Theory of Bubble and Foam Formation

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. 

Not only the equilibrium sruface tension but also the kinetic properties of a surfactant adsorption monolayer play an important role in various phenomena related to the stability of foams and emulsions [5,30], rising of bubbles and flotation [31]. Indeed, many processes are accompanied by disturbances (expansion, compression) of the adsorption monolayer or by formation of new surface of the solution. The surfactant solution has the property to damp the disturbances by diffusion of the surfactant from the bulk to the interface, or vice versa. The main subject of this section is the theory of adsorption and surface tension under such dynamic conditions. 

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]. 

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. 

Abstract. The stability of suspensions/emulsions is under consideration. Traditionally consideration of colloidal systems is based on inclusion only Van-der-Waals (or dispersion) and electrostatic components, which is refereed to as DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. It is shown that not only DLVO components but also other types of the inter-particle forces may play an important role in the stability and colloidal systems. Those contributions are due to hydrodynamic interactions, hydration and hydrophobic forces, steric and depletion forced, oscillatory structural forces. The hydrodynamic and colloidal interactions between drops and bubbles emulsions and foams are even more complex (as compared to that of suspensions of solid particles) due to the fluidity and deformability of those colloidal objects. The latter two features and thin film formation between the colliding particles have a great impact on the hydrodynamic interactions, the magnitude of the disjoining pressure and on the dynamic and thermodynamic stability of such colloidal systems. 

For a liquid to form a foam, it must be able to form a membrane around the gas bubble possessing a form of elasticity that opposes the thinning of the lamellae as a result of drainage. Foaming does not occur in pure liquids because no such mechanism for the retardation of lamellae drainage or interfacial stabilization exists. When amphiphilic materials or polymers are present, however, their adsorption at the gas-liquid interface serves to retard the loss of liquid from the lamellae and, in some instances, to produce a more mechanically stable system. Theories related to such film formation and persistence, especially film elasticity, derive from a number of experimental observations about the surface tension of liquids ... 

---