Foam drainage gravitational

Approximate calculations for a gravitational foam drainage have shown [5] that at a 0.1 mm, a =30 mN/m and film thickness h = 0.1-0.2 pm (this state is usually reached 5-7 min after foam formation), the amount of liquid in Plateau borders exceeds more than 10 times that in films. This applies for a foam layer situated 4-5 cm above the porous plate. For low dispersity foams (a = 1 mm or more) the condition h r a is fulfilled at this height even at h 1 pm, i.e. practically at the moment of foam formation by the barbotage method. 

The rate of foam drainage at large pressure drops in the foam liquid phase is much higher than that in gravitational field. 

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...

There are several works [1,14,23,61] treating qualitatively the influence that foam structure and foaming solution properties, such as viscosity, surface tension and temperature, exert on the drainage rate. A possibility to treat quantitatively the role of various factors on drainage rate of low expansion ratio foams in gravitational field is given by Eq. (5.46). 

Another example of a lack of correlation is the behaviour of a foam consisting of multilayer films, obtained from concentrated surfactant solution with a solubilised oil [59]. The viscosity of such foaming solutions is much higher compared to that of the solutions of the individual components and the foam drainage rate is considerably lower. However, in gravitational field as well as under applied pressure drop, the stability of such foams is strongly reduced. 

These studies were performed at high surfactant concentrations (> 0.01 mol dm 3) where the appearance of -potential was hard to explain. Unclear remained the fact that f-potential did not depend on the electrolyte concentration (NaCl) up to a value of 3%. Anomalously high values of -potential ( 0.5 V) have been reported by Laniquielli and Galembeck [64] who studied electrokinetic phenomena occurring at gravitational foam drainage. They attribute this anomaly to the electrolyte accumulation in the electrode space. The analysis of these results indicates that reliable values of the -potential in foams can be derived with an especially developed theory of the electrokinetic phenomena in foams that accounts for the peculiarities of these systems. Furthermore, new apparatus is needed to allow the study of foams with controlled border sizes and precisely defined border profile. 

Liquid foams evolve mainly by two processes coarsening and drainage. The first process is slow and becomes effective after many minutes, hours or even days. Drainage under the influence of gravitation is faster and it is usually the main process, which destabilizes foams. 

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. 

A mobile capillary micromanometer that can be set at various levels of the foam column, thus allowing pressure measurements at different points, can be used to evaluate the dependence of the radius of border curvature on the coordinates of either gravitational field or pressure gradient [51,68]. For a simultaneous determination of both the pressure and the radius of border curvature at non-steady state flow (for example, gravitational drainage) a system of a number of micromanometers can be used. 

This formula is analogous to the one obtained previously about gravitational drainage of low expansion ratio foams [21,52]. 

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... 

The accumulation ratio //min increases with the decrease in bubble size only if n = const (Eq. (10.11)). If the expansion ratio changes simultaneously with dispersity (as is often the case under the conditions of gravitational drainage since the decrease in foam bubble size results in a decrease in the rate of drainage and in the formation of a low expansion ratio foam at the outlet of the foam generator), then a decrease in bubble size leads to a decrease in mm rather than to its increase [23,67]. 

Fig. 10.5. Change in the liquid content with time in foams from protein solution at gravitational drainage...

A negative influence of the foam collapse on accumulation ratio and extraction coefficient has also been found [51,67] when separation of foams of various stability was carried out under gravitational drainage. Over a wide range of concentrations of the surfactant being extracted these coefficients decreased as a result of internal foam collapse. 

---