Thin films stability

Foams are thermodynamically unstable. To understand how defoamers operate, the various mechanisms that enable foams to persist must first be examined. There are four main explanations for foam stabiUty (/) surface elasticity (2) viscous drainage retardation effects (J) reduced gas diffusion between bubbles and (4) other thin-film stabilization effects from the iateraction of the opposite surfaces of the films. 

Sharma, A. (1993) Relationship of thin film stability and morphology to macroscopic parameters of wetting in the apolar and polar systems. Langmuir,... 

Figure 1. Schematic diagram showing the possible mechanisms of thin film stabilization, (a) The Marangoni mechanism in surfactant films (b) The viscoelastic mechanism in protein-stabilized films (c) Instability in mixed component films. The thin films are shown in cross section and the aqueous interlamellar phase is shaded.

Thin films stabilized by SDS were selected as the test system during the construction and... 

The solution diffusion properties of FITC-labelled BSA were measured by FRAP [12], The results showed that the protein diffused freely in solution with a diffusion coefficient of approximately 3xl0 7 cm2/s. This was in reasonable agreement with previously published values [36]. FRAP measurements were also made on thin films stabilized by FITC-BSA. The films were allowed to drain to equilibrium thickness before measurements were initiated. Thin films covering a range of different thicknesses were studied by careful adjustment of solution conditions. BSA stabilized films that had thicknesses up to 40 nm showed no evidence of surface diffusion as there was no return of fluorescence after the bleach pulse in the recovery part of the FRAP curve (Figure 14(c)). In contrast, experiments performed with thin films that were > 80 nm thick showed partial recovery (55%) of the prebleach level of fluorescence (Figure 14(b)). This suggested the presence of two classes of protein in the film one fraction in an environment where it was unable to diffuse laterally, as seen with the films of thicknesses < 45 nm, and a second fraction that was able to diffuse with a calculated diffusion coefficient of lxlO 7 cm2/s. This latter diffusion coefficient was 3 times slower than that... 

Correa and Saramago [282] describe the calculation of disjoining pressures for non-aqueous films. In this case the dispersion forces were found to be the most important in determining thin-film stability. 

In pure liquids, gas bubbles will rise up and separate, more or less according to Stokes law. When two or more bubbles come together coalescence occurs very rapidly, without detectable flattening of the interface between them, i.e., there is no thin-film persistence. It is the adsorption of surfactant, at the gas-liquid interface, that promotes thin-film stability between the bubbles and lends a certain persistence to the foam structure. Here, when two bubbles of gas approach, the liquid film thins down to a persistent lamella instead of rupturing at the point of closest approach. In carefully controlled environments, it has been possible to make surfactant-stabilized, static, bubbles, and films with lifetimes on the order of months to years [45],... 

Sharma, B (1998) Equilibrium and Dynamics of Evaporating or Condensing Thin Fluid Domains Thin Film Stability and Heterogeneous Nucleation, ACS J. Langmuir, vol. 14, pp. 4915-4928. 

Some applications have already been discussed in sec. 3.9 in connection with fig. 4.23 we briefly touched on the thin film stability problem and in chapter 5 the role of Gibbs monolayers in wetting will be addressed. Volumes IV and V will also contain several applications. Let us therefore select one illustration, the preparation of (macro-) emulsions, as a paradigm. This topic is of great practical relevance and it is typical in that, as in most applications, Gibbs monolayers under dynamic conditions are involved. 

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. 

Figure 11.5 Thin-film stability dispersion curve of polystyrene film A y 40x10...

Fig. 13.18. The steric repulsive force measured by Lyklema and van Vliet (1978) for thin films stabilized by poly(vinyl alcohol) in water poly(vinyl alcohol) molecular weight I, 143000 2, 42 500.

The deficit in knowledge of thin-film stability makes purely theoretical modeling of emulsion dynamics impos-... 

Shimamoto, K., Hatabayashi, K., Hirose, Y., Nakao, S., Fukumura, T., and Hasegawa, T. (2013) Full compensation of oxygen vacancies in EuTiOs (001) epitaxial thin film stabilized by a SrTiOs suriace protection layer. Appl Phys. Lett, 102 (4), 042902. 

Thin films stabilized by surface forces under static (equilibrium) conditions in dilute surfactant... 

J. I. Martin, Z.-G. Wang, and M. Schick, Effects of polymer brush self-assembly on spreading and thin film stability, Langmuir, 12, 4950 [1996]. 

Equations 11.4 predict X and x as well as N for instabilities driven by nonretard van der Waals interaction, and are widely used for the interpretation of thin film stability and determination of interaction potentials from measurements of X, obtained from experimental results [65, 91]. 

Figure 12.3 (a) Thin film stabilized by surfactants with hydrophilic head groups and... 

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