Foam destruction, mechanisms

All the results presented so far give reason to conclude that the avalanche-like destruction of a foam column at definite temperature, pressure drop and foam dispersity, depends mainly on the equilibrium pressure reached. However, in order to establish the mechanism of action of the critical pressure drop, further studies of single foam films and foams are required. They should be performed under conditions that reveal the role of all elements of the foam (films, borders and vertexes) in the process of foam destruction. 

A detailed discussion on many aspects of the mechaiusms of antifoaming can be found in review articles by Garrett, Wasan and Christiano, as well as in the books by Exerowa and Kruglyakov and Kralchevsky and Nagayama. In the present section, we compare the mechanisms of foam destruction by oils and oil-silica compounds. The discussion is based on results... 

The behavior of foam in porous media is intimately related to the connectivity and geometry of the medium in which it resides. Porous media have several attributes that are important to foam flow. First, they are characterized by a size distribution of pore-bodies (sometimes called pores) interconnected through pore-throats of another size distribution. Body and throat size distributions are important as is their possible correlation to determine the distribution of body to throat size ratios. Foam generation and destruction mechanisms in porous media depend strongly on the body to throat size aspect ratio. 

Foam Destruction. Net foam generation cannot continue unchecked. It is balanced by foam destruction processes. Chambers and Radke (26) enunciated two basic mechanisms of foam coalescence capillary-suction and gas diffusion. Because capillary-suction coalescence is the primary mechanism for lamellae breakage, we focus on it, and only briefly touch upon foam coarsening by gas diffusion. 

Denkov, N.D., Marinova, K., Tcholakova, S., Deruelle, M. Mechanism of foam destruction by emulsions of PDMS-silica mixtures, in Proceedings of 3rd World Congress on Emulsions, 24-27 Sept., 2002, Lyon, France, paper l-D-199. 

Some observations are clearly inconsistent with the mechanism. For example, Kulkarni et al. [53] describe an experiment where a drop of PDMS is added to foam formed from sodium lauryl sulfate in a tray. This caused no foam destruction despite the obvious inference that spreading from the drop would occur, which would in turn cause collapse if the spreading mechanism is effective in this context. 

The mechanisms of antifoaming and defoaming are not so well understood. However, these compounds interfere in one or more ways with the stabilization methods discussed earlier. Potential mechanisms are to increase surface tension or to decrease elasticity, bulk- or surface viscosity, or electrostatic repulsion in the lamellae. Defoamers are formulations (multicomponent products) and typically contain various oils (e.g. silicone oUs) and hydrophobic particles making the possible interactions quite versatile. Some of the basic requirements of a defoamer are limited solubility in aqueous phase so that it goes to the interface, surface tension should be below the value of foam liquid to enhance entry and spreading at air-hquid interphase, and low interfacial tension with foaming liquid to enhance spreading at air-hquid interface. Formation and use of oil lenses is an often used route to foam destruction. 

The foam height does not depend only on the adsorption kinetics. Indeed, if a foam is very unstable, the bubbles are destroyed just after being formed, and the amount of foam produced is small. Foaming and foam stability carmot, therefore, be considered separately. In the following, we will recall the two main mechanisms leading to foam destruction ripening and coalescence. 

FIG. 1 Formation of asymmetric oil-water-air films (shaded areas) in two of the possible mechanisms of foam destruction by oil drops or lenses bridging-stretching (a-c-d) and (b-c-d) [11,12] bridging-dewetting (a-c-e) and (b-c-e) [2-6],... 

In this chapter we present a brief overview of the results obtained so far by the FTT with various oils and surfactants in relation to antifoaming. As shown here, the critical capillary pressure, determined in the FTT experiments, has a close relation to the actual process of foam destruction by oil drops. Several conclusions about the mechanism of antifoaming and the antifoam activity of the oils have been drawn and presented in quantitative terms by using the concept of the critical capillary pressure, P , and the FTT results. 

As shown in the following, the threshold value of Pc is related to a transition from one mechanism of foam destruction (rapid rupture of the foam films) to another, much slower mechanism, which includes a compression of the antifoam globules in the Gibbs-Plateau borders of the foam. 

One important question for any mechanism of foam destruction by oils is which structural element of the foam (foam film or Gibbs-Plateau border)... 

The fact that the real foams containing oil drops are unstable (although at least several minutes are needed before the foam destruction starts), while the foam films in the capillary ceU remain stable for much longer time, indicates that the mechanism of foam destruction by oil drops does not occur through rupture of the foam films. Another mechanism that is in agreement with the results obtained for these systems is discussed in Sections 111.A.4 and 111.A.5. 

Although silicone oils by themselves or hydrophobic particles (e.g., specially treated silica) are effective antifoams, combinations of silicone oils with hydrophobic silica particles are most effective and commonly used. The mechanism of film destruction has been studied with the use of surface and interfacial tensions, measurements, contact angles, oil-spreading rates, and globule-entering characteristics for PDMS-based antifoams in a variety of surfactant solutions.490 A very recent study of the effect of surfactant composition and structure on foam-control performance has been reported.380 The science and technology of silicone antifoams have recently been reviewed.491... 

The prevention of foaming and the destruction of existing foams is often a matter of practical importance for example, polyamides and silicones find use as foam inhibitors in water boilers. Antifoaming agents act against the various factors which promote foam stability (described above) and, therefore, a number of mechanisms may be operative. 

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... 

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