Bubbles in foam

Sphere, flow across, 15 72 It Sphere-of-influence (SOI), 19 355-356, 358 Spherical bubbles, in foams, 12 7-8 Spherical fillers, phenolic resin,... 

A foam is a colloidal dispersion in which a gas is dispersed in a continuous liquid phase. The dispersed phase is sometimes referred to as the internal (disperse) phase, and the continuous phase as the external phase. Despite the fact that the bubbles in persistent foams are polyhedral and not spherical, it is nevertheless conventional to refer to the diameters of gas bubbles in foams as if they were spherical. In practical occurrences of foams, the bubble sizes usually exceed the classical size limit given above, as may the thin liquid film thicknesses. In fact, foam bubbles usually have diameters greater than 10 pm and may be larger than 1000 pm. Foam stability is not necessarily a function of drop size, although there may be an optimum size for an individual foam type. It is common but almost always inappropriate to characterize a foam in terms of a given bubble size since there is inevitably a size distribution. This is usually represented by a histogram of sizes, or, if there are sufficient data, a distribution function. 

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. 

Geometrical shape of gas bubbles in foam depends on the ratio of gas and liquid volumes, on the degree of polydispersity and on bubble packing. The results discussed below apply also for concentrated emulsions (considering density and interfacial tension). 

As it is well known, the contacts between drops (in emulsions), solid particles (in suspensions) and gas bubbles (in foams) are accomplished by films of different thickness. These films, as already discussed, can thin, reaching very small thickness. Observed under a microscope these films reflect very little light and appear black when their thickness is below 20 nm. Therefore, they can be called nano foam films. IUPAC nomenclature (1994) distinguishes two equilibrium states of black films common black films (CBF) and Newton black films (NBF). It will be shown that there is a pronounced transition between them, i.e. CBFs can transform into NBFs (or the reverse). The latter are bilayer formations without a free aqueous core between the two layers of surfactant molecules. Thus, the contact between droplets, particles and bubbles in disperse systems can be achieved by bilayers from amphiphile molecules. 

The action of nucleating agents in forming small bubbles in foam formation may be likened to the use of "boiling chips" added to... 

There are three scenarios for the occurrence of a two-particle collision in a dispersion depending on the type of particle-particle interactions. (1) If the repulsive forces are predominant, the two colliding particles will rebound and the colloidal dispersion will be stable. (2) When at a given separation the attractive and repulsive forces counterbalance each other (the film formed upon particle collision is stable), aggregates or floes of attached particles can appear. (3) When the particles are fluid and the attractive interaction across the film is predominant, the film is unstable and ruptures this leads to coalescence of the drops in emulsions or of the bubbles in foams. 

A complicated many-layer structure of foam cells is formed when gas bubbles are sparged into solutions of surfactants. According to [429], each bubble has a two-sided envelope which is a layer of the solvent containing hydrophilic polar parts of surfactant molecules (see Figure 7.2). Nonpolar hydrophobic parts of molecules on the inner surface of the envelope are oriented toward the bubble, and on the outer surface, outward the envelope. Between two cells, each of which is a capsule with envelope, there is a lamella, that is, an interlayer of a complicated structure. In the middle of the lamella, there is a liquid layer that is a continuous phase. On each of two surfaces of this layer, there is a monolayer of the surfactant. The hydrophobic parts ( tails ) of surfactants molecules in each monolayer and the tails of the envelope form two direct plate micellae [413], which separate the envelope and the continuous liquid film at the center. Thus, gas bubbles in foam are separated at least by five distinct layers. The multilayer structure of a foam lamella is well seen in photographs (e.g., see [429], p. 54). This fact is also confirmed by the ladder-type shape of the disjoining pressure... 

Matzke, E. B., The three-dimensional shape of bubbles in foam-an analysis of the role of surface forces in three-dimensional cell shape determination, Amer. J. Botany, Vol. 33, No. 1, pp. 58-80, 1946. 

Coalescenceis especially typical in concentrated emulsions. In such systems coalescence mainly determines the lifetime of emulsions prior to phase separation. In finely dispersed emulsions, both dilute and concentrated, the average size of drops may noticeably increase due to Ostwald ripening. At the same level of dispersion Ostwald ripening of emulsion droplets is a slower process than mass transfer of bubbles in foams [60]. This is due to a rather low interfacial energy, and consequently, low difference in chemical potentials of substance in droplets of different size, as well as due to a lower mutual solubility of liquids as compared to the solubility of gases in liquids. 

Two problems are inherent in many technologies. The interfaces involved are freshly formed and have an age of only seconds, sometimes even less than a millisecond. On the other hand the movement of interfaces, for example of drops in emulsions or bubbles in foams, disturbs the equilibrium of their adsorption layers. 

Ice crystals increase the viscosity of ice cream because they are solid particles suspended in the matrix cf. equation 2.11). However, because the ice crystals can interact during flow (since their volume fraction is quite high) and because they are not spherical, the viscosity is greater than equation 2.11 predicts. Similarly, fat droplets in emulsions and gas bubbles in foams with a relatively low gas-phase volume can also be... 

EDLs can be present at the gas-liquid interfaces between bubbles in foams. 

This gives the variation of P that passes through a maximum that occurs when a = 7 . Such a curve (Figure 7.6) is verified experimentally [21]. Inflation of balloons is only one example of elastic instability. This phenomenon also occurs in the generation of bubbles in foam production and the development of aneurisms in arteries in the human body. 

Finally, based on the Ostwald equation, the stability of a liquid in liquid anulsion is compared with that of a foam. As anile, the density in the gas phase is much lower than that in the liquid phase and it then follows from Equations 6.21 and 6.22 that droplets of a liquid in another liquid can be much smaller than air bubbles in a liquid. Emulsion droplets have dimensions typically in the range of some tenths of micrometers up to a few micrometers, whereas air bubbles in foams are usually in the millimeter range. 

The mechanism by which bubbles in foam are broken by at least partly hydrophobic silica particles is discussed in Chapter 4. Finely divided silica aggregates, made suffi ciently hydrophobic to be suspendable in a hydrocarbon or silicone oil, probably should retain some hydrophilic areas to be held at the bubble surface. The silica is treated with a silicone oil and heated to 245 C to react it with the surface, then suspended in an oil (634). A surfactant in the oil may be included (635). 

Thin liquid films can be formed between two coUiding emulsion droplets or between the bubbles in foam. Formation of thin films accompanies the particle-particle and particle-wall interactions in colloids. From a mathematical viewpoint, a film is thin when its thickness is much smaller than its lateral dimension. From a physical viewpoint, a liquid film formed between two macroscopic phases is thin when the energy of interaction between the two phases across the film is not negligible. The specific forces causing the interactions in a thin liquid film are called surface forces. Repulsive surface forces stabilize thin films and dispersions, whereas attractive surface forces cause film rupture and coagulation. This section is devoted to the macroscopic (hydrostatic and thermodynamic) theory of thin films, while the molecular theory of surface forces is reviewed in Section 4.4. 

Rheological properties of foams (elasticity, plasticity, and viscosity) play an important role in foam production, transportation, and applications. In the absence of external stress, the bubbles in foams are symmetrical and the tensions of the formed foam films are balanced inside the foam and close to the walls of the vessel [929], At low external shear stresses, the bubbles deform and the deformations of the thin liquid films between them create elastic shear stresses. At a sufficiently large applied shear stress, the foam begins to flow. This stress is called the yield stress, Tq- Then, Equation 4.326 has to be replaced with the Bingham plastic model [930] ... 

The interactions across a thin film, called the surface forces, to a great extent are determined by the surfactant adsorption at the film surfaces. From a physical viewpoint, a liquid film formed between two phases is termed thin when the interaction of the phases across the film is not negligible. Thin films appear between the bubbles in foams, between the droplets in emulsions, as well as between the particles in suspensions. Furthermore, the properties of the thin liquid film determine the stability of various colloids. 

---