Liquid film migration

Show schematically and explain a diagram of Gibbs free energy versus composition for liquid film migration. 

We listed many examples early in this chapter, and they can be quite different in practice, from the msting of iron to the toasting of dough to the roasting of ore. The reactant A could be a gas or a liquid, and the film could be a sohd or liquid. The migration of A through the reacted film could be diffusion of A dissolved in C or permeation of A through a porous film of C. We describe this by a diffusion coefficient D s, but the value of D/ s and the mechanism by which transport occurs will not be discussed here. 

As noted above, the best modern silicone gels now contain special organo-functional silane "coupling agents, which are able to form a strong, water resistant, chemical bond to the alumina and conductive surfaces, thereby preventing the formation of a liquid film on the adhesive which could provide a path for migration. 

Figure 8.7. Various microstructures observed after DIIM (a) faceted grain boundaries (Al203-Fe203)/ (b) zigzag migration in Mo-Ni/ and (c) migration reversal of liquid films in Mo-Ni. ...

Rhee, W. H., Song, Y. D. and Yoon, D. N., A critical test for the coherency strain effect on liquid film and grain boundary migration in Mo-Ni-(Co-Sn) alloy, Acta MetalL, 35, 57-60, 1987. 

Ko, J. Y., Migration of intergranular liquid films and formation of core-shell grains in TiC-WC-Ni system, MS Thesis, KAIST, Daejeon, 2003. 

The assumption that all the displaced fluid is completely removed from the film so that it remains plane parallel facilitates calculation but may not be entirely realistic. In practice, liquid may migrate only a small distance from the particle to form a bulge. Calculated rupture times may therefore be overestimates. 

With very slow reactions (such as between carbon dioxide and water) the dissolved molecules migrate well into the body of the liquid before reaction occurs so that the overall absorption rate is not appreciably increased by the occurrence of the chemical reaction. In this case, the liquid film resistance is the controlling factor, the liquid at the interface can be assumed to be in equilibrium with the gas, and the rate of mass transfer is governed by the molecular CO2 concentration-gradient between the interface and the body of the liquid. At the other extreme are very rapid reactions (such as those of ammonia with strong acids) where the dissolved molecules migrate only a very short distance before reaction occurs. The... 

The required sensitivity of the analytical procedure is indicated, for example, by the following considerations. Under the extractability test conditions proposed by the British Plastics Federation on thin films (<0.5 mm) of polymer containing 0.03% wiv of additive, each cubic centimetre volume of plastic is contacted with 20 cm of the extraction liquid. Thus, if all the additive present in the original polymer film migrates during the extraction test into the extraction liquid, at the end of the test this liquid will contain only approximately 15 ppm of additive. Consequently it was necessary to devise a procedure for determining extracted polymer additive in each of the extractants in amounts down to 3 ppm. 

The data in Table 13.9 indicate that, after it had migrated from the polyethylene film, lauric diethanolamide extensively hydrolysed to DEA, especially in the case of the 5% sodium carbonate and 5% citric acid extractants. Also, between 60% and 100% of the original lauric diethanolamide content of the film migrated from the film into the extraction liquid during ten days exposure at 60 °C. 

Of particular interest in the usage of polymers is the permeability of a gas, vapour or liquid through a film. Permeation is a three-part process and involves solution of small molecules in polymer, migration or diffusion through the polymer according to the concentration gradient, and emergence of the small particle at the outer surface. Hence permeability is the product of solubility and diffusion and it is possible to write, where the solubility obeys Henry s law,... 

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