Newton black soap films

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... 

Entropic confinement forces occur at ultrathin (<5 nm) surfactant films and between bilayers in solution. They are mainly responsible for the stability observed in so-called Newton black soap films. It arises from the steric repulsion occurring when adsorbed layers overlap. These forces operate by various modes, like undulation, peristaltic fluctuations, or by head-group overlap. ... 

Thermotropic smectic membranes are similar to the Newton black soap films in respecf to their structure, thickness, and because both are spanned on frames and can exchange molecules with the meniscus. However, the smectic membranes are more complex in the respect that the number N of monolayers is variable, whereas the Newton black films are usually bilayers. Smectic membranes are also similar to vesicles by means of their layered structures however, the vesicles are rather isolated unframed systems in the sense that the number of molecules is conserved, but the surface is free to evolve. ... 

The physicochemical properties of foam and foam films have attracted scientific interest as far back as a hundred years ago though some investigations of soap foams were carried out in the seventeen century. Some foam forming recipes must have been known even earlier. The foundations of the research on foam films and foams have been laid by such prominent scientists as Hook, Newton, Kelvin and Gibbs. Hook s and Newton s works contain original observations on black spots in soap films. 

The phenomena described above have been known for a long time indeed, Newton reported on the black spots in soap films. In the past 25 years, however, these thin, liquid structures have become a subject of intensive scientific studies. One of the main reasons is that the interaction forces between colloidal particles suspended in a liquid are of the same nature as those operating in soap films. Because the film geometry is well defined (i.e., a thin, flat liquid sheet, macroscopic in lateral extension), it is an attractive experimental subject for studying these forces, in particular with optical means. 

The film eventually reaches an equilibrium thickness in which both faces of the film are parallel (Fig. 1.18(c)). In this state there is no variation in intensity over the surface of the film. This is called the common black film. It occurs, typically, at a thickness of 300A (30nm). A further decrease in the film thickness, to another stable equilibrium state with a thickness of about 50A, is often possible and is known as the Newton black film. This film is darker than the first black film, the common black film, as the second of the two split rays, that is refracted into the film, travels through a thinner soap film than in the case of the common black film. Consequently the phase difference between the two split rays of the Newton black film is closer to tt than in the case of the common black film. Some films have only one equilibrium state while others have two or more equilibrium states. 

In a controlled environment a soap film will thin until its thickness becomes appreciably less than the wavelength of light so that it appears black when viewed by reflected light. Commonly it is found that the film reaches a stable thickness, in thermodynamic equilibrium, of about 300 A. This is known as the common black film. If a small amount of evaporation is allowed to take place the film will often thin to a new equilibrium thickness of about 50 A. This is the Newton black film. The thickness of these two equilibrium states can be obtained by measuring the intensity of the reflected light by using photoelectric detectors. The application of Eq. (2.26) will enable the thickness of the film, t, to be determined. 

Fig. 2.9 The molecular structure of soap films (a) the Newton black film (b) a stratified film consisting of two bimolecular leaflets .

The two minima in the potential energy function, Eq. (2.40) and Fig. 2.13, are associated with the stable black films known as the common black film and the Newton black film. This behaviour is typical of a simple mobile film. However it is possible that a soap film could give rise to only one minimum in the potential energy function V. This will depend on the relative importance of the various contributions, in (2.40), to the total potential energy. In the case of a nonionic soap the double layer repulsion will be absent and will be re-... 

The schematic of a soap film disjoining pressure isotherm is shown in Figure 2.12. It is important to note that thermodynamically metastable films can exist only in regions with negative slopes. Hence, the region with positive slope separates metastable regions between the common black films ( 50 nm) and the Newton black films ( 4 nm). Basically, common black films stability is due to the electric double-layer forces, while the Newton black films should be the results of entropic forces. 

AU physicists know that Newton used soap bubbles in his admirable research on the colors of thin films. The experiments that he carried out by this means, and which are described in his Optics (year 1704), are too well known for me to point them out here I will insist only on the following points Newton employed, not complete bubbles, but film caps resting on liquid he observed the black spot at the top, the small colored spots which go up and go down on the cap, as well as the little black spots which climb up to the top spot, with which they merge he noted the appearance of the blue of the 1st order only with one solution very charged with soap, and, in this case, he sometimes saw the blue in question invading aU the cap finally, one can infer from his description that the uniformity of color, and consequently the uiuformity thickness of the film, were shown sometimes also for colors other than the blue of the 1st order. 

One case, that of a black film, is particularly important Soap films, which are thirmer than 30 nm appear black [718, 749, 750, 752]. Newton was the first to have reported on black films. Very thin films appear black because the light reflected from the backside is phase shifted by X/2 and the path length in the film is negligibly small. For this reason, the phase difference between the two rays is k/2 and they interfere destructively. No light is reflected and the film appears black when viewed against a black background. 

Upon the contact of a water drop with the flat water/soap solution in n-decane interface, a liquid hydrocarbon film was formed which is surrounded by Newton rings that may be seen in reflected light. Upon thinning, the film itself changes its color from white to grey and then to black. 

The films are thinner at the top than at the bottom. After a while, the upper part often ends up becoming extremely thin, reducing to two soap monolayers sandwiching just a few molecules of water. Such films no longer produce optical interference effects instead, they look quite black. Newton studied these black films. Long before, the Assyrians had observed them and even used their unpredictable shapes (in horizontal geometries) to divine the future. 

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