Theories of foam stability

Whilst there is no single theory that can explain foam stability in a satisfactory maimer, several approaches have been considered and these are summarised below. 

In the film-near region, the liquid flows upwards, while near the frame the liquid flows downward. In between, partides show no or only restricted, more or less dr-cular, motion [9]. 

The above regeneration mechanism results in the formation of patches of thin film at the border, with the excess fluid flowing into the border channel. The edge effects determine the drainage, with the rate of thinning varying inversely with film width [7, 8]. This results in thickness fluctuations caused by capillary waves. 

Marginal regeneration is probably the most important cause of drainage in vertical films with mobile surfaces, i.e. with surfactant solutions at concentrations above the c.m.c. 

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The basic theories of foam stability have been discussed in detail in the first two chapters of this book from the perspective of aqueous foams. Much of what has been said there applies equally to nonpolar foams. However, some important differences exist between the two types of foams, and the following discussion will attempt to clarify these differences. 

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

Muller et. al. [421] have studied the behaviour of emulsion Newton bilayer films and compared it to that of foam films. They determined the dependence of the lifetime on surfactant concentration of emulsion films stabilised with 22-oxythylated dodecyl alcohol (see Section 3.4.1). Experimental data for both kinds of films proved to be in conformity with the theory of bilayer stability (see Section 3.4). The values of the equilibrium concentrations Ce calculated for emulsion films were higher (Ce 10 3 mol dm 3) than those for foam films (Ce 3 1 O 5 mol dm 3). It is worth noting that Ce value of foam films from certain surfactants is lower than CMC (C < CMC) while for emulsion films - Ce > CMC. That is why it is impossible to obtain thermodynamically stable films in the latter case. This result is of particular importance for the estimation of stability of aqueous emulsions with bilayer films between the drops of the organic liquid. 

Let us summarise the conditions of formation of a microscopic foam film in order to serve the in vivo situation. These are film radius r from 100 to 400 pm capillary pressure pa = 0.3 - 2.5-102 Pa electrolyte (NaCl) concentration Ce 0.1 mol dm 3, ensuring formation of black films (see Section 3.4) and close to the physiological electrolyte concentration sufficient time for surfactant adsorption at both film surfaces. Under such conditions it is possible also to study the suitable dependences for foam films and to use parameters related to formation and stability of black foam films, including bilayer films (see Section 3.4.4). For example, the threshold concentration C, is a very important parameter to characterise stability and is based on the hole-nucleation theory of bilayer stability of Kashchiev-Exerowa. As discussed in Section 3.4.4, the main reason for the stability of amphiphile bilayers are the short-range interactions between the first neighbour molecules in lateral and normal direction with respect to the film plane. The binding energy Q of a lipid molecule in the foam bilayer has been estimated in Section 11.2. 

Foams play an important role in several fields of human Hfe in food technology, medicine, cosmetics, oceanography, environmental technology, fire extinguishing technology, etc. Therefore, foams were investigated very early by natural scientists. Foam films and Plateau borders were characterized, and the formation and structure of foams were described. The mechanical, optical and electrical properties of foams and theories for foam stability were presented [1]. 

The mode of action of defoamers and antifoams can be understood on the basis of the theory of foaming, which is described extensively elsewhere [62-64]. The main factor of foam stabilization is the formation of a coherent surfactant layer that covers the air-liquid interfaces (Fig. 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. 

Likewise, the practical food foreman knows that by following certain manufacturer s recommendations and certain processing conditions in his plant, he is able to produce stabilized foam products to the satisfaction of his superiors and the public, most of the time yet when problems of instability and poor shelf life of the finished product are brought to his attention and all simple adjustments fail to produce a satisfactory result, he must turn to the food or colloid chemist for the theory and industrial application of foams. 

Figure 10. Comparison of the critical-capillary-pressure data of Khatib, Hirasaki and Falls (5) (darkened circles) to the proposed dynamic foam stability theory (solid line). Best fitting parameters for the constant-charge electrostatic model are listed.

For transporting foam, the critical capillary pressure is reduced as lamellae thin under the influence of both capillary suction and stretching by the pore walls. For a given gas superficial velocity, foam cannot exist if the capillary pressure and the pore-body to pore-throat radii ratio exceed a critical value. The dynamic foam stability theory introduced here proves to be in good agreement with direct measurements of the critical capillary pressure in high permeability sandpacks. 

The best fit between experimental results and theory is achieved when both the change in hydrostatic pressure along the height of the forming bubble at the moment of its detachment from the capillary orifice and the expansion of bubble during its rising are taken into account. Surface tension and density of foaming solution (see Eq. (1.9)) determine the size of bubbles when they are formed slowly. The surfactant kind and concentration affect both the rate of formation of adsorption layers at bubble surface and the stability of foam obtained. 

Details about the theory of stability of thin liquid films, including foam films, can be found in some monographs [3-6]. However, the literature reflecting the theory of black foam films is rather poor. For this reason it will be granted special attention here. The new theoretical and experimental results accumulated during the recent years have brought nearer... 

The main trends of the study of surface forces in foam films are briefly outlined here and the results obtained will be successively discussed in the next Sections. Furthermore, some earlier considerations will be commented, for instance, the first quantitative experimental verification of the DLVO-theory with the aid of foam films, since these results form the base of the further achievements in measurement and interpretation of surface forces and their role in the stability of foam films. 

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