Thin films drainage behavior

In summary, the results of our thin film drainage study as well as our investigation of oil spreading mechanisms and frequency dependence of dynamic interfacial tension all suggest that the C 2 0S system, which displays the m.ost unstable foam behavior in the presence of oil, should not perform as effectively as the Ci6A0S system in oil displacement experiments in porous media. 

When a quantitative estimate of residual soil is not called for and the suitabiUty of a metal surface for further finishing needs to be assessed, the water-break test is used. The term water-break refers to the behavior of a water film on a smooth greasy surface. When the film becomes sufficiently thin by drainage, it suddenly breaks into islands or droplets between which the surface appears dry. On the other hand, when a film drains from a clean water-wettable, nongreasy surface, it becomes progressively thinner and finally disappears by evaporation without ever breaking into droplets. Such a surface is said to be free from water-break. 

Figure 7. A photographic sequence showing the drainage behavior of a thin film formed from 2 mM SDS in 2 mM sodium phosphate buffer, pH 7.0 containing 0.1 M NaCl. See text for description.

Ivanov and Kralehevsky (1997) agree that the final emulsion type depends on the relative coaleseence rates of oil-in-water and water-in-oil dispersions, but suggest that the rates are determined by the behavior of the thin liquid films formed when two drops approaeh. Consider, for example, a thin film of water between two oil drops. If the surfaetant is preferentially soluble in water and has a low solubihty in oil, it can remain in the film, slow film drainage, and stabilize the film at some equilibrium thiekness or at least hinder film breakup, whieh leads to eoaleseenee. In contrast, if the solubility of the surfactant in oil is high, it can diffuse into the oil, where it is not available to slow film drainage and hinder breakup. A similar argument ean be made for the oil film between two water drops in a water-in-oil emulsion. 

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]. 

The behavior of thin liquid films formed between coalescing drops and bubbles has attracted considerable attention in an attempt to understand the stabilizing mechanisms of emulsions and foams. It is now generally recognized that the drainage of this film plays a crucial role in determining the stability of the dispersion. 

In summary, the interfacial tension, the interfaeial dilatational modulus, and the drainage of thin liquid films are fundamental principles for nnderstanding foams and emulsions (Figure 10.1). The correlation between these magnitudes and the behavior of the resulting colloidal dispersion provides a technological application to fundamental interfaeial seience. 

Whereas Tween 20 displaces completely p-casein from the surface, whole casein appears to be more resistant to the displacement. This feature is reflected in the foam stability and in the drainage of thin liquid films of whole casein/Tween 20 mixtures. The reason for this might be related to the more compact structure of K-casein and its resistance against displacement (Maldonado-Valderrama and Langevin, 2008), also suggesting that this fraction governs the foam stability of whole casein. This section illustrates the important effect of the nature of the components on foam stability of mixed protein/surfactants systems and the important relationship with surface behavior. 

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