Foam viscosity

Foam viscosity and stability can be enhanced viscosifying the continuous phase with thickening agents. These are mostly the same thickening agents used to prepare viscous fracturing base gels. 

Figure 10. Foam viscosity depends on percent internally dispersed gas (Mitchell quality). (Reproduced from ref. 363. U.S. Patent 3 937 283,1976.)...

The viscous properties of HIPEs and high gas fraction foams have also been studied extensively, using a two dimensional, monodisperse, hexagonal cell model. Khan and Armstrong [52] showed that, under steady shear flow (i.e. beyond the yield point of the system), the foam viscosity was inversely proportional to shear rate. At high rates of shear, a constant viscosity value was approached. Gas fraction, <)>, was assumed to be very close to unity. 

This work shows that high shear rates are required before viscous effects make a significant contribution to the shear stress at low rates of shear the effects are minimal. However, Princen claims that, experimentally, this does not apply. Shear stress was observed to increase at moderate rates of shear [64]. This difference was attributed to the use of the dubious model of all continuous phase liquid being present in the thin films between the cells, with Plateau borders of no, or negligible, liquid content [65]. The opposite is more realistic i.e. most of the liquid continuous phase is confined to the Plateau borders. Princen used this model to determine the viscous contribution to the overall foam or emulsion viscosity, for extensional strain up to the elastic limit. The results indicate that significant contributions to the effective viscosity were observed at moderate strain, and that the foam viscosity could be several orders of magnitude higher than the continuous phase viscosity. 

A subsequent analysis [66] also employed this model, with the inclusion of results for the shear strain. The dependence of the viscous effects on initial foam orientation was also noted. Further work [67] on monodisperse wet foams, where is between 0.9069 and 0.9466, demonstrated that, under shear flow, the foam viscosity increased with increasing < > (decreasing liquid content). In contrast, for small deformations, the viscous contribution to the overall stress was found to be independent of liquid content. 

General Functional Properties. These include solubility, emulsification, water and oil absorption, gelation or coagulation, foaming, viscosity, texture, adhesion or cohesion, and film formation. 

Foam viscosity of freshly made foams, and those allowed to stand for 60 min, was highest for suspensions in the pH range of... 

Figure 5. Foam viscosity and stability properties of glandless cottonseed flour...

Foam viscosities measured after one min were essentially the same at flour concentrations of 2 to 10%, increased at the 12% flour level, declined slightly at the 14 and 16% levels, then increased as the flour concentration was increased to 23 and 30% (Figure 5). Changes in foam viscosities after 60 min were more variable viscosities were highest at the 30% flour level, intermediate at the 6 to 8% levels, and lowest at the 16% flour 1 evel. 

Glycyrrhizin is 50-100 times sweeter than sucrose and has a slow onset of taste and a long aftertaste (Table 1). It exhibits a dark, sweet, woody flavor, which limits its use as a pure sweetener. Glycyrrhizin provides licorice flavor, enhances food flavors, masks bitter flavors, and increases the perceived sweetness level of sucrose. It also has the potential for providing functional characteristics, including foaming, viscosity control, gel formation, and possibly antioxidant characteristics (3,19,58). 

Since wet foams contain approximately spherical bubbles, their viscosities can be estimated by the same means that are used to predict emulsion viscosities. In this case the foam viscosity is described in terms of the viscosity of the continuous liquid phase (tjo) and the amount of dispersed gas (4>). In dry foams, where the internal phase has a high volume fraction the foam viscosity increases strongly due to bubble crowding, or structural viscosity, becomes non-Newtonian, and frequently exhibits a yield stress. As is the case for emulsions, the maximum volume fraction possible for an internal phase made up of uniform, incompressible spheres is 74%, but since the gas bubbles are very deformable and compressible, foams with an internal vol-... 

Foam viscosity was optimum in the pH range of 3.5 to 5.5 (Figure 4). Between these values, most proteins of glandless cottonseed flour were in the insoluble form. 

Figure 4. Foam viscosity and stability properties of cottonseed proteins at various pH values. ( —< ) 1 min, fO------------------------O) GO min.

The expression for foam viscosity Eq. (8.1 la) contains two terms x 0/y which is the elasticity component, related to the demolition of foam structure, and r, which is a dissipation term, related to the liquid flow through films and borders during the deformation process. The models of Khan and Armstrong [14] and Kraynik and Hansen [43] imply that the continuous phase is in the films, no liquid exchange occurs and the film surfaces are mobile, thus predicting a very small contribution of the viscous dissipation in the films, rj = 13[Pg.584]

The critical analysis of the results on foam rheology, proposed by Heller and Kuntamukkula [16], has shown that in most of the experiments the structural viscosity depends on the geometrical parameters of the device used to study foam flow. This means that incorrect data about flow regime and boundary conditions, created at the tube and capillary walls, etc., are introduced in the calculation of viscosity (slip or zero flow rate). Most unclear remains the problem of the effect of the kind of surfactant and its surface properties on foam viscosity and on the regime of foam flow (cross section rate profile and condition of inhibition of motion at the wall surface). 

The scope of possible foam applications in the field warrants extensive theoretical and experimental research on foam flow in porous media. A lot of good work has been done to explain various aspects of the microscopic foam behavior, such as apparent foam viscosity, bubble generation by capillary snap-off, etc.. However, none of this work has provided a general framework for modeling of foam flow in porous media. This paper attempts to describe such a flow with a balance on the foam bubbles. 

Low foam viscosity allows form to penetrate into substrate... 

In dry foams, where the internal phase has a high volume fraction, the foam viscosity increases strongly, because of bubble crowding or... 

Some foams that have a drop-size distribution that is heavily weighted toward the smaller sizes will represent the most stable foam. In such cases, changes in the size distribution curve with time yield a measure of the stability of the foams. The bubble size distribution also has an important influence on the viscosity. For bubbles that interact electrostatically or sterically, foam viscosity will be higher when bubbles are smaller (for a given foam quality). This condition results because the increased interfacial area and thinner films increase the resistance to flow. The viscosity will also be higher when the bubble sizes are relatively homogeneous, that is, when the bubble size distribution is narrow rather than wide (also for given foam quality). 

The foam-dilatational viscosity, K, arises because of two primary mechanisms (37) (1) viscous flow within the thin films, and (2) interfacial tension gradients acting along the foam bubble surfaces. The effect of interfacial tension gradients is to increase the foam viscosity as they impede flow near the surfaces of the thin foam films by contributing to a larger film stress. As in the wet foam (eq 6), the foam dilatational viscosity for a dry foam, K, is inversely proportional to film thickness as well (eq 9). 

Figure 11 displays the dependence of the foam dilatational viscosity upon the rate of foam expansion for Na = 1 (21). The foam viscosity, K, increases for foam expansion, as the foam films thin and velocity gradients occurring over the thickness of the film increase in magnitude, whereas, for opposite reasons, the foam viscosity, K, decreases as the foam is compressed. 

Figure 2 A plot of foam quality vs. foam viscosity. (Reproduced with permission from reference 6. Copyright 1975 Canadian Institute of Mining Metallurgy, and Petroleum.)...

Effective Foam Viscosity For foam flowing in porous media, the foam s effective viscosity is that calculated from Darcy s law. This value is an approximation because foams are compressible and are also usually non-Newtonian. 

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