Drainage of foam films

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... 

Under the action of an outer driving force, the flnid particles approach each other. The hydro-dynamic interaction is stronger at the front zones and leads to a weak deformation of the interfaces in this front region. In this case, the nsnal hydrodynamic capillary number, Ca = V[VJa, which is a small parameter for nondeformable surfaces, should be modified to read Ca = y VJiJch, where the distance, h, between the interfaces is taken into account. The shape of the gap between two drops for different characteristic times was calcnlated numerically by many authors. " Experimental investigation of these effects for symmetric and asynunetric drainage of foam films were carried out by Joye et In some special cases, the deformation of the fluid particle can be... 

By using the thin-film balance developed by Schedud-luko and Exerowa (6), the drainage of foam films has been extensively studied. The apparatus employed is shown in Figure 2.16. 

Figure 2.16. The thin-film balance method used for evaluating the stability and drainage of foam films (a) schematic representation of the geometry of films in a foam, which occurs in the measuring cell of the Scheludko-Exerowa system (b) schematic of the set-up used for studying microscopic thin aqueous films (c) a typical interferogram of photocurrent versus time of drainage for the thinning process (adapted from ref. (6)), with permission from Elsevier Science...

FIGURE 1.6 Axisymmetric drainage of foam film with immobile air-water surfaces, (a) Dimple and barrier ring that form, (b) Defining maximum dimple size where barrier ring is absent and where film is formed in a cylindrical Scheludko cell (references [13,39] see Section 2.3.1). Radius of curvature, of Plateau border air-water surface is approximated by radius, R, of cell—is radius of curvature of air-water surface of the dimple. (Reprinted with permission from Joye, J. et al., Langmuir, 8, 3083. Copyright 1992 American Chemical Society.)... 

Observation of the drainage of foam films containing drops of these alcohols using a Scheludko cell revealed that the oil drops were swept out of the films without causing film rupture. Indeed, the drainage behavior and stability of the films were apparently totally unaffected by the presence of the oils despite a reduction in equilibrium surface tension. Arnaudov et al. [7] then concluded that these alcohols must cause foam collapse by rupturing Plateau borders rather than foam films as described in Section 4.5.3. 

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 high bulk liquid viscosity simply retards the rate of foam collapse. High surface viscosity, however, involves strong retardation of bulk liquid flow close to the surfaces and, consequently, the drainage of thick films is considerably more rapid than that of thin films, which facilitates the attainment of a uniform film thickness. 

Any additives that can act to reduce the viscosity of foam films, and thereby increase the liquid drainage rate, will tend to reduce foam stability as a result. This includes any chemicals that can reduce surface viscosity and/or surface elasticity. Some alcohols can be use to produce these effects. 

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. 

Influence of the type of foam films on foam drainage... 

Fig. 5.12 depicts the lg Wit dependence for NaDoS foams with thin liquid films, h 16 nm, with CBF, h 8 nm and with NBF, h 4.2 nm. The differences between curve 1, 2 and 3, corresponding to the different foam film types, is clearly expressed and is valid not only for the curve slopes but also for t at which a plateau is reached, that itself corresponds to hydrostatic equilibrium. Fig. 5.12,a and 5.12,b plots the initial linear parts of the experimental Wit dependences where the black circles are for NBF, and the black squares are for CBF. It can be seen that the drainage rate is different for the different types of foam films. 

This finding is supported by the results obtained at constant NaDoS concentration and various NaCl concentrations. All points, depicted for NaCl concentration up to 0.32 mol dm 3 lay on curve 1, while for concentrations higher than that, lay on curve 2. Thus, a critical electrolyte concentration Cei,cr is determined which is decisive for the formation of the respective type of foam films. Its value, Cei,cr = 0.33 0.05 mol dm 3, is in a better agreement with the values obtained employing other techniques for foams (see Chapter 6) and foam films (Chapter 3). This result evidences that the foam film type affects the drainage process. However, a quantitative interpretation is not possible. This refers to the jump in the value of the drainage rate (initial slopes) in W(t) dependence for the different types of foam films but does not answer the question why the liquid from a CBF foam drains faster. The solution of these and other problems related to the type of foam films requires its correlating with the... 

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). 

The analysis of zp(Apo) and FI(li) dependences permit the conclusion that the course of zp(Apo) curves is determined by the behaviour of the different types of foam film. The initial course of these curves reflects the influence of foam drainage. Most explicitly the rp(Apo)... 

Along with the diffusion foam collapse and the structural rearrangement, the hydrostatic equilibrium of borders is disturbed. This leads to the drainage of the released excess liquid. At low surfactant concentrations (lower than CMC) this liquid is surfactant enriched, compared to the initial solution. The degree of its concentrating depends on the dispersity of the initial foam and on the type of foam films. That is why a stable foam (of a lifetime longer than several minutes) can be formed even at initial surfactant concentration lower than the concentration Cw at which black spots are observed [53,54]. 

The studies discussed expand the use of the method for assessment of foetal lung maturity with the aid of microscopic foam bilayers [20]. It is important to make a clear distinction between this method [20] and the foam test [5]. The disperse system foam is not a mere sum of single foam films. Up to this point in the book, it has been repeatedly shown that the different types of foam films (common thin, common black and bilayer films) play a role in the formation and stability of foams (see Chapter 7). The difference between thin and bilayer foam films [19,48] results from the transition from long- to short-range molecular interactions. The type of the foam film depends considerably also on the capillary pressure of the liquid phase of the foam. That is why the stability of a foam consisting of thin films, and a foam consisting of foam bilayers (NBF) is different and the physical parameters related to this stability are also different. Furthermore, if the structural properties (e.g. drainage, polydispersity) of the disperse system foam are accounted for it becomes clear that the foam and foam film are different physical objects and their stability is described by different physical parameters. 

The extent and rate of drainage of surplus solution from the interior of the lamellae is one of the important factors determining foam stability, since drainage causes thinning of the film, and when the film reaches a critical thickness (50-100 A), the film may rupture spontaneously. Drainage of the film occurs under two influences gravity and pressure difference. 

As shown in Section 3.4. the rate of drainage of liquid films depends on surface rheological properties. This is valid for foam and emulsion films. However, the drainage of the films between fluid particles is only the first common step in stability, the DLVO theory controls films and films stabilised in the second minimum of the it/A- isotherm. 

As mentioned earlier, heavy oil produced by solution gas drive often displays marked foaminess in wellhead samples. This feature is not surprising because the two key factors needed for nonpolar foam stability are present in the heavy-oil system. The viscosity of the liquid phase (heavy oil) is high enough to retard drainage of liquid films by capillary... 

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