Bulk foam lifetime

Foam Stability Measurements. The average lifetime (LF) is a property of bulk foams that is useful in ranking foams in order of stability. LF is defined as the area under the drainage profile divided by the initial foam height The apparatus used to determine these drainage profiles is shown in Fig. 9. 

Thus, it might be assumed that stabilisation of foam films will depend also on the action of other positive components of disjoining pressure. For example, equilibrium films are obtained from concentrated butyric acid solutions and, therefore, in this concentration range the foam lifetime also increases. On the basis of these concepts it should be expected that a foam consisting of films with equilibrium thicknesses at a constant capillary pressure pa = n, should be infinitely stable. In fact, a real foam decays both in bulk and as a disperse system, due to gas diffusion transfer and certain disturbances (shift of films and borders on structural rearrangement as a result of the collective effects , etc.)... 

As shown in Figure 10.3 for the natural gas-crude oil system, this study revealed a linear correlation (albeit with a correlation coefficient of only 0.84) between increasing bulk phase shear viscosity and increasing foam stability as indicated by the average foam lifetime, Lp, which is defined as... 

The comparison of lifetimes of an isolated and open foam at two values of the pressure drop shows that there is a little difference between them (no more than 15-20%). At Ap = 1 and 10 kPa, the lifetime of an isolated foam is, respectively, 95+5 min and 40 3 min. For an open foam these values are, respectively, 80+3 min and 30+3 min. The small increase in the lifetime of a foam that does not contact the bulk gas phase, and the decay of an open foam, initiated by the upper layers, indicate that the probability of surface film rupture is slightly increased. 

The systematic study of foam bilayers from phospholipids [28,38-40] reveals that they do not rupture spontaneously at any concentration allowing their formation. That is why in the case of phospholipid foam bilayer the dependence of their mean lifetime on the bulk amphiphile concentration cannot be measured in contrast to foam bilayer from common surfactants [41,42], This infinite stability of phospholipid foam bilayers is the cause for the steep W(d) and W(C) dependences. In the case of AF foam bilayers this high stability was confirmed by a very sensitive method [19,43] consisting of a-particle irradiation of foam bilayers. As discussed in Sections 2.1.6 and 3.4.2.2, the a-particle irradiation substantially shortens the mean lifetime of foam bilayers. The experiments showed that at all temperatures and dilutions studied (even at d,), the foam bilayers from AF did not rupture even at the highest intensity of irradiation applied, 700 (iCi. 

The experimental evidence of the role of liquid viscosity in determining nonaqueous foam stability has been provided by several researchers. Brady and Ross (2) found that the foam stability of refined mineral oils increased linearly with kinematic viscosity. McBain and Robinson (5) showed that a high bulk liquid viscosity was often associated with high foam stability. Callaghan and Neustadter (4) measured the foam stability of crude oil foams and reported that the average lifetimes of such foams are almost linearly dependent on bulk viscosity. 

An exception to this concerns the formation of monomolecular crystalline layers on the surfaces of molten alkane waxes at temperatures close to the melting point as reported by, for example, Gang et al. [32], This layer causes increases in the lifetimes of the transient froths formed by such materials. Presumably, the stress at the surface due to such a layer will result in the formation of velocity gradients and therefore shear stress to resist drainage. Gang et al. [32] state, however, that the stabilization of a foam by surface crystallization or surface-induced crystallization of the interior, by a nonsurfactant component, is clearly not common but may be an additional mechanism at work in complex systems where a monolayer crystallizes at a higher temperature than the bulk. ... 

Let us first evaluate the stability of foams formed by whole casein and P-casein under similar experimental conditions (Figure 10.2). It can be clearly seen in the figure that the foam of P-casein is more stable at this bulk concentration. The stability of the foam formed is often estimated by the half-lifetime of the foam (ty, the time taken by the foam to decay to half the original height after the air flow is stopped. Hence, the foam of P-casein (0.1 g/L) has a half-lifetime of 35min whereas the foam of whole casein (0.1 g/L) has a half-lifetime of 15 min. 

Let us first evaluate the behavior of the foam formed by the surfactant and the different effect caused by adding each protein into the bulk solution. The stability of the foams formed by each of the systems presented in this work is characterized by their half-lifetime. Figure 10.3 shows the halflifetime of the foam formed by the pure Tween 20 along with that of the mixed systems. These were obtained with a fixed concentration of protein of 0.1 g/L for both cases and increasing the concentration of Tween 20 between 10" and 10 M. 

Table 10.3 shows the half-lifetime of the foam formed by solutions P-casein as a function of bulk concentration. The foamability of P-casein is very dependant on the bulk concentration of protein. Stable foams of P-casein were only found above a concentration of protein of 0.05 g/L. Then, the stability of the foam increased very steeply with the bulk concentration having a much larger halflifetime for the foam formed with 0.1 g/L P-casein. 

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