Vertical Foam Films

Pure liquid films are highly unstable. Surfactants are added to increase their stability. Traditionally, the main surfactants used were soaps. Soaps are salts of fatty acids, which contain at least eight carbon atoms. Therefore, such films are sometimes called soap films. The more general term is foam film. Still, one should keep in mind that liquid films are always metastable at best and will eventually mpture. Here, we focus on aqueous films. 

Vertical foam films have a preferred direction due to gravitation. We focus on freestanding aqueous films since most of the experimental work has been done in that area to better understand the stability of foams. 

1) Robert Hooke, 1635-1703. English physicist, mathematician, biologist, and inventor. 

2) Sir Isaac Newton, 1643-1727. English mathematician, physicist, and astronomer. Founder of mechanics and geometrical optics. 

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. 

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When a vertical foam film is illuminated, part of the light is scattered by thermal fluctuational microwaves in the film surface. Measuring the intensity of the scattered light makes possible the calculation of the film tension and the energy of molecular interactions in the film [89,90]. 

Lyklema and Mysels [168] have studied equilibrium macroscopic vertical foam films from sodium octylsulphate aqueous solution containing KC1. The film thicknesses comprised a large range from 80 to 8 nm and electrolyte concentrations from 103 to 1 mol dm3 (Fig. 3.18). 

Additional data about the structure of black films are obtained by X-ray diffraction method. The first steps [336,338] have been performed with vertical foam films in a frame in a horizontal scanning diffractometer. Black films from decyltrimethyl ammonium decyl sulphate and NaBr solutions have been studied. The film thickness was calculated using a model of the mean electron density projection on the film normal. However, there was no indication whether the films were CBF or NBF. Platikanov et al. [339,340] used a new device for investigation of a horizontal black films from aqueous NaDoS solution (see Section 2.2.6). They found essentially different X-ray diffraction traces for the three types of black films CBF, NBF and stratified black films. This indicates their different structure. Precise X-ray reflectivity measurements with CBF and NBF films from NaDoS and NaCl aqueous solutions [341-343] provided more details about their structure. The data obtained for the thicknesses of the respective layers which detail the film structure are given below... 

Figure 15. Color interference bands in vertical foam film formed from a latex suspension with particle diameters of 150 nm.

The influence of surface-shear viscosity on the drainage of vertical foam film is also investigated in four different model foam films as shown in Table 2 [161]. The (dyt/dOmax values for these four systems were adjusted in the range... 

A related phenomenon to asymmetric drainage in horizontal cylindrical foam films is that of the so-called marginal regeneration. This process occurs in vertical foam films subject to both capillary suction from the Plateau borders and gravity. It is manifest as an apparent turbulent motion on the margin of foam films where thin elements of film are drawn out of the Plateau border and thick elements are sucked... 

It should be clear then that asymmetric drainage of horizontal cylindrical films and marginal regeneration in vertical foam films, although often apparent, are not always exhibited. Liquid flow by capillary suction from the Plateau border is. 

FIGURE 2.11 Schematic of example of a cell for study of stability and rate of formation of vertical foam films. (Reprinted from Colloid Surf. A, 307, Garrett, P.R., Wicks, S.P., Fowles, E., 282-283. Copyright 2006, with permission from Elsevier.)... 

This polyethoxy-polypropoxy block copolymer is more snrface active than the protein and radically alters the vertical foam film drainage behavior of solutions of the latter. The extremely slow rigid film drainage behavior (presumably with suppressed marginal regeneration) of the BSA solntions was shown by Nemeth et al. [132] to be replaced by relatively rapid, apparently mobile, drainage behavior in the presence of copolymer. These vertical films were, however, unstable with lifetimes of the order of a... 

FIG. 23 Aqueous core thickness ( 2) of vertical foam films for 1 mM CmE8 m= 10, 12, 14, and 16) solutions as a function of time at different temperatures. 

The process of hole formation and expansion inside the foam films was directly observed with larger vertical foam films (2X3 cm ) by using a high-speed video camera [11]. Solutions of 10 mM AOT and siUca-PDMS compound were used in these experiments. The results unambiguously demonstrated that the fast antifoams ruptured the foam films (not the GPBs). 

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