Model film rupture

Fig. 8.2 Strain-generated active path mechanisms, (a) Often referred to as the film rupture model and (b) the slip step dissolution model. In both cases growth is by dissolution film rupture is the rate controlling step, not the mechanism of crack growth...

Figure 8.3 Surface correlated wave model for water film rupture.

Schematic representation of crack propagation by the film rupture model (Jones)5...

Contemporary understanding of liquid film rupture is based on the Linear Stability Theory and the concept of existence of fluctuational waves on liquid surfaces [81]. According to this model the film is ruptured by unstable waves, i.e. waves the amplitudes of which increase with time. The rupture occurs at the moment when the amplitude Ah or the root mean... 

Another approach to the rupture of thin liquid films, proposed by Tsekov and Radoev [84,85], is based on stochastic modeling of this critical transition. Autocorrelation functions for steady state [84] and for thinning [85] liquid films were obtained. A method for calculation of the lifetime At and hcr of films was introduced. It accounts for the effect of the spatial correlation of waves. The existence of non-correlated subdomains leads to decrease in At and increase in hcr as a result of the increase in the possibility for film rupture. Coupling of dynamics of surface waves and rate of drainage v leading to stabilisation of thinning films has also been accounted for [86,87]. 

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... 

Logan [42] suggested a film rupture or sHp-step dissolution model for SCC mechanism where the crack is initiated by locahzed anodic dissolution. This mechanism postulates that plastic strain in the metal at the crack tip causes a fracture of the oxide barrier film. The film rupture model (FRM) assumes a repetitive, cycling process of film rupture, dissolution of the underlying metal, and repassivation. 

The flocculation and coalescence processes of a polydis-persed lamella or film can be divided into two processes film drainage and film rupture. To model the film-mpture process of polydispersed emulsions, film stress-relaxation experiments were carried out. In these experiments, the film was quickly expanded and then the relaxation of the film was measured. To characterize the film-drainage process, dynamic film-tension measurements were conducted in which the film was continuously and slowly expanded while the film tension was monitored. Single interfaces were also studied by forming a drop at the eapillary (7). 

An idealised model of the surface of a thin liquid film is one of a monolayer of evenly-distributed surfactant molecules. However, a more realistic model is one where the molecules are not evenly distributed therefore, the surface concentration depends on surface position. The result of this heterogeneous distribution is that gradients of surface concentration, and therefore surface tension, are present. One example of this was pointed out in Section 5.1 on the effect of Marangoni instabilities on film rupture. Regarding film drainage, a surface tension gradient exerts a surface stress that can either impede or... 

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... 

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