Dynamic foams stability

Figure 10. Comparison of the critical-capillary-pressure data of Khatib, Hirasaki and Falls (5) (darkened circles) to the proposed dynamic foam stability theory (solid line). Best fitting parameters for the constant-charge electrostatic model are listed.

For transporting foam, the critical capillary pressure is reduced as lamellae thin under the influence of both capillary suction and stretching by the pore walls. For a given gas superficial velocity, foam cannot exist if the capillary pressure and the pore-body to pore-throat radii ratio exceed a critical value. The dynamic foam stability theory introduced here proves to be in good agreement with direct measurements of the critical capillary pressure in high permeability sandpacks. 

Figure 2.17 Illustration of a dynamic foam stability test apparatus in which foam is generated by flowing gas through a porous diffuser. Not drawn to scale.

The foaming capability and foam stability obtained from sparkling wines is usually tested by a dynamic foam stability method, as discussed in Section 2.6.2. Because these foams are evanescent and not really very stable, at least compared with the foams found in other industries, dynamic rather than static foam tests are the most suitable. In one version of the dynamic foam test, the Mosalux method, the foam heights are automatically measured using infrared beams and sensors [848],... 

Those factors that were previously mentioned that produce finer-textured foams also produce more stable foams. Factors such as surfactant type, concentration, increasing pressure, and higher inputs of mechanical energy generate more stable foams. For higher temperatures such as those that exist downhole, dynamic foam stability relies upon surfactant type and concentration rather than the addition of thickeners (polymer stabilizers). It is not known what rates are necessary to maintain dynamic stability in fractures, or whether those conditions typically exist. 

Foaminess. A measure of the persistence of a foam (the time an average bubble exists before bursting). Ideally independent of the apparatus and procedure used, and characteristic of the foaming solution being tested. In practice these ideals have not been achieved but some approaches to determining foaminess using dynamic foam stability tests have been reviewed by Bikerman [J3]. See also Dynamic Foam Test. 

The dynamic foam stability is usually measured by the volume of foam at a specific equilibrium flow rate, while the static foam stability is measured by the rate of collapse. Dynamic measurements are particular relevant for transient foams, while for foams of high stability, the static or equilibrium methods are usually more useful, particular for highly stabilized foams such as protein-stabilized foam systems. 

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

Any of several methods for assessing foam stability in which one measures the rate of collapse of a (static) column of foam. See also Dynamic Foam Test, Foaminess. 

Dynamics and stability of thin foam films have been and continue to be an object of intensive research [e.g. 28-35]. Model studies with vertical large macroscopic films with linear sizes of the order of centimeters as well as with horizontal circular microscopic films with radius of the order of millimeters were performed. The kinetics of thinning of vertical macroscopic films in described in detail in [33]. Some of the results presenting an interpretation of the dynamic properties of films and foam are considered in Chapter 7. Microscopic foam films offer certain advantages with respect to treatment of stability of foams and foam films, since the systems studied behave under strictly defined conditions. 

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. 

Dynamic foams are these in which a state of dynamic equilibrium between the rate of foam formation and the rate of foam collapse is reached. The behaviour of such foams as well as the reasons for their stability has been considered in many monographs [e.g. 13,25,39-41,113,114], An analysis of the stability of dynamic foams based on the properties of foam films is presented here. 

As a stability characteristic of dynamic foams Bikerman has proposed the average time of gas retention in the foam (see Eq. (7.19)). In order to determine the foam stability by Bikerman s method along with cylindrical vessels (A = const), conical shaped foam vessels (a... 

The surface elasticity force is considered as the most important factor of stability of steady-state foams [113]. In the model of Malysa [123] it is assumed that a dynamic foam is a non-equilibrium system and phenomena occurring in the solution have an influence on the formation and stability of the foam. The foam collapse takes place only at the top of the foam bubbles at thickness larger than 100 nm, where fl = 0. So, the lifetime of the bubbles at the... 

In the study of the accumulation of gelatine and its mixtures with NaDoS it has been established [80] that the maximum degree of accumulation is reached at gelatine/NaDoS ratio of 1.74. This corresponds to formation of gelatine-surfactant complexes [81,82]. The formation of complexes raises considerably foam stability [80]. Under static conditions and using a porous plate cell, the accumulation ratio reaches 100-150 at the isoelectric point of gelatine (pH = 4.8). In the absence of NaDoS, however, and under the optimum conditions (concentration 0.01-0.02% and pH = 4.8), = 10-24. Under dynamic conditions (in a moving... 

In order to investigate foam stability, it is important to consider the role of the plateau border under dynamic and static conditions. Foam films with intermediate life times - that is, between unstable and metastable foams - should also be considered. 

The adsorbed surfactant film is assumed to control the mechanical-dynamical properties of the surface layers by virtue of its surface viscosity and elasticity. This concept may be true for thick films (>100 run) whereby intermolecular forces are less dominant (i.e., foam stability under dynamic conditions). Surface viscosity reflects the speed of the relaxation process which restores the equilibrium in the system after imposing a stress on it. Surface elasticity is a measure of the energy stored in the surface layer as a result of an external stress. 

Experimental Assessment of Foam Stability. Usually foam stability has been tested by one of three methods (4, 6, 13) (1) the lifetime of single bubbles (2) the steady-state (dynamic) foam volume under given conditions of gas flow, shaking, or shearing or (3) the rate of collapse of a (static) column of foam generated as described. 

In a typical dynamic foam test, foam is generated by flowing gas through a porous orifice into a test solution, as shown in Figure 15. The steady-state foam volume maintained under constant gas flow into the column is then measured. There are many variations of this kind of test (4, 40), including an ASTM standard test (41). This technique is frequently used to assess the stability of evanescent foams. 

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