Viscosity foam stabilization

The presence of a third phase can promote or impair foam stability, and in some cases, even prevent foaming. As mentioned previously, stable foams can be formed from mixtures of an isotropic liquid with a liquid-crystal phase The foam lamellae become covered with layers of liquid-crystal the foam stability is increased through surface viscosity. Foam stability can also be affected by the presence of other dissolved species, an additional liquid phase such as oil in an aqueous foam, or fine solids. In these cases, whether the effect is one of stabilizing or destabilizing depends on several factors. First, it depends on whether or not the third-phase species have a strong affinity for the liquid phase, and therefore whether they tend to accumulate at the gas—liquid interface. Second, once accumulated, any effect they may have on the interfacial properties is important. 

Both high bulk and surface shear viscosity delay film thinning and stretching deformations that precede bubble bursting. The development of ordered stmctures in the surface region can also have a stabilizing effect. Liquid crystalline phases in foam films enhance stabiUty (18). In water-surfactant-fatty alcohol systems the alcohol components may serve as a foam stabilizer or a foam breaker depending on concentration (18). 

Bendure indicates 10 ways to increase foam stability (1) increase bulk liquid viscosity, (2) increase surface viscosity, (3) maintain thick... 

Cocamide DEA (or MEA or TEA) is used as a foaming agent, to make lather. The other surfactants generate a certain amount of suds, but this foaming agent is added to get the amount just right. In addition to its foam-stabilizing effects, it is also a viscosity booster—it s thick. 

At high bulk viscosity, lowering the surface tension is not relevant for the mechanism of stabilization of foams, but for all other mechanisms of foam stabilization a change of the surface properties is essential. A defoaming agent will change the surface properties of a foam upon activation. Most defoamers have a surface tension in the range of 20 to 30 mNm . The surface tensions of some defoamers are shown in Table 21-2. 

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. 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

The surface and bulk viscosities not only reduce the draining rate of the lamella but also help in restoration against mechanical, thermal, or chemical shocks. The highest foam stability is associated with appreciable surface viscosity (qs) and yield value. 

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

In addition to the film elasticity, other factors that may affect foam stability arc surface shear viscosity, bulk viscosity of the foaming liquid, and the presence of particulate matter. 

The foam stability of /3-cas foams progressively decreased with added Tween 20. In contrast, there was a very sharp transition in equilibrium film thickness at R = 0.5. Surprisingly, surface diffusion of /3-cas was not detected at any R value in these films. This was unexpected since it has been reported that adsorbed layers of /3-cas are characterized by a very low surface viscosity [3], signifying that protein-protein interactions in /3-cas films are very weak. We had expected to observe surface diffusion either in the films stabilized by... 

The main factor determining the stability of such foams is the rate and extent of drainage from the thin liquid film. In general, this type of foam is relatively unstable. The stability may be enhanced by increasing the viscosity of the liquid by increasing the dry matter content or adding certain hydrocolloids. The foam stability may also be enhanced with hydrocolloids, in particular microcrystalline cellulose. 

Bendure indicates 10 ways to increase foam stability (1) increase bulk liquid viscosity, (2) increase surface viscosity, (3) maintain thick walls (higher liquid-to-gas ratio), (4) reduce liquid surface tension, (5) increase surface elasticity, (6) increase surface concentration, (7) reduce surfactant-adsorption rate, (8) prevent liquid evaporation, (9) avoid mechanical stresses, and (10) eliminate foam inhibitors. Obviously, the reverse of each of these actions, when possible, is a way to control and break foam. 

Biliaderis, 1989 Oomah and Mazza, 1998b). Flaxseed gum exhibited good foam stability at a level of 1% and maximum viscosity at pH 6.0-8.0 (Mazza and Biliaderis, 1989). Oomah and Mazza (1998b) reported that lipid removal significantly increased apparent viscosity values of flaxseed gum. Furthermore, viscosity of seed, cake, and flake samples was significantly related to protein (r = 0.97) and carbohydrate (r = 0.91) fractions, which were related to mucilage fraction of the seed. 

The stability of foams in constraining media, such as porous media, is much more complicated. Some combination of surface elasticity, surface viscosity and disjoining pressure is still needed, but the specific requirements for an effective foam in porous media remain elusive, partly because little relevant information is available and partly because what information there is appears to be somewhat conflicting. For example, both direct [304] and inverse [305] correlations have been found between surface elasticity and foam stability and performance in porous media. Overall, it is generally found that the effectiveness of foams in porous media is not reliably predicted based on bulk physical properties or on bulk foam measurements. Instead, it tends to be more useful to study the foaming properties in porous media at various laboratory scales micro-, meso-, and macro-scale. 

To the extent that viscosity and surface viscosity influence foam stability, one would predict that stability would vary according to the effect of temperature on the viscosity. Thus some petroleum industry processes exhibit serious foaming problems at low process temperatures, which disappear at higher temperatures. Ross and Morrison [25] cite some examples of petroleum foams that become markedly less stable above a narrow temperature range that may be an interfacial analogue of a melting point. 

Adamson [15] and Miller et al. [410] illustrate some techniques for measuring surface shear viscosity. Further details on the principles, measurement and applications to foam stability of interfacial viscosity are reviewed by Wasan et al. [301,412], It should be noted that most experimental studies deal with the bulk and surface viscosities of bulk solution rather than the rheology of films themselves. 

Any additives that can act to reduce the viscosity of foam films, and thereby increase the liquid drainage rate, will tend to reduce foam stability as a result. This includes any chemicals that can reduce surface viscosity and/or surface elasticity. Some alcohols can be use to produce these effects. 

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