Drainage and Thinning of Foam Films

To investigate foam stability one must consider the role of the plateau border under dynamic and static conditions. One should also consider foam films with intermediate lifetimes, i.e. between unstable and metastable foams. 

Film drainage can also be decreased by increasing the surface viscosity and surface elasticity. This can be achieved, for example, by addition of proteins, polysaccharides and even particles. These systems are applied in many food foams. 

For convenience, the drainage of horizontal and vertical films will be treated separately. 

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The extent and rate of drainage of surplus solution from the interior of the lamellae is one of the important factors determining foam stability, since drainage causes thinning of the film, and when the film reaches a critical thickness (50-100 A), the film may rupture spontaneously. Drainage of the film occurs under two influences gravity and pressure difference. 

Lobo and Wasan (81) observed the drainage and stability of pseudoemulsion films from nonionic surfactant solutions (Enordet AE1215-30 ethoxylated alcohol) at concentrations much above the CMC. They observed that, for a 4 wt% surfactant system, the film thinned stepwise by stratification (Figure 27), in a fashion similar to the foam films from micellar solutions (Figure 14). Three thickness transitions were observed (81) at 4 wt% concentration with n-octane as oil, which was the same number of steps as observed by Nikolov et al. (54) in foam films at the same concentration. This observation on the micellar layering in the pseudoemulsion film confirms, again, the universality of the stratification phenomenon. 

Finally, we consider the hydrodynamic theory of thin liquid film rupture. The stability of the liquid films to a great extent is ensured by the property of the adsorbed surfactant to damp the thermally excited fluctuation capillary waves representing peristaltic variations in the film thickness [6]. In addition to the theory of stability of free foam and emulsion films, we consider also the drainage and stability of wetting films, which find application in various coating technologies [7]. 

Fig. 5.12 depicts the lg Wit dependence for NaDoS foams with thin liquid films, h 16 nm, with CBF, h 8 nm and with NBF, h 4.2 nm. The differences between curve 1, 2 and 3, corresponding to the different foam film types, is clearly expressed and is valid not only for the curve slopes but also for t at which a plateau is reached, that itself corresponds to hydrostatic equilibrium. Fig. 5.12,a and 5.12,b plots the initial linear parts of the experimental Wit dependences where the black circles are for NBF, and the black squares are for CBF. It can be seen that the drainage rate is different for the different types of foam films. 

The studies discussed expand the use of the method for assessment of foetal lung maturity with the aid of microscopic foam bilayers [20]. It is important to make a clear distinction between this method [20] and the foam test [5]. The disperse system foam is not a mere sum of single foam films. Up to this point in the book, it has been repeatedly shown that the different types of foam films (common thin, common black and bilayer films) play a role in the formation and stability of foams (see Chapter 7). The difference between thin and bilayer foam films [19,48] results from the transition from long- to short-range molecular interactions. The type of the foam film depends considerably also on the capillary pressure of the liquid phase of the foam. That is why the stability of a foam consisting of thin films, and a foam consisting of foam bilayers (NBF) is different and the physical parameters related to this stability are also different. Furthermore, if the structural properties (e.g. drainage, polydispersity) of the disperse system foam are accounted for it becomes clear that the foam and foam film are different physical objects and their stability is described by different physical parameters. 

It was found that in each case the foam stability was effected by a completely different mechanism. In the first case, where the foam film containing oil is solubilized within the micelle to form a microemulsion, the normal micellar interactions are changed. It had been earlier demonstrated that micellar structuring causes stepwise thinning due to layer-by-layer expulsion of micelles and that this effect was found to inhibit drainage and increase the foam stability. Generally, this stratification phenomenon was found to be inhibited by the oil solubilized within the micelle which decreased the micelle volume (representing a decrease in the repulsion between the micelles). 

By using the thin-film balance developed by Schedud-luko and Exerowa (6), the drainage of foam films has been extensively studied. The apparatus employed is shown in Figure 2.16. 

Figure 2.16. The thin-film balance method used for evaluating the stability and drainage of foam films (a) schematic representation of the geometry of films in a foam, which occurs in the measuring cell of the Scheludko-Exerowa system (b) schematic of the set-up used for studying microscopic thin aqueous films (c) a typical interferogram of photocurrent versus time of drainage for the thinning process (adapted from ref. (6)), with permission from Elsevier Science...

A related phenomenon to asymmetric drainage in horizontal cylindrical foam films is that of the so-called marginal regeneration. This process occurs in vertical foam films subject to both capillary suction from the Plateau borders and gravity. It is manifest as an apparent turbulent motion on the margin of foam films where thin elements of film are drawn out of the Plateau border and thick elements are sucked... 

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]. 

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