Three-phase foam thinning, film

Our objective in this study is to elucidate the complex phenomena occurring during the process of three phase foam thinning, to identify the interaction mechanisms between the oil droplets, the thinning foam film and the Plateau-Gibbs borders and the role of surface and interfacial tension gradients in foam stability, and to examine the implications upon crude oil displacement by foam in pourous media. 

During the process of three phase foam thinning, three distinct films may occur foam films (water film between air bubbles), emulsion films (water between oil droplets) and pseudoemulsion films (water film between air and oil droplets) (Figure 1). To study the behavior of these films and particularly the oil droplet-droplet, oil droplet-air bubble and oil droplet-foam frame interactions it is necessary to utilize numerous microscopic techniques, including transmitted light, microinterferometric, differential interferometric and cinemicrographic microscopy. 

In such concentrated disperse systems three types of liquid films form foam films (G/L/G), water-emulsion films (O/W/O) and non-symmetric films (O/W/G). The kinetics of thinning of these films, their permeability as well as the energy barrier impeding the film rupture determine the stability of these systems. They might be subjected to internal collapse, i.e. coalescence of bubbles or droplets and increase in their average size, or to destruction as a whole, i.e. separation into their initial phases - gas, oil and water. 

The thin liquid films formed in foams or emulsions exist in permanent contact with the bulk liquid in the Plateau border, encircling the film. From a macroscopic viewpoint, the boundary between the film and Plateau border is treated as a three-phase contact line the line, at which the two surfaces of the Plateau border (the two concave menisci sketched in Fig. 16), intersect at the plane of the film—see the right-hand side of Fig. 16. The angle, 0o, subtended between the two meniscus surfaces represents the thin-film contact angle. 

At this point, the concept of the linear collapsed Plateau border is introduced. The Plateau border is the area of bulk continuous phase between three adjacent droplets or cells in an emulsion or foam respectively. The collapsed border is, therefore, an extremely thin version, which can be represented macro-scopically, as the line of intersection of three films of zero thickness, at angles of 120°. 

In concentrated systems with highly mobile interfaces (foams and emulsions) capillary phenomena of the first kind, related to the surface curvature in regions of film - macroscopic phase contact or in the regions where three films come into contact, may play a significant role in the energy and dynamics of film thinning. As shown in Fig. VII-2, a concave surface is formed in these types of regions. Under this surface the pressure is lowered by the amount equal to capillary pressure (see Chapter I, 3),... 

So far we have been concerned mainly with dispersions of solids in which all three dimensions of the particles of the dispersed phase are in the colloid size range. An important group of colloidal phenomena involve systems having only one dimension in this range (see Figure 1.1). This chapter deals with thin liquid films, present in isolation, as the basic components of liquid foams or as the film between two emulsion droplets in contact. Other examples of such colloidal systems are those in which the solid particles are thin plates, as in many clay systems, and solid foams. These will, however, be omitted from the present discussion. 

A foam is a dispersion of gas bubbles in a relatively small volume of a liquid or solid continuous phase. Liquid foams consist of gas bubbles separated by thin liquid films. It is not possible to make a foam from pure water the bubbles disappear as soon as they are created. However, if surface active molecules, such as soap, emulsifiers or certain proteins, are present they adsorb to the gas-liquid interfaces and stabilize the bubbles. Solid foams, e.g. bread, sponge cake or lava, have solid walls between the gas bubbles. Liquid foams have unusual macroscopic properties that arise from the physical chemistry of bubble interfaces and the structure formed by the packing of the gas bubbles. For small, gentle deformations they behave like an elastic solid and, when deformed more, they can flow like a liquid. When the pressure or temperature is changed, their volume changes approximately according to the ideal gas law (PF/r= constant). Thus, foams exhibit features of all three fundamental states of matter. In ice cream, the gas phase volume is relatively low for a foam (about 50%), so the bubbles do not come into contact, and therefore are spherical. Some foams, for example bubble bath. 

Many foams of practical interest contain large proportions of the gas phase (more than 90% by voliune). The gas cells are polyhedral since such high volume fractions are impossible in dispersions of spheres having moderate drop size distributions. The cells are separated by thin liquid films of uniform thickness. The junctions of the films are called Plateau bordCTs. A balance of forces requires that three films meet at a plateau border and that the angle between adjacent films be 120°, as illustrated in Figure 5.8a. 

During compression of polymeric foams, three characteristic stages of deformation are commonly observed. At low deformations, the polymer foam is in the linear elastic response regime, i.e., the stress increases linearly with deformation and the strain is recoverable. The second phase is characterized by continued deformation at relatively constant stress, known as the stress collapse plateau. And the final phase of deformation is densification where the foam begins to respond as a compacted solid. At this point the cellular structure within the material is collapsed, and further deformation requires compression of the solid foam material (Ouellet et al. 2006). As mentioned above, a specific technique is required to obtain stress-strain curves of ferroelectrets in thickness direction because the thickness in ferroelectrets is normally very thin, corresponding to very small defiections. Dansachmiiller et al. developed an experimental technique that allows obtaining the stress-strain curves in ferroelectret films (Dansachmiiller et al. 2005). This method may also be used to obtain the stress-stain curve for a polymer foam film without oriented macro-dipoles. The schematic of the experimental setup is shown in Fig. 4. 

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