Foam film

In an interesting medical application, the formation of a stable black foam film from amniotic fluid can be used as an assessment of fetal lung maturity [206]. 

Fig. XIV-16. A photomicrograph of a two-dimensional foam of a commercial ethox-ylated alcohol nonionic surfactant solution containing emulsified octane in which the oil drops have drained from the foam films into the Plateau borders. (From Ref. 234.)...

Ion Flotation and Foam Separation. Ions and dissolved surfactant molecules can be removed from solutions by the agency of foam. In this case ions are sandwiched in foam films. The scientific basis of these processes is weU understood and successes of metal ion recovery from solutions including U, Pt, Au, as weU as different surfactants (detergents) have been reported in the Hterature. 

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. 

Both high bulk and surface shear viscosity delay film thinning and stretching deformations that precede bubble bursting. The development of ordered stmctures in the surface region can also have a stabilizing effect. Liquid crystalline phases in foam films enhance stabiUty (18). In water-surfactant-fatty alcohol systems the alcohol components may serve as a foam stabilizer or a foam breaker depending on concentration (18). 

AH these mechanisms except high bulk viscosity require a stabilizer in the surface layers of foam films. Accordingly, most theories of antifoaming are based on the replacement or modification of these surface-active stabilizers. This requires defoamers to be yet more surface active most antifoam oils have surface tensions in the 20 to 30 mN/m range whereas most organic surfactant solutions and other aqueous foaming media have surface tensions between 30 and 50 mN/m(= dyn/cm). This is illustrated in Table 3. 

One useful method of aqueous defoaming is to add a nonfoam sta-bihzing surfac tant which is more surface-active than the stabilizing substance in the foam. Thus a foam stabilized with an ionic surfactant can be broken by the addition of a very surface-active but nonstabihzing sihcone oil. The sihcone displaces the foam stabilizer from the interface by virtue of its insolubility. However, it does not stabilize the foam because its foam films have poor elasticity and rupture easily. 

If, by one of the above procedures, a few or even many bubbles have been introduced into a liquid, there is still no foam. In a foam, films between the bubbles are thin otherwise, the system is a gas emulsion. How, then, can a true foam be achieved If it is assumed that, because of some kind of stirring, two bubbles move to meet each other and the liquid layer between them gets thinner and thinner and if this process continues for a sufficient time, the two bubbles will touch and, eventually, coalesce. Many such encounters would destroy the foam. It is clear, therefore, that bubbles should be free to approach each other closely, but should be unable to cross the last short fraction of the initial distance. 

V. Bergeron and C.J. Radke Equilibrium Measurements of Oscillatory Disjoining Pressures in Aqueous Foam Films. Langmuir 8, 3020 (1992). 

LB. Ivanov, A.S. Dimitrov, A.D. Nikolov, N.D. Denkov, and P.A. Kralchevsky Contact Angle Film and Line Tension of Foam Films. II. Film and Line Tension Measurements. J. Colloid Interface Sci. 151, 446 (1992). 

V. Bergeron Disjoining Pressures and Eihn Stabflity of Alkyltrimethylammonium Bromide Foam Films. Langmuir 13, 3474 (1997). 

The production of foamed films and sheets from polypropylene and polystyrene is discussed, with particular reference to packaging applications. Advantages of foamed materials for this application are examined, and the chemical and physical foaming processes are described. Extrusion technology for film and sheet by chemical and physical foaming processes is discussed, and recent developments in the coextrusion of multilayer packaging trays for the food industry are considered. 

Espoo, Technical Research of Finland, 1998, pp.l03. 10 ins. 15/4/99. VTT Publn. 361 NEW TECHNOLOGY TO MANUFACTURE POLYPROPYLENE FOAM SHEET AND BIAXIALLY ORIENTED FOAM FILM Raukola J 1... 

Chicago, II., 29th Aug-2nd Sept.l993,p.403-6. 6A MULTILAYER EXTRUSION DIE GEOMETRY FOR THERMOFORMED POLYPROPYLENE FOAM SHEETS AND BIAXIALLY ORIENTED FOAM FILMS Raukola J Savolainen A Tampere,University of Technology (TAPPI)... 

FIGURE 8.3 Thickness of foam films and added ions. 

Measurements have been carried out on the excess tensions, equilibrium thicknesses, and compositions of aqueous foam films stabilized by either n-decyl methyl sulfoxide or n-decyl trimethyl ammonium-decyl sulfate, and containing inorganic electrolytes. 

Surface viscosity is reduced (thus destabilizing the foam films). 

Nikolov AD, Wasan DT, Denkov ND, Kralchevsky PA, Ivanov IB (1990) Drainage of foam films in the presence of nonionic micelles. Prog Colloid Polym Sci 82 87-98... 

Figure 7. Photomicrographs on the various drainage stages of a foam film containing 2 wt% sodium caseinate, at 40 °C. Film diameter 0.35 mm.

The structure and stability of foamed emulsions, such as whipped cream, ice cream or whipped toppings, strongly depend on the interparticle interactions and on the orientation of drops/particles at the foam films. Further development of the surface force balance and... 

One of the central questions in the stability of foams is why are liquid films between two adjacent bubbles stable, at least for some time In fact, a film of a pure liquid is not stable at all and will rupture immediately. Formally this can be attributed to the van der Waals attraction between the two gas phases across the liquid. As for emulsions, surfactant has to be added to stabilize a liquid film. The surfactant adsorbs to the two surfaces and reduces the surface tension. The main effect, however, is that the surfactant has to cause a repulsive force between the two parallel gas-liquid interfaces. Different interactions can stabilize foam films [570], For example, if we take an ionic surfactant, the electrostatic double-layer repulsion will have a stabilizing effect. 

The interaction between the two gas-liquid interfaces across a foam film can directly be measured by a thin-film balance (TFB) [571-573], A single thin foam film is formed in a hole drilled through a porous glass plate (Fig. 12.19). The plate, and therefore the liquid in the foam, is connected to a reservoir with a constant reference pressure Pr by a capillary tube. The film holder is placed into a closed cell. A constant pressure Pg is applied to the gas in the... 

Edited by T. Kajiyama and M. Aizawa Vol. 5 Foam and Foam Films. By D. Exerowa and P.M. Kruglyakov Vol. 6 Drops and Bubbles in Interfacial Research. Edited by D. Mobius and R. Miller Vol. 7 Proteins at Liquid Interfaces. Edited by D. Mobius and R. Miller Vol. 8 Dynamic Surface Tensiometry in Medicine. By V.M. Kazakov, O.V. Sinyachenko,... 

Mileva, E. Exerowa, D. Foam films as instrumentation in the study of amphiphile self-assembly. Adv. Colloid Interface Sci. 100-102, 547-562 (2003). 

Electric double layers can be present at the gas/liquid interfaces between bubbles in foams. In this case, since the interfaces on each side of the thin film are equivalent, any interfacial charge will be equally carried on each side of the film. If a foam film is stabilized by ionic surfactants then their presence at the interfaces will induce a repulsive force opposing the thinning process. The magnitude of the force will depend on the charge density and the film thickness. 

Figure 5.8 shows a hypothetical jt(h) isotherm for a foam film, illustrating the primary and secondary minima. A slightly more complicated isotherm is shown in Figure 5.15. 

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