Stabilization of foams

The importance of the thin film between the mineral particle and the air bubble has been discussed in a review by Pugh and Manev [74]. In this paper, modem studies of thin films via SFA and interferometry are discussed. These film effects come into play in the stability of foams and froths. Johansson and Pugh have studied the stability of a froth with particles. Small (30-/ m), moderately hydrophobic 6c = 65°) quartz particles stabilized a froth, while more hydrophobic particles destabilized it and larger particles had less influence [75]. 

The main performance characteristics of light-duty liquids or dishwash blends are soil emulsification and foaming. Figueroa-Raulli [73] published a comparative study of AOS vs. alkyl sulfates (AS), alcohol ether sulfates (AES), and linear alkylbenzenesulfonates (LAS) in light-duty liquid blends. The blends were evaluated for flash foam, the stability of foam under accumulating soil loads, and for the number of plates washed. Blend performance is expressed as a percentage of a dishwash reference standard. 

Stabilization of emulsions Stabilization of suspensions Stabilization of foams Control of crystal growth... 

While drilling low-pressure reservoirs with nonconventional methods, it is conunon to use low-density dispersed systems, such as foam, to achieve underbalanced conditions. To choose an adequate foam formulation, not only the reservoir characteristics but also the foam properties need to be taken into account. Parameters such as stability of foam and interactions between rock-fluid and drilling fluid-formation fluid are among the properties to evaluate while designing the drilling fluid [13]. 

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. 

Dynamic Stability of Foam Lamellae Flowing Through a Periodically Constricted Pore... 

MC is used as an adhesive in ceramics to provide water retention and lubricity in cosmetics to control rheological properties and in the stabilization of foams in foods as a binder, emulsifier, stabilizer, thickener, and suspending agent in paints, paper products, plywood as a rheology control for the adhesive in inks, and in textiles as a binder, and for coatings. 

Table 6.3 Effect of protein self-assembly, induced by interaction with lecithin, on the stability of foams stabilized by complexes of sodium caseinate (1 % w/v) with soy phospholipids Lipoid S-21 (1(T5 M) (Istarova et al., 2005 Semenova, 2007). Values of Mw and A 2 are presented for the protein with and without surfactant at three pH values. Also shown are photographs of foams recorded 9 minutes following foam preparation. In each of the images the volume of the glass vessel containing die foam is 10 ml. 

Coke, M., Wilde, P.J., Russell, E.J., Clark, D.C. (1990). The influence of surface composition and molecular diffusion on the stability of foams formed from protein detergent mixtures. Journal of Colloid and Interface Science, 138, 489-504. 

The stability of foams is usually measured by the volume of liquid drained from a foam during a specific time at room temperature (10. 15. 17) or by a decrease in foam volume over time. Methods employed to measure foam stability include the rate of fall of a perforated weight through a column of foam (20. 22), the penetration of a penetrometer cone (17), or the ability to support a series of specific weights (277. 

The properties of the finished beer vary with the type of beer and place of origin. The figures in Table 1 do not, however, show much about the quality of the beer this can only pardy be expressed in figures based on objective measurements. The quality consists of aroma, taste, appearance, (color, clarity) formation, and stability of foam. Of these, the first two are still inaccessible to objective measurement. Although the aroma of a product is determined by the quantity of volatile alcohols, etc, the quality of the product cannot be expressed in those terms. Appearance, foam formation, and foam stability can be evaluated more easily. For judgment on taste and aroma, taste-testing panels are the only method. 

Tamsma, A., Mucha, T. J. and Pallansch, M. J. 1962. Factors related to flavor stability of foam-dried milk. II. Effect of heating milk prior to drying. J. Dairy Sci. 45, 1435— 1439. 

We have used film interferometry to reveal a new mechanism for the stabilization of foams and emulsions due to layering inside the thinning films, as will be discussed below. 

The structure and stability of foamed emulsions, such as whipped cream, ice cream or whipped toppings, strongly depend on the interparticle interactions and on the orientation of drops/particles at the foam films. Further development of the surface force balance and... 

Tween 20 was considerably more effective at reducing the stability of foams of a-la than was the case with /3-lg. There was a significant decrease in a-la foam stability in the presence of Tween, at R values as low as 0.05. Minimal foam stability was observed at R = 0.15. There was no observed change in film drainage behavior or onset of surface diffusion in the adsorbed protein layer up to this R value. The only observed change was a progressive decrease in film thickness. Therefore, it is likely that disruption of adsorbed multilayers is responsible for a reduction in the structural integrity of the adsorbed protein layer and that this increases the probability of film rupture. 

One of the central questions in the stability of foams is why are liquid films between two adjacent bubbles stable, at least for some time In fact, a film of a pure liquid is not stable at all and will rupture immediately. Formally this can be attributed to the van der Waals attraction between the two gas phases across the liquid. As for emulsions, surfactant has to be added to stabilize a liquid film. The surfactant adsorbs to the two surfaces and reduces the surface tension. The main effect, however, is that the surfactant has to cause a repulsive force between the two parallel gas-liquid interfaces. Different interactions can stabilize foam films [570], For example, if we take an ionic surfactant, the electrostatic double-layer repulsion will have a stabilizing effect. 

To gain more insight into the stability of foams let us have a closer look at the pressures involved. We refer to Fig. 12.18. Outside the foam the pressure is fixed. Usually this will be atmospheric pressure. The pressure in one of the top compartments is determined by the radius of curvature of the liquid film according to the Laplace equation. If we take water as a liquid with surfactant present and a typical surface tension of 40 mN/m the pressure inside one of the top compartments with a radius of curvature R = 1 mm is AP = 47/R 160 Pa. The factor 4 is due to the fact that we do not have a bubble (then we would have AP = 2-y/R) but a soap film with two sides. For compartment A the curvature is indicated by a semicircle. 

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. 

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