Foams film rupture mechanisms

FIGURE 4.18 Foam film rupture mechanism by bridging oil drop proposed by Frye and Berg involving elimination of oil-water surface. (From Frye, G.C., Berg, J.C., J. Colloid Interface Sci., 130, 54,1989.)... 

Denkov, N. D., Cooper, P. and Martin, J.-Y., Mechanism of action of mixed solid/liquid antifoamers 1. Dynamics of foam film rupture, Langmuir, 15, 8514-8529 (1999). 

Early speculations about the mode of action of PDMS-based antifoams assume that duplex film spreading from drops in foam films induces shear in the intralamellar liquid, which leads to foam film rupture. Clearly the rate of spreading would be a key aspect of that mechanism. However, as we describe in Chapter 4, this view of antifoam mechanism is now somewhat discredited. Nevertheless, other aspects of antifoam action, such as the effect of antifoam viscosity on deactivation during prolonged interaction with foam generation (see Chapter 5), could be determined by spreading rates. It is therefore appropriate to briefly review this topic here. 

Yet another possibility is described by Denkov et al. [54] and again ascribed to Frye and Berg [75]. In this mechanism, the oil lens is assumed to be non-deformable so the lens shape is preserved after the lens bridges a foam film. The situation is depicted in Figure 4.19. Again provided B>0 and 0 > 90°, this configuration would result in foam film rupture as the second air-water surface peels off the lens to produce a hole when the two three-phase contact lines become coincident. Denkov et... 

These observations concerning the role of the oil in foam film rupture are clearly not consistent with a Marangoni spreading mechanism. As we have seen, presence... 

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. 

Film Rupture. Another general mechanism by which foams evolve is the coalescence of neighboring bubbles via film mpture. This occurs if the nature of the surface-active components is such that the repulsive interactions and Marangoni flows are not sufficient to keep neighboring bubbles apart. Bubble coalescence can become more frequent as the foam drains and there is less Hquid to separate neighbors. Long-Hved foams can be easHy... 

For example, a synergistic defoaming occurs when hydrophobic solid particles are used in conjunction with a liquid that is insoluble in the foamy solution [652]. Mechanisms for film rupture by either the solid or the liquid alone have been elucidated, along with explanations for the poor effectiveness, which are observed with many foam systems for these single-component defoamers. 

In order to understand the basis for the prevention of bubble coalescence and hence the formation of foams, let us examine the mechanical process involved in the initial stage of bubble coalescence. The relatively low Laplace pressure inside bubbles of reasonable size, say over 1 mm for air bubbles in water, means that the force required to drain the water between the approaching bubbles is sufficient to deform the bubbles as illustrated in Figure 8.2. The process which now occurs in the thin draining film is interesting and has been carefully studied. In water, it appears that the film ruptures, joining the two bubbles, when the film is still relatively thick, at about lOOnm thickness. However, van der Waals forces, which are attractive in this system (i.e. of air/water/air), are effectively insignificant at these film thicknesses. 

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