Foams factors affecting stability

For foams of low stability the effect of the different factors on foam expansion ratio is difficult to anticipate, since the accelerated internal collapse and the partial destruction of the foam column during foaming lead to an increase in the amount of excess liquid in the foam, to bubble expansion and to increase in surfactant concentration that also affect the rate of drainage. 

The factors affecting the stability and growth of bubbles in aqueous foams have been reviewed in depth by deVries (3). In order to disperse a given volume of gas in a imit volume of liquid, one must increase the free energy of the system by an amount of energy AF as follows ... 

This chapter will provide information on PLA foaming. It will focus on the foam production with emphasis on foaming agent types and processes used in foam production, factors affecting the creation, growth, and stabilization of cells, as well as the properties of PLA foams. 

Although there is a general agreement on the factors affecting foam stability there is still some disagreement about the relative importance of the various factors [18,19]. 

Bates, R. P., 1964. Factors affecting foam production and stabilization of tropical fruit products. Food Technol. 18(1) ... 

As stated above, one of the main factors affecting foam stability is coalescence of air bubbles (Langevin, 2000,2008). This de stabilizing mechanism implies the rupture of the liquid film separating two bubbles to form a bigger. The interfacial elasticity of an adsorbed layer measures the resistance of the film to deformation (Langevin, 2000 Wilde, 2000). Therefore, the surface elasticity is expected to determine ultimately the rupture of the foam film, and hence, the stability of the foam. 

In general the foam density reduces as the amount of blowing agent is increased, with a lower limit set by foam stability. It is possible to model the factors which affect the final density Mahapatro and co-workers (206) used a regular Kelvin foam model to analyse the expansion of PE foams. The foam has uniform sized cells, each with eight hexagonal faces and four square... 

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. 

In view of the importance of macroscopic structure, further studies of liquid crystal formation seem desirable. Certainly, the rates of liquid crystal nucleation and growth are of interest in some applications—in emulsions and foams, for example, where formation of liquid crystal by nonequilibrium processes is an important stabilizing factor—and in detergency, where liquid crystal formation is one means of dirt removal. As noted previously and as indicated by the work of Tiddy and Wheeler (45), for example, rates of formation and dissolution of liquid crystals can be very slow, with weeks or months required to achieve equilibrium. Work which would clarify when and why phase transformation is fast or slow would be of value. Another topic of possible interest is whether the presence of an interface which orients amphiphilic molecules can affect the rate of liquid crystal formation at, for example, the surfaces of drops in an emulsion. 

One of the most important factors regulating the rate of foam collapse (especially coalescence process) is the surfactant kind, along with the additives, both affecting the equilibrium film thickness, film stability, rate of film thinning and rate of drainage. Unfortunately, data about the kinetics of internal foam collapse for foams from various surfactants under comparable conditions are very poor. 

The results of the experiments discussed and the conclusions drawn indicate that foam films play an extremely important role in foam stability. However, there is a number of other factors, such as rheological, that also affect foam stability, though in most cases the stability can be explained when the properties of foam films are taken into account. In this sense the correlation foam film/foam should be regarded as a general trend. 

Other factors (pH, temperature, foam dispersity, foam column height, rate of gas and liquid feed, etc.) also affect the accumulation effectiveness (parameter //). Some of these, such as pH, temperature, type and concentration of the collector, are changing the adsorption, others, like dispersity and foam column height, are changing the drainage rate that determines foam stability and expansion ratio. The book of Rusanov et al. [23] summarises the results on the effect of these factors on foam accumulation of surfactants. 

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. 

A method to quantify the stability of a foam is to measure its half-life. The half-life is the time required for the foam to drain half its liquid volume. The longer it takes to drain the liquid, the more stable the foam will be. A major factor that strongly affects the stability is the method of foam generation. In addition, the experimentally measured half-life is a function of the height of the foam column. 

Other factors can also affect the stability of foam among these are temperature, surfactant structure, surface viscosity, rate of drainage and bulk viscosity. 

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