Foam stability kinetic factors

Stability of foams and other disperse systems holds a central position in colloid chemistry. Nevertheless, many aspects of this problem are not completely clear. There is no theory as yet that can offer a thorough quantitative explanation of the behaviour of foams with time. It is hardly possible to create such a unified theory, since the dominant thermodynamic and kinetic factors of stability can be different for the different foams. 

The kinetic factors of foam stability as well as of the stability of any disperse system, are mainly determined by the stabilising ability of the surfactant adsorption layers. This action is spread over all structural elements of the foam. 

One of the most important factors regulating the rate of foam collapse (especially coalescence process) is the surfactant kind, along with the additives, both affecting the equilibrium film thickness, film stability, rate of film thinning and rate of drainage. Unfortunately, data about the kinetics of internal foam collapse for foams from various surfactants under comparable conditions are very poor. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

The equilibrium and dynamic aspects of surface tension and adsorption of surfactants at the air-water interface are important factors in foam film stability [82]. Dynamic adsorption models with the diffusion-controlled and mixed-kinetic mechanisms are discussed in some surfactant solution litera-... 

An important factor that controls the final morphology of foams is the crystallization kinetic of the polymer. In the extrusion process, once the melt exits the die, bubbles nucleate first and then they grow in not isothermal conditions. The viscosity increase occurring during the cooling/ crystallization of the polymer is necessary to stabilize the cellular structure and to avoid the collapse and/or the coalescence of the bubbles. For this reason, the investigation of the crystallization behaviour of the polymer should be known in order to optimize die temperature profile in the extruder and at the die. 

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