Drainage, in foams

The theoretical analysis indicated that asymmetric drainage was caused by the hydrodynamic instability being a result of surface tension driven flow. A criterion giving the conditions of the onset of instability that causes asymmetric drainage in foam films was proposed. This analysis showed as well that surface-tension-driven flow was stabilised by surface dilational viscosity, surface diffusivity and especially surface shear viscosity. 

Recently Joye et at. [74] have reported a numerical simulation of instability causing asymmetric drainage in foam films. The results obtained confirmed the rapid increase in drainage rate. 

FIGURE 12.4. Film drainage in foams can be modeled as a thin film of liquid of thickness, 8, between two parallel vertical plates of width, w. The rate of hydraulic (gravity-driven) drainage will depend on the viscosity of the liquid as well as other factors, as is found in real foams [Eq. (12.3)]. 

Abbreviations of prominent use properties of the various classes of commercial surfactants are shown in Table 1. Antimicrobial activity includes germicidal, bactericidal, and bacteriostatic effects emolliency describes lubrication or a soft feel imparted to skin by surfactants a hair conditioner is a substantive surfactant appHed from aqueous solution to impart a lubricating or antistatic effect and opacifters are used to thicken hand-dishwashing products and cosmetic preparations to convey an appearance of high concentration and to retard solvent drainage from foam. 

Diverse foam structure applications In foam rubber, foamed polymers, shaving foams, milk shakes, and whipped creams, slowly draining thin liquid films (TLF) are needed. Accordingly, the rate of drainage is the most important factor in such industrial foam applications. 

Nikolov AD, Wasan DT, Denkov ND, Kralchevsky PA, Ivanov IB (1990) Drainage of foam films in the presence of nonionic micelles. Prog Colloid Polym Sci 82 87-98... 

To understand drainage we have to discuss the pressure inside the liquid films. At the contact line between liquid films, a channel is formed. This is called the Plateau border. Due to the small bending radius (rP in Fig. 12.18), there is a significant Laplace pressure difference between the inside of the compartment and the liquid phase. The pressure inside the liquid is significantly smaller than in the gas phase. As a result, liquid is sucked from the planar films into the Plateau s border. This is an important effect for the drainage of foams because the Plateau borders act as channels. Hydrodynamic flow in the planar films is a slow process [574], It is for this reason that viscosity has a drastic influence on the evolution of a foam. Once the liquid has reached a Plateau border the flow becomes much more efficient. The liquid then flows downwards driven by gravitation. 

One of the simplest representations for foam drainage, in terms of the drained volume of liquid, is given by ... 

If measurements are carried out in the same sample at consecutive time intervals the rate of drainage, the rate of foam collapse, the changes in foam volume and the gas fraction in the foam can be determined in addition to the evolution of the bubble-size distribution. 

The techniques employed in foam drainage investigations involve the determination of liquid quantity that drains from the foam per unit time. Different foam numbers were introduced as indirect drainage characteristics of low expansion ratio foams produced by the... 

Fig. 5.10 shows the change of qexp /qtb ratio with decrease in border radius during drainage of foams with CBF and NBF. The size of border radius at which a total immobility of border surfaces is achieved is determined by extrapolation of the qtxp (rmin) dependence to < cxP/< th = 1. It is found that qexf> = qA for a foam with CBF when r, 3.5 Jim. 

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]... 

Analysis of experimental data about drainage of low expansion ratio foams [24,67] in which the flowing process does not start at the moment of foam formation, as well as the fact that the AVL t (t) curve lacks an inflection point (or, respectively, a maximum of rate dVJdx), proves that Eq. (5.46) is one of the simplest and physically well grounded kinetic dependences of foam drainage in gravitational field. 

Fig. 5.13. Kinetic curves of foam drainage in gravitational field curve 1 - foam from 2% saponin...

Rate of drainage of foams from surfactant solutions with various viscosities (the experimental conditions are given in Fig. 5.17)... 

Increase in foam expansion ratio under constant dispersity leads to decrease in drainage rate because the radius of Plateau borders diminishes. Indeed, Eq. (5.60) indicates that the change in drainage rate is inversely proportional to the square of expansion ratio and... 

The foam column height influences strongly the drainage rate. As it was mentioned, the initial moment of liquid outflow from the foam (drainage initiation) is determined by the critical values of expansion ratio, dispersity and height of the foam. It follows from Eq. (5.60) that if two foams have equal expansion ratio and dispersity, then their initial drainage rates should not depend on foam column height. However, with the decrease in foam column... 

Thus, at equal expansion ratio and dispersity, respectively, the drainage rate will strongly decrease with the decrease in foam column height. 

With temperature lowering the drainage rate increases despite of the increase in foaming solution viscosity. This can be explained by the fact that with the temperature lowering not only viscosity but surface tension also increases, leading to expansion of foam bubbles and increase in drainage rate, according to Eq. (5.60). 

In the presence of some coagulated soles it is possible to produce stable foams in which there are no surfactants [89]. Usually it is assumed [83] that the presence of solid particles in foams leads to delay of drainage because the Pateau borders become narrower, their walls become rougher and the particles form local caps. An analogous delay of drainage results from emulsion drops that enter the Plateau borders [90]. However, as pointed out in [84], the influence of solid particles on foam drainage rate should not be considered synonymously. 

Drainage of foam containing oil drops is discussed in Section 7.11. 

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. 

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