Foam formation mechanism

A study of the effect of pore geometry on foam formation mechanisms shows that snap-off" bubble formation is dominant in highly heterogeneous pore systems. The morphology of the foams formed by the two mechanisms are quite different. A comparison of two foam injection schemes, simultaneous gas/surfactant solution injection (SI) and alternate gas/surfactant solution injection (GDS), shows that the SI scheme is more efficient at controlling gas mobility on a micro-scale during a foam flood. 

The exchange processes taking place at the atmosphere-ocean border were experimentally studied by Kiseleva (1990) and Zaitsev (1988), among others. They showed that at high wind speeds the rate of gas exchange sharply increases. This is connected with the mechanism of foam formation on wave crests as well as the intense activity of air bubbles being trapped and held beneath the water surface. The dependence of the amount of spray Q on height over the water surface can be approximated well by a linear function. For instance, at a wind speed of V = 11.1 m/s this approximation is (Kiseleva, 1990) ... 

Any substance that acts to reduce the stability of a foam it can also act to prevent foam formation. Terms such as antifoamer or foam inhibitor specify the prevention of foaming, and terms such as defoamer or foam breaker specify the reduction or elimination of foam stability. Example Poly(dimethylsiloxane)s, (CH3)3SiO[(CH3)2SiO]xR, where R represents any of a number of organic functional groups. Antifoamers can act by any of a number of mechanisms. 

The method of beating up involves foam formation by reciprocating a perforated plate or gauze (fixed on a piston) in the foaming solution [1,8]. The piston is moved either manually or mechanically. Several other methods based on mechanical agitation are also employed [9]. 

Detailed analysis of these approaches is presented by Garrett [19]. As far as these processes are valid for homogeneous defoaming, the impossibility for a complete inhibition of foam formation in many systems, even when the solutions are saturated with antifoams, indicates that these kinetic phenomena are not sufficient for heterogeneous defoaming. Therefore, none of the considered points of view gives a satisfactory explanation of the mechanism of action of antifoams in heterogeneous systems. 

Stability is the most important property of a bubble-containing product once the desired characteristics for the product have been achieved, the structure must be kept at least until product consumption. Destabilization processes can combine in various ways and at various rates between foam formation and phase separation thus the reference to foam stability must be accompanied by the specific mechanism under observation (Hailing 1981). In general, the following phenomena can be distinguished when a liquid foam is allowed to stand (Prins 1986) ... 

In contrast to protein stabilized foams the foam formation and stabilization mechanisms in whipping cream are supposed to depend on the bubble stabilization by means of fat rather than proteins. The fat content can therefore not be less than 30% butterfat, and whipping cream is expected to fulfil certain criteria, namely whipping time, foam firmness, foam volume increase (or overrun), and volume of dripping-off (or drainage). 

The rapid industrial growth of polyurethanes and related polymers in the last 20 years has provided a surge of research dealing with both practical and theoretical aspects. The chemistry of isocyanates has been reviewed by Saunders and Slocombe (1) and by Arnold et al. 2). The extensive developments in isocyanate chemistry will be again brought up to date in a forthcoming book by Saunders and Frisch (3). The study of the mechanism of reaction has been emphasized particularly by Baker and co-workers (4) and by Dyer and co-workers (5). Detailed aspects of polyurethane foam formations are given comprehensive treatment by Dombrow 6) and by Saunders (7). 

When a char layer is formed on the outside surface of the foam the inside virgin foam is protected by the char layer having a high degree of heat insulation. This char-formation mechanism is shown in Figure 64 (14). 

The first requirements for foam formation are thus surface tension lowering and surface elasticity. A greater elasticity tends to produce more-stable bubbles. But if the restoring force contributed by surface elasticity is not of sufficient magnitude, then persistent foams may not be formed because of the overwhelming effects of the gravitational and capillary forces. These foams are termed evanescent foams. Important properties that determine the stability will include bubble size, liquid viscosity, and density difference between gas and liquid. More-stable foams may require additional stabilizing mechanisms. 

Foam Formation. Three fundamental pore-level generation mechanisms exist snap-off, division, and leave-behind. 

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