Real foam stability

Individual structural elements of the foam, such as films and borders, can be under hydrostatic equilibrium and can correspond to a true metastable state. Therefore, when there is no diffusion expansion of bubbles in a monodisperse foam, its state can be regarded as metastable in the whole disperse system. Krotov [5-7] has performed a detailed analysis of the real hydrodynamic stability of polyhedral foam by solving two problems determination of... 

Let us consider the simplest model of a real foam built up of different by size bubbles but having equilibrium films. If the diffusion expansion of bubbles runs slowly, then the real (aggregative) stability with respect to coalescence will be preserved but the column height and dispersity will decrease as a result of gas diffusion transfer between bubbles in the foam and from the upper bubbles to the surrounding medium (if the foam is open). 

The formation of ordered microstructure in thin liquid films offers a new mechanism for the stabilization of foams. As a proof of microstructuring in real foams, Figure 17 shows a photograph of an aqueous foam system stabilized because of the stratification in the foam bubble lamellae. The practical importance of the film microstructuring is that the lifetimes of foams with stratifying films are much longer. 

A number of fundamental works are dedicated to the investigation of foam stability [6, 17] but the interest of the investigators in this problem is persistent, which is evidenced by recent review papers, e.g. [20]. In this section data are given which can be of interest for the practical application of foams. First of all, several processes take place simultaneously in real foams, which lead to foam collapse. The main processes are redistribution of the disperse medium in differently high foam column layers and the change of the mean radius of the foam cells [12]. 

Changing the shape of the dispersed species while flowing also has an impact. Since emulsion droplets and foam bubbles are not rigid spheres, they may deform in shear flow. In the cases of electrostatically interacting species, or those with surfactant or polymeric stabilizing agents at the interface, the species will not be noninteracting, as is assumed in the theory. Thus, Stokes law will not strictly apply and may underestimate or even overestimate the real terminal velocity. 

Monolayers of micro- and nanoparticles at fluid/liquid interfaces can be described in a similar way as surfactants or polymers, easily studied via surface pressure/area isotherms. Such studies provide information on the properties of particles (dimensions, interfacial contact angles), the structure of interfacial layers, interactions between the particles as well as about relaxation processes within the layers. Such type of information is important for understanding how the particles stabilize (or destabilize) emulsions and foams. The performed analysis shows that for an adequate description of II-A dependencies for nanoparticle monolayers the significant difference in size of particles and solvent molecules has be taken into account. The corresponding equations can be obtained by using a thermodynamic model developed for two-dimensional solutions. The obtained equations provide a satisfactory agreement with experimental data of surface pressure isotherms in a wide range of particle sizes between 75 pm and 7.5 nm. Moreover, the model can predict the area per particle and per solvent molecule close to real values. Similar equations were applied also to protein monolayers at liquid interfaces. 

The correlation between the stability of single O/W emulsion films, single drops under oil/water interfaces and real emulsions found in [514,516] also deserves attention. As revealed in the beginning of this Section the correlation between emulsions and emulsion films was studied in various aspects and always provide information about stability of such systems. Model studies of emulsion systems are worth further development especially if the correlation films/real emulsion is done at definite conditions which are as close as possible in both cases, for example, at equal capillary pressure, film size, emulsion dispersity, etc., as it is done in the correlation foam films/foam (see Chapter 7). 

The real challenge in polyurethane foam formation is to control the chemical and physiochemical processes up to the point where the material finally sets. The sequence and the rate of the chemical reactions are predominately a function of the catalyst and the reactivity of the basic raw materials, polyol and isocyanate. The physiochemical contribution to the overall stability and processability of a system is provided by the silicone surfactants. Optimum foaming results will be achieved only if the correct relationship between chemistry and physics exists [4]. 

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