Foam film area

The stabihty of a single foam film can be explained by the Gibbs elasticity E which results from the reduction ia equiUbrium surface concentration of adsorbed surfactant molecules when the film is extended (15). This produces an iacrease ia equiUbrium surface tension that acts as a restoring force. The Gibbs elasticity is given by equation 1 where O is surface tension and is surface area of the film. 

The sturdy 16 x 20 foot communication outposf was set up inside the much larger film area. For added safety, the floor was covered with inch-thick foam rubber. The furnishings included two bunk beds, walkie-talkie, telephone and switchboard, radio-telephone, an event log, a small desk, a worktable, chairs and a draped-off chemical toilet. 

When the number and volume of the polyhedral compartments are given, the optimal structure of the foam is the one that creates the smallest total film area. This condition constitutes a formidable but straightforward mathematical optimization problem. Solution as an average, the polyhedra consist of 13.4 sides. Experimentally it was indeed found that the polyhedra most commonly found in foams have 14 sides, followed by 12 sides as a second choice. 

Equalising Eqs. (1.32) and (1.33) and accounting for Eq. (1.34), an expression is derived which indicates that excess (capillary) pressure in bubbles is determined by both specific foam surface area and solution surface tension but does not depend on the shape of films and bubbles... 

This method gained a significant improvement with the introduction of the contemporary infrared technique with a Fourier transformer (FT-IR), permitting to obtain measurable values of adsorption of the infrared light even from single black foam films. The thickness of the aqueous core is derived from the adsorption at 3400 cm 1 which is related to the OH stretching vibration of the water molecules. Umemura et al. [114] have employed the polarised Fourier transformed infrared spectra for the study of the water content of NaDoS black films. The cell used to form films of ca. 2 cm2 area is illustrated in Fig. 2.19. By fitting the calculated curved of polarised FT-IR spectra to the respective experimentally obtained... 

Black foam films appear in thermodynamically non-equilibrium films in the form of black spots (see Section 3.2.2.2). These clearly expressed thin regions (but not holes as named in [e.g. 35,381]) expand, fuse and occupy the whole film area. Thus, CBF and NBF reach an equilibrium state. This process can be most distinctly observed in microscopic foam films (see Fig. 3.14). 

Data on emulsion film formation from insoluble surfactant monolayer are rather poor. It is known, however, that such films can be obtained when a bubble is blown at the surface of insoluble monolayers on an aqueous substrate [391,392]. Richter, Platikanov and Kretzschmar [393] have developed a technique for formation of black foam films which involves blowing a bubble at the interface of controlled monolayer (see Chapter 2). Experiments performed with monolayers from DL-Py-dipalmitoyl-lecithin on 510 3 mol dm 3 NaCl aqueous solution at 22°C gave two important results. Firstly, it was established that foam films, including black films, with a sufficiently long lifetime, formed only when the monolayer of lecithin molecules had penetrated into the bubble surface as well, i.e. there are monolayers at both film surfaces on the contrary a monolayer, however dense, formed only at one of the film surfaces could not stabilize it alone and the film ruptured at the instant of its formation. Secondly, relatively stable black films formed at rather high surface pressures of the monolayer at area less than 53A2 per molecule, i.e. the monolayer should be close-packed, which corresponds to the situation in black films stabilized with soluble surfactants. 

The molecular model of amphiphile bilayers with holes in them is a good basis also for the description of the rupture of NB foam films by a-particle irradiation [331,415,416]. The mean lifetime ra of the foam bilayer shortens dramatically under irradiation, and probability considerations [416] show that only a small area Sh S of the bilayer is active for the passage of the a-particles. Assuming that 5 is the overall area of those holes in the foam bilayer which are large enough to be irradiation-active makes it possible to represent Sh as... 

The main features of this technique are the absence of contact between the foam and the ambient space (i.e. no foam/gas interface) and constant capillary pressure along the whole foam height. This technique allows to study the kinetics of internal foam destruction at various capillary pressures, i.e. decrease in the specific foam surface area without destruction of the foam column. Thus, the influence of surface foam films on foam lifetime and the character of foam destruction can be estimated. 

A conclusion has been drawn in [67] that the extinction of the luminous flux (7//o, where, 7o is the intensity of the incident light and 7 is the intensity the light passed through the foam) is a linear function of the specific foam surface area. A similar dependence has been used also for the determination of the specific surface area of emulsions [68]. Later, however, it has been shown [69,70] that the quantity 7//o depends not only on the specific surface area (or dispersity) but also on the liquid content in the foams, i.e. on the foam expansion ratio, that during drainage can increase without changing the dispersity. Since foam expansion ratio and dispersity are determined by the radii of border curvatures and film thicknesses, all the structural elements of the foam will contribute to the optical density of foams. This means that... 

Figures 5.37 and 5.38 show the critical thicknesses of rupture, Rp for foam and emulsion films, respectively, plotted vs. the film radius." In both cases the film phase is the aqueous phase, which contains 4.3 x 10 M SDS + added NaCl. The emulsion film is formed between two toluene drops. Curve 1 is the prediction of a simpler theory, which identifies the critical thickness with the transitional one." Curve 2 is the theoretical prediction of Equations 5.270 to 5.272 (no adjustable parameters) in Equation 5.171 for the Hamaker constant the electromagnetic retardation effect has also been taken into account. In addition, Eigure 5.39 shows the experimental dependence of the critical thickness vs. the concentration of surfactant (dodecanol) for aniline films. Figures 5.37 to 5.39 demonstrate that when the film area increases and/or fhe electrolyte concentration decreases the critical film thickness becomes larger.

Factors such as micellar concentration, micelle size, chain lengths, film area, electrolyte concentration, temperature, presence of solubilized oil on the micellar film structuring, and thus foam stability are highlighted. 

An oscillation of the foam film can be performed such that the film thickness changes with the area, or can be even kept constant by well-adjusted simultaneous oscillations of... 

The ability to control foam stability or coalescence rate of bubbles is important in many industrial applications. Foams can persist for a few minutes to several days depending on storage conditions. To effectively utilize foams in any of these situations, it is important to have some control over their stability. Therefore it is very important to deepen our understanding of the mechanisms involved in foam persistence and decay. Most works in this area have been rather empirical and many experimental data are rendered useless because important parameters such as bubble size have not been measured. In this chapter we attempt to summarize the quantitative analysis on foam film stability in aqueous systems in terms of surface tension measurements. 

The optical properties of single foam films have been extensively studied, but those of the foam as disperse system are poorly considered. The extinction of luminous flux 1/ lo, where / is the intensity of the light passing through the foam and lo is the intensity of the incident light) is conduded to be a linear function of the specific foam area. This could be used to determine the specific surface area of a foam. 

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