interference colours produced by white light and the corresponding thicknesses of the soap film. 

Interference colours produced by a vertical soap film, shortly after formation, using white light... 

The educational value of soap films and bubbles should not be overlooked in these concluding remarks. The shapes, motions, and colours of films and bubbles provide a simple means of demonstrating many interesting phenomena to students at all academic levels, from primary school to university. It is my hope that this volume will encourage greater use of soap film and bubble demonstrations and experiments in our schools, educational institutions, and universities. 

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