Foam films mechanical model

Mechanical model of the foam film and the adjacent meniscus [21]... 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

The mechanism of Ca2+ binding is not clear yet. However, increase in repulsive double layer forces between neutral diacylphosphatidylcholine bilayer in aqueous media in the presence of divalent ions has been identified by other methods as well [293-296]. These systems differ from the foam- film model by virtue of their interface ordered lipid phase/water in place of the air/water interface of foam films. Nevertheless, the CaCb concentration where the transition from NBF to silver films is observed in experiments with foam films is very close to the concentrations where increase in the distance between the bilayers was found [293,294,296]. Results with microscopic films are also in good agreement with the established increase in the free energy of formation of macroscopic films stabilised with lysolecithin in the presence of CaCl2 [287]. 

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... 

A single model of foam-oil interaction cannot account for all situations. Certain foam—oil sensitivity models can be reconciled with both microvisual studies and core-flood foam effectiveness measurements, all for a wide variety of foams, oils, porous media, and other experimental conditions. However, exceptions are readily found. In an earlier section, the models of emulsification—imbibition, pseudoemulsion film thinning, entering, and spreading were introduced. Cases in favor of, and exceptions to, the applicability of each of these can be found in the literature. Although this situation prompts some inclination to search for additional mechanisms, the truth may be that all the models presented have some validity and that one or another valid mechanism is most significant in a given situation. 

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-... 

One possible explanation of the observed dependence might be that the film rupture in om systems occurs by passing below the barrier II (Fig. 13c). Indeed, Bergeron [45] showed with large planar foam films (studied by the porous plate method) that in some systems II aI corresponded to an actual maximum of the calculated curve IlAs(fj), whereas in other systems ns was well below the maximum of the calculated nAs(fj) cmves (for a possible explanation see Ref. 45). Such a possibility is offered by different theoretical models of film rupture, in which the formation of imstable spots in large liquid films by various mechanisms is considered [40,45-47]. However, all these models are developed for large planar films and cannot be applied directly to om system without a careful analysis of the role of film curvature in the film rupture process. Fmther experimental and theoretical work is under way to reveal the actual mechanism of film rupture, to develop an adequate model of this process, and to explain the observed linear dependence of Has versus 1// eef-... 

In order to understand the nature and mechanisms of foam flow in the reservoir, some investigators have examined the generation of foam in glass bead packs (12). Porous micromodels have also been used to represent actual porous rock in which the flow behavior of bubble-films or lamellae have been observed (13,14). Furthermore, since foaming agents often exhibit pseudo-plastic behavior in a flow situation, the flow of non-Newtonian fluid in porous media has been examined from a mathematical standpoint. However, representation of such flow in mathematical models has been reported to be still inadequate (15). Theoretical approaches, with the goal of computing the mobility of foam in a porous medium modelled by a bead or sand pack, have been attempted as well (16,17). 

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

In contrast to bulk foam, in a porous medium the majority of films move separately so their rupture under certain critical pressure does not occur simultaneously, i.e. the films are not affected by the rupture of other films (there is no collective effect). On the other hand, moving films are subjected to the oscillation of thickness as well as to other mechanical effects. Hence, their critical pressure should be lower than that of the static foam and should depend also on the rate of movement due to the dynamic effects. Analogous reduction in the critical pressure is observed when a bulk foam advances [47] and when a foam is placed in a centrifugal field [183]. The influence of the dynamic effects on the critical pressure has been explained by the model of Jimenez and Radke [175]. 

The polyurethane foam model of a shark used in the Jaws films became dirty with both use and degradation and appears discoloured as a result (upper image). Mechanical cleaning of selected areas improved its appearance (lower image). 

Atto-engineering for more than a whole century is in permanent and almost infinite development. Theoretical background is related to the surface physics and chemistry, quantum and wave mechanics, and quantum electrodynamics. Discrete and constrained discrete models are convenient for describing related events. Tools and equipment are nano- and atto-dispersions and beams (demons, ions, phonons, infons, photons, electrons), ultra-thin films and membranes, fullerenes and bucky tubules, Langmuir-Blodgett systems, molecular machines, nano-electronic devices, and various beam generators. Output is, generally, demonstrated as finely dispersed particles (plasma, fluosol-fog, fluosol-smoke, foam, emulsion, suspension, metal, vesicle, dispersoid). 

Of the three mechanisms, hydrodynamic drainage due to gravity is usually the most rapid and, if the foam is particularly unstable, leads to total collapse before other mechanisms can become important. In those cases, once the loss of liquid from the lamellar layer produces a critical thickness of 5-15 nm, the liquid film can no longer support the pressure of the gas in the bubble, and film rupture occurs. As a model for gravity drainage, a film may be treated as a vertical slit of thickness S (not to be confused with the solubility parameter... 

It is known that foam becomes more stable when the surfactant concentration increases above a value c at which Newton black films start to form [17]. This is correlated with the rapid increase in surface coverage and the Gibbs elasticity above a concentration a called the Szykowski concentration, c a. It can be conjectured that the rupture below c = a occurs via the Vrij-Scheludko model, whereas other interpretations need to be foimd for the rupture of the Newton black films. Exerowa and coworkers [17] proposed a mechanism involving the nucleation of vacancies. In this model, it is assumed that the chemical potential of the surfactant is different in the... 

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