Foam instability

The proteins in the liquid film should 1) be soluble in the aqueous phase, 2) readily concentrate at the liquid-air interface, and 3) denature to form cohesive layers possessing sufficient viscosity and mechanical strength to prevent rupture and coalescence of the droplet. That is, the polypeptides of the denatured proteins in the liquid film should exhibit a balance between their ability to associate and form a film and their ability to dissociate, resulting in foam instability. 

In the case where foam instability is desirable, it is essential to choose surfactants that weaken the Gibbs-Marangoni effect. A more surface-active material such as a poly(alkyl) siloxane is added to destabilize the foam. The siloxane surfactant adsorbs preferentially at the air/liquid interface, thus displacing the original surfactant that stabilizes the foam. In many cases, the siloxane surfactant is produced as an emulsion which also contains hydrophobic silica particles. This combination produces a synergetic effect for foam breaking. 

Foam instability is expressed mainly by the processes of excess (referring to equilibrium quantity) liquid outflow, diffusion gas transfer from smaller to larger bubbles and coalescence. If the vapour pressure of the solvent in the surrounding medium of foam is lower than its saturated vapour pressure, then the process of evaporation influences significantly foam collapse. Finally, if a foam is produced from a gas phase, the main component of which is the solvent vapour, condensation of these vapours appears to be the determining process of bubble expansion. 

Another mechanism of foam instability is due to Ostwald ripening (disproportionation), the driving force for which process is the difference in Laplace pressure between the small and larger foam bubbles. The smaller bubbles have a higher... 

In the case of a polyhedral foam with planar hquid lamella, the pressure difference between the bubbles is not large, and consequently Ostwald ripening is not the mechanism for foam instability in this case. With a polyhedral foam, the main driving force for foam collapse is the surface forces that act across the liquid lamella. 

Stability. The static stability of a foam is the ability to resist bubble breakdown resulting from bubble collapse or coalescence. Foam instability can be caused by the drainage of liquid from the foam resulting from increased quality above 0.95 because of pressure reduction, heating, bubble rupture, or coalescence. 

Another major point of consideration is obviously the processing latitude of silicone surfactants. This phrase describes the variability of catalyst concentration without running into either closed, dead foam or observing first signs of foam instability indicated by splits in the foam. Not only will a different silicone surfactant result in a different processing window, e.g., amount of stannous octoate variation between the two extremes, but also the position of the processing window might be different. 

The foregoing discussion leads to the question of whether actual foams do, in fact, satisfy the conditions of zero resultant force on each side, border, and comer without developing local variations in pressure in the liquid interiors of the laminas. Such pressure variations would affect the nature of foam drainage (see below) and might also have the consequence that films within a foam structure would, on draining, more quickly reach a point of instability than do isolated plane films. 

The general mode of action for the control of foam is to displace organic solutes or other materials that are absorbed into the bubble film at the gas-BW interface. The new film tends to lack elasticity and is therefore prone to rupturing and instability. 

Likewise, the practical food foreman knows that by following certain manufacturer s recommendations and certain processing conditions in his plant, he is able to produce stabilized foam products to the satisfaction of his superiors and the public, most of the time yet when problems of instability and poor shelf life of the finished product are brought to his attention and all simple adjustments fail to produce a satisfactory result, he must turn to the food or colloid chemist for the theory and industrial application of foams. 

At autopsy, a ruptured plaque features a thin fibrous cap overlying cell-rich regions with a lipid-rich necrotic core (<3 mm ) containing cell debris. The indicators for plaque instability include an inflammatory infiltrate with lipid-rich macrophages (foam-cells) and a decreased collagen and smooth muscle cell (SMC) content in the fibrous caps as well as at the shoulders of the atheroma, respectively. Therefore, one of the challenges of modern medicine is the design of... 

Conditions sometimes exist that may make separations by distillation difficult or impractical or may require special techniques. Natural products such as petroleum or products derived from vegetable or animal matter are mixtures of very many chemically unidentified substances. Thermal instability sometimes is a problem. In other cases, vapor-liquid phase equilibria are unfavorable. It is true that distillations have been practiced successfully in some natural product industries, notably petroleum, long before a scientific basis was established, but the designs based on empirical rules are being improved by modern calculation techniques. Even unfavorable vapor-liquid equilibria sometimes can be ameliorated by changes of operating conditions or by chemical additives. Still, it must be recognized that there may be superior separation techniques in some cases, for instance, crystallization, liquid-liquid extraction, supercritical extraction, foam fractionation, dialysis, reverse osmosis, membrane separation, and others. The special distillations exemplified in this section are petroleum, azeotropic, extractive, and molecular distillations. 

Suspended particles may consist of complex inorganic hydroxides and silicates or, sometimes, organic debris. Particles too small to be easily distinguished can cause difficulties when a drink is carbonated, acting as minute centres of instability resulting in a loss of carbonation, foaming (gushing) at the filler-head and variable fill volumes. 

Major changes in all three measured parameters in Figure 15 occur at R = 0.9 - 1.0. It is significant that this is also the point where instability is first observed in the bulk foam [13,22]. Thus, there is strong evidence that suggests a link between changes in the adsorbed... 

Dispersion Characteristics The chief characteristics of gas-in-liquid dispersions, like those of liquid-in-gas suspensions, are heterogeneity and instability. The composition and structure of an unstable dispersion must be observed in the dynamic situation by looking at the mixture, with or without the aid of optical devices, or by photographing it, preferably in nominal steady state photographs usually are required for quantitative treatment. Stable foams may be examined after the fact of their creation if they are sufficiently robust or if an immobilizing technique such as freezing is employed [Chang et al., Ind. Eng Chem., 48, 2035 (1956)]. 

The role of surfactants in stabilization/destabilization of foam (air/liquid dispersions) is similar to that for emulsions. This is due to the fact that foam stability/instability is determined by the surface forces operative in liquid films between air bubbles. In many industrial applications, it is essential to stabilize foams against collapse, e.g., with many food products, foam in beer, fire-fighting foam, and polyurethane foams that are used for furniture and insulation. In other applications, it is essential to have an effective way of breaking the foam, e.g., in distillation... 

The theoretical analysis indicated that asymmetric drainage was caused by the hydrodynamic instability being a result of surface tension driven flow. A criterion giving the conditions of the onset of instability that causes asymmetric drainage in foam films was proposed. This analysis showed as well that surface-tension-driven flow was stabilised by surface dilational viscosity, surface diffusivity and especially surface shear viscosity. 

Recently Joye et at. [74] have reported a numerical simulation of instability causing asymmetric drainage in foam films. The results obtained confirmed the rapid increase in drainage rate. 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

In the study of the mechanism of heterogeneous defoaming along with the spreading coefficients the so-called enter coefficient (destruction coefficient) is used to estimate the instability of aqueous foam films... 

The rate of decreases in the specific surface area (or average dispersity) is a basic parameter characterising the aggregative instability of a foam. 

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