Macroscopic foam films

Macroscopic foam films are obtained either when glass or platinum frames are drawn out from a solution or when two uniaxial cylinders are drawn apart or when two concentric cylinders are withdrawn from a solution (or when the liquid level is lowered). Furthermore,... 

The simplest and the most widely used method of formation of macroscopic foam films involves drawing out a large film in a frame from a solution (Fig. 2.13). 

Various physical methods are employed in the study of macroscopic foam films obtained with a frame. 

The CBF/NBF transition can also be found by studying the properties of macroscopic foam films with respect to their dependence on certain parameters. For instance, h(Cei) and h(T) dependences of NaDoS macroscopic films have been plotted in [308]. The ranges of electrolyte concentration and temperature where CBF and NBF are stable were determined. 

Elasticity can be measured either with macroscopic vertical or horizontal foam films [94-97] or with individual foam bubbles [98]. 

Dynamics and stability of thin foam films have been and continue to be an object of intensive research [e.g. 28-35]. Model studies with vertical large macroscopic films with linear sizes of the order of centimeters as well as with horizontal circular microscopic films with radius of the order of millimeters were performed. The kinetics of thinning of vertical macroscopic films in described in detail in [33]. Some of the results presenting an interpretation of the dynamic properties of films and foam are considered in Chapter 7. Microscopic foam films offer certain advantages with respect to treatment of stability of foams and foam films, since the systems studied behave under strictly defined conditions. 

Early studies of rupture of unstable thin films have been performed with macroscopic emulsion films [94] and foam films [53]. Very high values for hcr were obtained (of the order of 10 pm). Systematic investigations with microscopic films [e.g. 29,64,73] have shown that their critical thickness is considerably smaller. The probability character of rupture is illustrated by the curves in Fig. 3.12. As it is seen the most probable critical thickness increases with the increase in film radius. The most probable critical thickness of rupture is 30 nm (r = 0.1 mm). Usually such a thickness is reached by films from aqueous solutions of low molecular fatty alcohols at which the surfactant concentration is chosen so that the surface tension is equal in all cases [29,73]. Aniline films exhibit a higher hcr 42 nm. 

Lyklema and Mysels [168] have studied equilibrium macroscopic vertical foam films from sodium octylsulphate aqueous solution containing KC1. The film thicknesses comprised a large range from 80 to 8 nm and electrolyte concentrations from 103 to 1 mol dm3 (Fig. 3.18). 

The mechanism of Ca2+ binding is not clear yet. However, increase in repulsive double layer forces between neutral diacylphosphatidylcholine bilayer in aqueous media in the presence of divalent ions has been identified by other methods as well [293-296]. These systems differ from the foam- film model by virtue of their interface ordered lipid phase/water in place of the air/water interface of foam films. Nevertheless, the CaCb concentration where the transition from NBF to silver films is observed in experiments with foam films is very close to the concentrations where increase in the distance between the bilayers was found [293,294,296]. Results with microscopic films are also in good agreement with the established increase in the free energy of formation of macroscopic films stabilised with lysolecithin in the presence of CaCl2 [287]. 

The model approach implies bearing in mind that the foam is not a sum of individual films and the results obtained on model films cannot be directly applied to foams. Moreover, the different types of foam films (macroscopic, microscopic, horizontal and vertical, as well as foam bubbles) yield very different information about foam stability that has to be considered when the respective foam properties are compared. 

This review focuses on two different types of polyelectrolyte membrane. The first part deals with thin free-standing liquid films formed from aqueous polyelectrolyte/surfactant solutions. Free-standing films are interesting in two respects. First, the film can be considered as the building block of a foam so that its properties affect the behavior of the whole macroscopic foam. In this context it is the molecular structure at and near the film surfaces rather than the structure of the film core which is important. Second, the free-standing film presents a slit pore which enables study of the effect of geometrical confinement on the structuring of polymers. The liquid free-... 

Prins and Clint et al. developed a method of contact angle measurement for macroscopic flat foam films formed in a glass frame in contact with a bulk liquid. They measured the jump in the force exerted on the film at the moment when the contact angle is formed. A similar experimental setup was used by Yamanaka for measurement of the velocity of motion of the three-phase contact line. 

An extended work, experimental as well as theoretical, on mechanisms in thin microscopic foam lamellas and macroscopic foams has been published by Krugljakov Exerowa (1990). In the theory of Exerowa Kashiev (1986) thin-film stability is explained in terms of lateral diffusion of vacancies in a lattice like adsorption layers. As an example, selected experimental results of Exerowa et al. (1983) are shown in Fig. 3.18. 

As will become evident from the discussion that follows, essentially only the value of Aafh) in Equation 1.32 is of interest to us. For this reason, Aa h) can be viewed as a primary characteristic describing film properties and the interactions between surfaces. Unfortunately, for solid planar parallel surfaces, such measurements are nearly impossible the experimental integration of n(fi) between macroscopic surfaces requires that the surfaces remain flat and parallel to the precision of fractions of an angstrom in the course of measuring very small forces. While this is impossible for solid surfaces, such measurements are quite possible and indeed broadly utilized in the investigation of liquid films emulsion films, foam films, and wetting films. In all of these cases, a flat and parallel state can be maintained because of the high mobility of the interfaces. 

This effect could concern the size of the films (of 2 cm ), which are likely to be significantly larger than those of foam films generated by, for example, sparging. This could mean the presence of high relative numbers of antifoam entities in the macroscopic films so that even a small probability of one being effective could lead to film rupture. 

The agreement between the drainage and the models based only the flow inside the Plateau borders and the vertex show that the thin films do not participate directly in the liquid transport, and can be neglected when one considers the drainage of a macroscopic foam. This has also been found in numerical simulations, and it is consistent with the fact that the liquid contained inside the films is always much smaller than the one inside the Plateau border network. However, there is some coupling between the downward flow inside the PB and circulation motions often seen inside the films. The result of this... 

The chapter addresses the ordering of Si nanoparticles in thin liquid films. While hydrophilic Si nanoparticles order in the film core perpendicular to the outer film surfaces, partially hydrophobized Si particles can also adsorb at the (fiuid) surface of foam films or emulsion films. Lateral ordering becomes important for the stability of the respective macroscopic systems (foams and emulsions). 

The area of colloids, surfactants, and fluid interfaces is large in scope. It encompasses all fluid-fluid and fluid-solid systems in which interfacial properties play a dominant role in determining the behavior of the overall system. Such systems are often characterized by large surface-to-volume ratios (e.g., thin films, sols, and foams) and by the formation of macroscopic assembhes of molecules (e.g., colloids, micelles, vesicles, and Langmuir-Blodgett films). The peculiar properties of the interfaces in such media give rise to these otherwise unlikely (and often inherently unstable) structures. 

Because of the non-regularity of the polyhedral foam structure (lack of long-range order) the foam becomes macroscopically isotropic, the specific surface area (per unit volume) accepting the luminous flux, is uniformly distributed in direction normal to the films... 

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