Column destruction

In the process of foam evaporation the films on the exterior protect those in the interior from rupturing until they themselves reach the exterior. That is why the rate of foam column destruction vh caused by evaporation would depend on the rate of film evaporation and the liquid volume fraction in films tp /, i. e. 

However, the kinetic dependence of foam column decay proposed by Schwarz appears to be better grounded [8]. No attempts to relate the constants characterising the rate of foam column destruction with both physicochemical characteristics of the foaming solution and foam structure have been reported. The stepwise kinetics of decay of the foam column is typical for aqueous as well as non-aqueous foams. Manegold [32] points out that coarse foams decay in larger steps than finely disperse foams. 

In several cases the lowest foam layers decay very slowly, which seams to be a characteristic feature of the kinetics of foam column destruction. The decrease in border capillary pressure can be regarded as the main reason for such a decrease in the rate of decay in gravitational field (see Section 6.5.2). At low surfactant concentrations the lower foam layers are stabilised, because the surfactant concentrates in them as a result both of internal foam collapse and decay of the upper layers. When the foam is destroyed by addition of antifoams, the delay of this process occurs because the antifoam solubilises in the surfactant solution during the breaking of the foam (see Sections 9.1 and 9.3). 

The most important factor regulating the rate of foam column destruction are the surfactant kind, electrolyte concentration and additives that determined the structural characteristics of the foam (dispersity, film type and thickness, etc.) and foam film stability. 

Influence of Plateau border pressure on foam column destruction... 

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

Fig. 6.11. Kinetic curves of foam column destruction under pressure drop surfactant Triton-X-100 (0.5...

In order to collect information about the influence of the pressure drop on the lifetime of foams with different types of foam films, foam columns of small heights (2-3 cm) were studied. It was found that the time needed to reach hydrostatic equilibrium pressure (the outflow of the excess liquid ceases) was considerably smaller (5-6 times) than the foam lifetime. This is realised at small pressure drops (up to 5-10 kPa). For that reason it is believed that in these experiments the foam column destruction runs mainly under equilibrium conditions (referring to hydrostatic pressure and drainage). 

A more detailed study on foam behaviour and the features of foam column destruction has been performed in [69-71]. Various kinds of surfactants, different foam column heights, foam dispersity and temperatures, were investigated at Ap pgH, including the range of critical pressure drops pcr. The kinetics of establishing a capillary pressure was also accounted for. Used were ionic (NaDoS) and nonionic (Triton-X-100) surfactants as well as some silicon-organic compounds which differed by the number of siloxane, dimethylsiloxane, oxyethylene and oxypropylene groups (KS-1, BS-3 and KEP-2). 

Other experiments showed that the foam column destruction was initiated by the rupture of individual films within the range of maximum pressure (close to the porous plate). For a Triton-X-100 foam this critical pressure was 8-9 kPa. This means that under critical pressure drop corresponding to an avalanche-like foam column destruction, the equilibrium... 

A new experimental technique has been introduced by Kruglyakov et al. [82] and Vilkova and Kruglyakov [83] for the kinetic study of the foam expansion ratio increase and foam column destruction in centrifugal field. It involves increase in the excess pressure in the foam liquid phase and reduction of the time needed to reach high excess pressures. 

A more important fact is the change in the mechanism of foam column destruction with the increase in the applied pressure drop. For example, at small pressure drops a slow diffusion bubble expansion along with the corresponding slow rate of structural rearrangement (either zero or very slow rates of coalescence) occurs in a NaDoS foam with CBF or NBF. This is expressed in the layer-by-layer reduction of foam column height ending with the disappearance of the last bubble layer. In such a foam the critical pressure of the foam column destruction is not reached at any dispersity, and only the foam column height and the rate of internal foam collapse determine the foam lifetime. 

The kinetics of foam collapse, i.e. the process of gas and liquid separation, is characterised by the rate of reduction of foam volume with time or by the rate of decline in its height, if the cross-sectional area is equal along the whole foam column. The stability of the foam as a whole can be characterised quantitatively at any moment by the reciprocal quantity of the rate of foam column destruction. Most often, however, the estimation of the stability of the foam column, is expressed by an integral characteristic time of decay of the whole foam column or a part of it. The relation between the internal foam collapse and the destruction of the foam column is discussed in Section 6.5. Sometimes foam stability is considered in terms of foaming ability of the solution. In general the latter characteristic involves the easiness of foam formation, foam volume and stability. Such an interpretation, however, makes this characteristic rather indefinite. For example, Abramson [12] indicates that for the estimation of the foaming ability of surfactants it is necessary to know the quantity and stability of the foam obtained from a particular surfactant as well as the conditions under which the surfactant acts as a foam stabiliser. That is why it has been repeatedly emphasised that foaming ability... 

Various methods are employed to estimate and compare foam stability with respect to foam column destruction. Most often they are reduced to determination of the lifetime of the foam column (or part of it) up to its complete disappearance. Several earlier works [13,14] dealing with the stability of flotation foams (froths) involved the use of a coefficient of stability... 

The time for foam column destruction under a-particle irradiation is another characteristic of the stability of a foam, subjected to external disturbances. As already mentioned in Chapter 3, the a-particle beam causes the destruction of the foam films that can be easily and precisely dosed. This is demonstrated in Figs. 7.2 and 7.3 [17,18]. 

These physicochemical constants deal with the properties of the foam films. The foaming agents can be described more fully by the parameters related to the foam itself, e.g. foam lifetime xp at constant pressure and the time of foam column destruction by a-particle irradiation Xm [17,75], Table 7.4 presents xp and Xr of various surfactants. 

---