Foam films diffusion process

Separating the total rate of bubble expansion into its constituents, i.e. the rates of the elementary processes, is a complex issue that can only be solved in some special cases. For example, if the foam films are very stable, the average bubble size will increase mainly as a result of diffusion. If the films are very unstable, the internal collapse will be caused by coalescence. 

The analysis as to whether coalescence or diffusion play a decisive role, using the variation in the distribution function is much more complicated. The curves in Fig. 6.6 show that the polydispersity of the foam rises considerably with time but from the curve type it is not clear which process regulates the course and what is the contribution of each process in the foam collapse. Some indirect data as well as the observations presented in [9], allow to assume that in the initial stage of collapse of polydisperse foams the diffusion transfer is the decisive process, since at the moment of foam formation the films are rather thick and cannot rupture spontaneously. 

The contemporary level of knowledge in foam science enables the solution of many problems related to the kinetics of various processes in a foam (such as film thinning and rupture, foam drainage, diffusion, development of film deformation accounting for the Gibbs and dynamic elasticity, etc.) and to establish the equilibrium conditions of the individual foam elements (films and borders). Thus, it allows the qualitative, and sometime semi-quantitative, interpretation of foam stability. 

Foam films are usually used as a model in the study of various physicochemical processes, such as thinning, expansion and contraction of films, formation of black spots, film rupture, molecular interactions in films. Thus, it is possible to model not only the properties of a foam but also the processes undergoing in it. These studies allow to clarify the mechanism of these processes and to derive quantitative dependences for foams, O/W type emulsions and foamed emulsions, which in fact are closely related by properties to foams. Furthermore, a number of theoretical and practical problems of colloid chemistry, molecular physics, biophysics and biochemistry can also be solved. Several physico-technical parameters, such as pressure drop, volumetric flow rate (foam rotameter) and rate of gas diffusion through the film, are based on the measurement of some of the foam film parameters. For instance, Dewar [1] has used foam films in acoustic measurements. The study of the shape and tension of foam bubble films, in particular of bubbles floating at a liquid surface, provides information that is used in designing pneumatic constructions [2], Given bellow are the most important foam properties that determine their practical application. The processes of foam flotation of suspensions, ion flotation, foam accumulation and foam separation of soluble surfactants as well as the treatment of waste waters polluted by various substances (soluble and insoluble), are based on the difference in the compositions of the initial foaming solution and the liquid phase in the foam. Due ro this difference it is possible to accelerate some reactions (foam catalysis) and to shift the chemical equilibrium of some reactions in the foam. The low heat... 

This brief review includes definition of the structural features of foams. A summary of the processes occurring in foam films follows with particular emphasis on the factors that determine the stability of those films. Finally, we include an outline of the processes of drainage and diffusion-driven coarsening, which concern the entire body of a foam and not just the constituent parts. 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

As diffusion to the surface of a polymer is one of the limiting steps in extraction, the particle size or film thickness of a sample is also important [278,333,337-340]. With the typical diffusion coefficients of additives in polymers a particle diameter of about 0.3 mm is required for an extraction time of about 1000 s at 40 °C. An exception to this is the extraction of thin films and foams, for which the shortest dimension is small. It is not surprising that no more than 50 % of antioxidants could be extracted from PP pellets as opposed to 90 % recoveries from the same polymer extruded into film [341]. Grinding of the polymer is usually an essential step before extraction. Care should be taken to avoid loss of volatile additives owing to the heat generated in such processes. Therefore, cryogrind-ing is preferred. 

If the kinetics of bubble expansion in a foam is determined by coalescence and is described by N = N0cxp(-r/rn), then the probability for film rupture does not depend on bubble area. This contradicts the theories of film rupture [e.g. 57,58] and probably means that there are other processes (along with coalescence) influencing bubble expansion (diffusion or... 

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