Surfactant-stabilized lamellae

Foam generation does not continue unchecked. Surfactant-stabilized lamellae are only metastable. Coalescence ensues when a translating lamella moves out of a sharply constricted pore-throat into a pore-body, and the lamella is stretched too rapidly for healing flow of foamer solution. Whereas foam generation by capillary snap-off is independent of surfactant formulation, coalescence of foam lamellae strongly depends on surfactant formulation, concentration, and salinity. 

In pure liquids, gas bubbles will rise up and separate, more or less according to Stokes law. When two or more bubbles come together coalescence occurs very rapidly, without detectable flattening of the interface between them, i.e., there is no thin-film persistence. It is the adsorption of surfactant, at the gas-liquid interface, that promotes thin-film stability between the bubbles and lends a certain persistence to the foam structure. Here, when two bubbles of gas approach, the liquid film thins down to a persistent lamella instead of rupturing at the point of closest approach. In carefully controlled environments, it has been possible to make surfactant-stabilized, static, bubbles, and films with lifetimes on the order of months to years [45],... 

Figure 2 illustrates what is coined a discontinuous-gas foam (2, 9), in that the entire gas phase is made discontinuous by lamellae, and no gas channels are continuous over sample-spanning dimensions. Gas is encapsulated in small packets or bubbles by surfactant-stabilized aqueous films. These packets transport in a time-averaged sense through the porous medium (20). 

Nevertheless, it is important to point out that a lamella cannot be created directly at a pore-throat. Rather, a lens forms first with lamella creation occurring upon expansion into the adjacent pore-body, provided surfactant is available (see the discussion of foam-generation mechanisms). During two-phase flow without stabilizing surfactant present, lenses are still created by snap-off in Roof sites (54, 60) followed by expansion and rapid coalescence in the downstream pore-body, once the lens thins to a film. If stabilized lamellae are pictured to rupture before exiting the immediate downstream pore-body, they are not much longer lived than unstable lenses. Such processes are accounted for in measurements of continuum relative permeabilities. 

The role of various surfactant association structures such as micelles and lyotropic liquid crystals (372), adsorption-desorption kinetics at liquid-gas interfaces (373) and interfacial rheology (373) and capillary pressure (374) on foam lamellae stability has been studied. Microvisual studies in model porous media indicate... 

Fig. 22. ETEM at 180°C in N2, illustrating the stability of gold nanorods, for nanoelectronics and catalysis applications. Gold atomic layers and surface atomic structures are visible. Surface of gold nanorod at room temperature showing twin defect lamellae on the atomic scale. They indicate interaction of the surfactant with the (110) surface forming twins to accommodate the shape misfit between the two.

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [411]. For example, in some cases the addition of a small amount of non-ionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer, which is possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [25,110], In general, some very stable foams can be formed from systems in which a liquid crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid crystal phase is present, stable foams are not produced. In this connection foam phase diagrams may be used to delineate compositions that will produce stable foams [25,110],... 

When the lamella between two droplets thins and breaks, the droplets on either side coalesce into a single, larger droplet (41,72). Continuation of this backward" process eventually leads to the disappearance of the dispersion, if it is not balanced by the forward" mechanisms of snap-off and division. Lamellae are thermodynamically metastable, and there are many mechanisms by which static and moving thin films can rupture. These mechanisms also depend on the molecular packing in the film and, thus, on the surfactant structure and locations of the dispersed and dispersing phases in the phase diagram. The stability and rupture of thin films is described in greater detail in Chapter 7. 

A critical literature review on foam rheology is given elsewhere (6). The injection of foam-like dispersions or C02 foams is a useful method in enhanced oil recovery ( 7). This method of decreasing the mobility of a low-viscosity fluid in a porous rock requires the use of a surfactant to stabilize a population of bubble films or lamellae within the porespace of the rock (8). The degree of thickening achieved apparently depends to some extent on the properties of the rock itself. These properties probably include both the distance scale of the pore space and the wettability, and so can be expected to differ from reservoir to reservoir, as well as to some extent within a given field (9,10). 

In the presence of anionic surfactants, it is reasonable to expect that the hydrophobic groups of the poljrmer and of the surfactant would combine to form a mixed film at the liquid-air interface. The interactions between the cationic groups of the polymer and the anionic groups of the surfactant would further strengthen the interactions in the monolayer. These effects can be expected to increase the surface and sub-solution viscosity in lamellae and in turn enhance their stability. 

Figure 12.2 Regions of stability of spheres, cylinders, and lamellae in oil predicted by a mean-field film theory using Eq. (12-2). The hatched region has coexisting spheres and cylinders. Above the region of spheres, there is a region of emulsification failure, where the sphere phase coexists with an excess water phase. In the ordinate, 8 is the thickness of the surfactant film, which is roughly the surfactant molecular length, 4>a and arc the volume fractions of surfactant (or amphiphile) and water, and / opt = (1 + 0.5k/k)/Ho. (From Safran 1994, with permission from Addison-Wesley Publishing Company, Copyright 1994.)...

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