Foams film rupture

Another approach to explain foam film rupture has been developed by de Vries [101] who proposed to consider film rupture as a result of fluctuational formation of holes (black spots) in it - nuclei of critical size (see Section 3.4). This idea was used in the analysis of the... 

The experiments indicated that foam films rupture at pressures lower than nmax is not due to occasional reasons. Critical pressure pcr was observed with different types of films (common foam, CBF and NBF) stabilised with various kinds of surfactants [171,303]. Similar effect has been observed by Black and Herrington [261] who studied films stabilised with three anion-active surfactants. However, details on the critical pressure of film rupture will not be discussed here, since a satisfactory theoretical explanation of this effect has not been proposed so far. There are some hypothesis on the matter. Nevertheless, this parameter has been successfully employed in clarifying the role of foam films in foam stability (see Chapter 7). No doubt, this parameter provides information about the stability of the different types of foam films and is awaiting its qualitative interpretation. 

Under a-particle irradiation a crater forms in the foam similar to their traces in Wilson s chamber [19]. Fig. 7.3 shows the light flashes created when the foam films rupture. 

Fig. 7.3. Light flashes at foam films rupture and foam borders disappearance under a-particle...

It is necessary to mention that an avalanche-like destruction is also observed in a NaDoS foam but it occurs at significantly higher pressure drops with respect to the equilibrium pressure pa (see Fig. 6.12,a). That is why it is important to distinguish between the destruction at equilibrium critical Apcr,e and at non-equilibrium critical Apcrne pressures since probably the causes are different. Foam destruction at Apcre is perhaps due to foam film rupture while at Apcr ne the destruction results from other phenomena occurring in the disperse... 

The examples given with the two representatives of non-ionic surfactants, NP20 and Cio(EO)4 clearly indicate that the isoelectric state at the solution/air interface leads to foam film rupture and, respectively, to decrease in foam stability. This fact supports the idea about the role of foam films in the stability of foams. On the other hand, it provides an opportunity to regulate foam stability. 

Critical Entry Pressure for Foam Film Rupture... 

Figures 4.46 and 4.47 show the critical thicknesses of rupture, for foam and pulsion films, respectively, plotted versus the film radius [722]. In both cases the film phase is the aqueous phase, which contains 4.3 x 10"" M SDS + added NaCl. The emulsion film is formed between two toluene drops. Curve 1 is the prediction of a simpler theory, which identifies the critical thickness with the transitional one [720]. Curve 2 is the theoretical prediction of Equations 4.281 through 4.283 (no adjustable parameters) in Equation 4.182 for the Hamaker constant the electromagnetic retardation effect has also been taken into account [404]. In addition, Eigure 4.48 shows the experimental dependence of the critical thickness versus the concentration of surfactant (dodecanol) for aniline films. Figures 4.46 through 4.48 demonstrate that when the film area increases and/or the electrolyte concentration decreases the critical film thickness becomes larger. Figure 4.49 shows the critical thickness of foam film rupture for three concentrations of SDS in the presence of 0.3 M NaCl [605]. The dashed and dash-dotted lines, for 1 and 10 pM SDS, respectively, are computed assuming only the van der Waals attraction (no adjustable parameter). The deviation of the predicted values of h ...

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). 

Foam film stability is, as we have seen, determined in part by the lack of balance between the disjoining pressure and the capillary pressure applied to the films by the Plateau borders. The capillary pressure also drives the process of film thinning, which precedes film rupture. This in turn influences the frequency of foam film rupture. The relative magnitudes of the capillary pressure and the hydrostatic head in the foam also determine the bulk drainage behavior of the foam. If the capillary pressure at the top of the foam balances the hydrostatic head, then bulk drainage will not occur. As we show in later chapters, the stability of the films between antifoam entities and the gas liquid surface—the so-called pseudoemulsion films [60]— may also be determined by the lack of balance between the disjoining pressure in the pseudoemulsion film and the Plateau border capillary pressure. It is therefore important to clearly define the nature of the pressure distribution in the continuous phase of a foam as represented by the system of Plateau border channels. In this, we follow closely the arguments of Princen [61]. 

Once antifoam oils emerge into the air-liquid surface of the foaming medium, they must exhibit certain types of spreading behavior if they are to be effective in causing foam film rupture. Finally, then, we review the types of spreading behavior to be expected together with brief descriptions of appropriate experimental methods for distinguishing between them. 

This arrangement has been found to be particularly useful in studying the behavior of antifoam drops in draining foam films. It permits observations of both the movement of the drops as they interact with draining films and also the actual events of foam film rupture induced by those drops [24, 37]. 

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. 

FIGURE 4.10 Foam film rupture caused by spreading from antifoam particle— Marangoni... 

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... 

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.)... 

---