Foam column decay

Usually foam column decay begins from the upper layers, because they release more rapidly the excess liquid and the foam films reach the equilibrium thickness faster. Moreover, the highest capillary pressure is in the upper foam layers (and equals the disjoining pressure in the films at equilibrium). Finally, if the vapour pressure over the foam is smaller than the saturated vapour pressure, the probability for surface film rupture increases due to evaporation. 

Foam column decay is also caused by gas diffusion from the upper layer bubbles into the ambient space and by surface coalescence, i.e. rupture of the surface films. The decrease in foam volume can be achieved layer by layer (each internal layer starts decaying only after the... 

Three typical types of foam column decay due to gravity can be distinguished linear, logarithmic and stepwise kinetics [32], Besides, the foam often decays as a whole after a certain induction period without any visible signs of volume reduction [41]. 

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. 

Fig. 6.10. Kinetic curves of foam column decay in gravitational field curve 1 - NaDoS (103 mol dm 3) +...

The total degradation time is often employed as a characteristic of the kinetics of foam column decay. Sometimes the time needed for breaking a certain part of the foam column (for example, 1/2 or 1/5 of its height) is also used. Obviously, either of these characteristics depend strongly on foam column height and foam dispersity. 

Characteristics of Stability Related to the Foam Column Decay... 

For stable foams with a not very high expansion ratio (at Ap - 1 kPa) a quantitative relation between the rates of internal foam collapse and foam column decay during its entire lifetime is also established (see Eq. (6.50)). Thus, the foam lifetime can be calculated on the basis of the regularities of its internal collapse mainly from the data about the diffusion bubble... 

When the foam is stable and the time for the process is short, the accumulation caused by internal collapse is not substantial. For foams of low stability and prolonged process, the accumulation resulting from internal foam collapse and foam column decay is comparable with that occurring in the adsorption layers. In addition, if accumulation is carried out with increase in the expansion ratio in the course of gravitational drainage or by the method of capillary foam drying, then the excess of the substance that accumulates in the bulk sections... 

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). 

Within 14 min the resistance of a layer with AZ = 1.5 cm increases from 30 to 100 kf2 (curve 1) and the rise is practically linear. This corresponds to an increase in the expansion ratio from 6 103 to 20 103. Even during the first minute the expansion ratio increases about 100 times. This part of the Rf(t) dependence corresponds to the reduction in the foam liquid content and to the increase in the foam capillary pressure without a change in the foam layer thickness, which is confirmed also by the direct observation of the foam state after the centrifuge is stopped. Further on the resistance sharply increases (part A B B of curve 1) due to the decay of the foam column. Within minutes the resistance increases to several Mf2 and... 

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... 

When the foam decays in gravitational field the capillary pressure in its upper parts reduces due to the diminish of the foam column height. Hence, the time of decay of the local layers would be different while to total lifetime of the whole foam column would be an integral characteristic which accounts for the effect of local pressure and the total height of the foam column Ho-... 

For a 10 cm foam column, = 628 min. In order to compare the stability of foams decaying in gravitational field at a capillary pressure corresponding to the particular layer... 

The result of these processes is a gradual decrease in the foam column height (H) in a layer-wise foam collapse or a avalanche -like decay of the foam volume when reaching a critical size of the polyhedral foam cells. 

The experimental determination of the least residual concentration in the process of foam accumulation was conducted in a separation column of 2 cm diameter [84]. The column height was 35 cm, the volume of the foaming agent solution was 20-25 cm3, and a sintered glass filter (POR-100), was used to disperse the gas. The volumetric rate of gas feed was 10015 cm3 min 1. The process of surfactant extraction begins with formation of a steady-state foam layer that has a constant volume V. It depends on the rate of gas feed when the rate of foam formation equals the rate of its decay. The average lifetime of the gas in the foam is... 

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