Foam films spherical

The spherical foam films can be obtained by blowing a bubble from a vertical capillary tube. The principle of formation of such a bubble is illustrated in Fig. 2.22. A vertical capillary tube is placed in a vessel with the surfactant solution so that its upper orifice is close to the solution surface. When a gas (air) with a definite pressure is introduced into the tube over the solution surface a foam film is formed acquiring the shape of a hemisphere. 

The same principle of formation of a spherical foam film is used in the device (Fig. 2.23) for direct measurement of the film tension y in static [126] and dynamic [127] conditions. The lower part of the capillary is placed in a closed space filled up with a gas, whose pressure can be measured precisely and by means of a special pump can be varied at different rates. The manometer registers the difference with the atmospheric pressure Ap, which is equal to the capillary pressure pa of the spherical foam film... 

Fig. 2.25 illustrates a device for the study of the deformation of a spherical foam film in an electric field, proposed in [130]. The surfactant solution is fed into the glass cell through the tube 2, so that its level reaches the porous plate 4. When air is blown through the tube 3, a foam bubble forms at the capillary orifice. An electric field is created between electrode 1 and 6, which deforms the foam film (bubble). The bubble transforms from spherical to ellipsoidal shape. The value of the deformation A/ depends on the surface tension of the film... 

The existence of ACg at constant temperature means that there is a difference Ap in the pressures on either side of the films, i.e. the film is deformed and the capillary pressure due to this curvature equals exactly the difference Ap. Because of their small thickness, the foam films have a negligible mass and the shape of the deformed film is practically spherical. 

A spherical foam film can be formed at the top of a foam bubble floating freely at the solution surface during the process of film thinning. Under gravity such a microscopic bubble,... 

Fig. 9.7. Effect of a solid hydrophobic spherical particle on the behaviour of a foam film (a) ...

FIG U RE 4.43 Foam film rupture caused by spherical particle with contact angle > 90°... 

FIGURE 4.64 Effect of rugosities on total capillary pressure in film formed between particle and air-water surface of a foam film subject to Plateau border suetion, p . (a) Smooth spherical particle of radius (b) Spherical particle also of radius but with rugosities of radius r, where r. r.B. 

FIGURE 5.7 Schematic illustration of excluded volume effect on movement of spherical antifoam entity in a draining plane-parallel foam film (of 100 microns radius) with immobile surfaces. Film would exhibit drainage according to Reynolds equation (Equation 5.20) with characteristic parabolic velocity profile. Asymmetry of shear forces means that particles will also rotate except when equidistant from each surface of film. 

Some authors [348,349] suggested that a possible explanation of the phenomenon can be the formation of a surfactant lamella liquid-crystal structure inside the film. Such lamellar micelles are observed to form in surfactant solutions, however, at concentrations much higher than those used in the experiments with stratifying films. The latter fact makes the explanation with a lamella liquid crystal problematic. Nikolov et al. [280,350,351] observed stratification not only with micellar surfactant solutions but also with suspensions of latex particles of micellar size. The stepwise changes in the film thickness were approximately equal to the diameter of the spherical particles contained in the foam film [280—282,352]. The experimental observations show that stratification is always observed. 

A foam is defined as a coarse dispersion of a gas in a liquid, where the volume fraction of gas is greater than that of the liquid. Solid foams (for example foam rubber or polystyrene foam) are also possible, but here we focus on more common liquid foams. These are always formed by mixtures of liquids (usually containing a soap or surfactant) and never by a pure liquid. If the volume fraction of gas is not too high, the bubbles in the foam are spherical, but at higher gas volume fractions the domains are deformed into polyhedral cells, separated by thin films of liquid (Fig. 3.12). Typically the gas bubbles are between 0.1 and 3 mm in diameter. 

In a wet foam, nearly spherical bubbles press against each other, creating small circular facets separated by thin liquid films, with both elastic and viscous interactions. Elastic forces in a wet foam are associated with the potential energy stored... 

Structural forces have meanwhile been directly measured in foam films using the thin film balance and caused by surfactant micelles [779] or polyelectrolytes [795, 796]. In fact, stratification in foam film in the presence of high surfactant concentrations has been observed much earlier [752] and is a common phenomenon. Structural forces have also been observed between partides induced by addition of polymer [1431,1444,1445], induced by surfactant micelles [1306,1443,1446], or in dispersions with smaller spherical particles [1447]. Measurements have been carried out by the surface force apparatus [1443], atomic force microscopy [1444,1445], total internal reflection microscopy [1431, 1446], and optical tweezers [1447]. 

A foam can be considered as a type of emulsion in which the inner phase is a gas, and as with emulsions, it seems necessary to have some surfactant component present to give stability. The resemblance is particularly close in the case of foams consisting of nearly spherical bubbles separated by rather thick liquid films such foams have been given the name kugelschaum by Manegold [175]. 

Fig. 3. Two-dimensional schematic illustrating the distribution of Hquid between the Plateau borders and the films separating three adjacent gas bubbles. The radius of curvature r of the interface at the Plateau border depends on the Hquid content and the competition between surface tension and interfacial forces, (a) Flat films and highly curved borders occur for dry foams with strong interfacial forces, (b) Nearly spherical bubbles occur for wet foams where...

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