Structural parameters, foams

FOAM STRUCTURAL PARAMETERS AND RELATED PROPERTIES TECHNIQUES FOR DETERMINATION... 

It has been established that the rise of the NaCl concentration in the Volgonate and NaDoS solutions leads to a decrease in surface tension and initial drainage rate and to an increase in foam dispersity and lifetime. The addition of dodecanol has the most significant effect on all foam structural parameters, rate of drainage processes and increase in bubble size. Fig. 10.12 depicts the dependence xR versus dodecanol concentration of a foam from alkylsulphonate (C = 0.2%). 

Chapter 4 Foam Structural Parameters and Related Properties Techniques 345... 

Foam is a disperse system in which the dispersed phase is a gas (most commonly air) and the dispersion medium is a liquid (for aqueous foams, it is water). Foam structure and foam properties have been a subject of a number of comprehensive reviews [6, 17, 18]. From the viewpoint of practical applications, aqueous foams can be, provisionally, divided into two big classes dynamic (bubble) foams which are stable only when gas is constantly being dispersed in the liquid 2) medium and high-expansion foams capable of maintaining the volume during several hours or even days. In general, the basic surface science rules are established in foam models foam films, monodisperse foams in which the dispersed phase is in the form of spheres (bubble foams) or polyhedral (high-expansion foams). Meanwhile, real foams are considerably different from these models. First of all, the main foam structure parameters (dispersity, expansion, foam film thickness, pressure in the Plateau-Gibbs boarders) depend... 

The rapid coalescence can proceed differently, depending on the foam structure parameters (see Fig. 6.5). For foams whose initial expansion is higher than the equilibrium expansion, the decrease in dispersity does not lead to a local expansion change. Those foams the expansion of which is lower than the equilibrium expansion are characterized by a simultaneous sharp increase in the mean equivalent cell radius and the local expansion. For foams whose expansion is higher than the equilibrium expansion obtained experimental data indicate that the rapid decrease in foam dispersity corresponds to the time before the establishment of hydrostatic equilibrium when there is a rapid outflow of the dispersion medium from a low-expansion foams and its redistribution through the foam column height. 

At atmospheric pressure the foam structure depends on the formulation and processing parameters. Foamed sheets and imitation leathers can be manufactured by this process. 

Along with all the advantages, the refined blend structure is more sensitive towards the foaming parameters as foaming temperature and time. The higher number of cells and the elastomeric PB block promote rapid cell coalescence and collapse at elevated foaming temperatures and times. These phenomena can result in an increase in foam density and open-celled or inhomogeneous foam structures. 

Usually the estimation of structural parameters of foam formed by dispersion through gauzes is done on the basis of liquid and gas material balance [36,38]. Such calculations do not account for the properties of foaming solution and capillary pressures during the process of foam formation. That is why they cannot give reliable results. 

The polyhedral foam consists of gas bubbles with a polyhedral shape the faces of which are flat or slightly bent liquid films, the edges are the Plateau borders and the edge cross-points are the vertexes (see Chapter 1). In the study of the physicochemical characteristics of foams there are several techniques that involve the analytical dependences of these characteristics and the structural parameters of foams. 

Relation between the Geometrical (Structural) Parameters of a Foam and its Physicochemical Characteristics... 

In order to compare the structural parameters of the foam model studied by Kruglyakov et al. [18] with the respective parameters of a real polydisperse foam (individual bubbles of different degree of polyherdisity) Kachalova et. al. [19] performed measurements of the average border radius of curvature of foams with variable expansion ratio. The foam studied, generated by the set-up shown in Fig. 1.4, was obtained from a nonionic surfactant solution of Triton-X-100 (a commercial product) to which NaCl (0.4 mol dm 3) was added. The expansion ratio was determined conductometrically with correction of the change in electrolyte concentration due to the internal foam destruction. The electrolyte concentration... 

The theory of luminous flux extinction by a foam, introduced by Krotov and Kruglyakov [63] (see Section 8.4.), gives the relation between optical density and structural parameters of foam (for border foam )... 

This constant wo characterises the initial volumetric flow rate referring to a unit cross-sectional foam area. A rigorous analytical dependence of the constant wq on the structural parameters of a low expansion ratio foam and the properties of the solution (dp IdH, H, r, R, n, a) has not been derived, so that this constant cannot be calculated. 

A semi-quantitative estimation of the influence of the structural parameters and physicochemical properties of the foaming solution on the initial drainage rate can be obtained from the equation describing the drainage in a homogenous polyhedral foam, the liquid of which flows out only through the borders [7]... 

Special cells have been constructed for the study of the optical properties of foams [75], They allow a simultaneous determination of the foam optical density, the pressure in Plateau borders and the electrical conductivity of the foam. Furthermore, the foam produced by these cells is homogeneous with respect to all structural parameters (border radius and bubble size). The homogeneity of the foam formed is achieve by glass filters which provide foam drying and control on the formation of air spaces. 

The relation between the structural parameters of low expansion ratio foams is given by Eqs. (4.24) and (4.26)... 

These expressions give only qualitative indication for the possible direction in which the structural parameters can be regulated. For example, it follows from them that the same expansion ratio can be achieved in a foam with fine bubbles and, respectively, thin borders and films as well as in a coarsely disperse foam with thick borders and films. 

The initial expansion ratio and dispersity of polyhedral foams are related through the quantitative dependence, given by Eq. (4.9). There at Ap > 103 Pa the content of the liquid phase in the films can be neglected. Thus, the connection of the structure parameters n, a and r can be expressed by the simple relation in Eq. (4.10). It follows from it that under given foaming conditions a definite expansion ratio can be reached by changing the border pressure, foam dispersity and surface tension of the foaming solution. 

Equations connecting the accumulation ratio to parameters of foam structure (dispersity, expansion ratio, radius of curvature of Plateau borders) can be derived from the balance equations of the surfactant and the liquid phase, and the data on foam structure. 

As mentioned in Chapter 5, the analytical dependence of the initial volumetric rate of drainage w0 on the structural parameters of a foam is unknown. Therefore, a semi-quantitative estimation of the effect of both the structural parameters of a foam and the properties of a foaming solution can be done using the Eq. (5.60) of the flow rate in a homogeneous polyhedral border foam [59]. 

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