Drainage foam films

It is more than 30 years since Callaghan and Neustadter [4] made a study of oil foaming, which combined snrface rheological and thin film observations. Perhaps it is time this approach is revisited with some emphasis on the difficult issue of measurements on live crude oils at elevated temperatures. Such measurements could be combined with application of modern theories abont foam drainage, foam film drainage, and rupture (see Chapter 1 for a brief introduction to these topics). 

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... 

Figure 7. Photomicrographs on the various drainage stages of a foam film containing 2 wt% sodium caseinate, at 40 °C. Film diameter 0.35 mm.

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. 

Reynolds relation requires liquid drainage from the film to follow strictly the axial symmetry between parallel walls. Rigid surfaces ensure such drainage through their non-deformability, while non-equilibrium foam films are in fact never plane-parallel. This is determined by the balance between hydrodynamic and capillary pressure. Experimental studies have shown that only microscopic films of radii less than 0.1 mm retain their quasiparallel surfaces during thinning, which makes them particularly suitable for model... 

Langevin et al. [35,71] have proposed a simplified hydrodynamic model of thinning of microscopic foam films that accounts for the influence of surface elasticity on the rate of thinning in a large range of thicknesses and Ap. However, as noted by the authors, in view of the rapid loss of surfactant molecules at the surface during film drainage, the elasticity would not correspond to the actual bulk surfactant concentration but to lower values since the system is very far from equilibrium. Frequency dependence of surface elasticity has been considered by Tambe and Sharma [72]. 

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. 

Fig. 3.11. depicts the dependence of drainage time on shear viscosity for NaDoS microscopic foam films in the presence of C12H25OH. From a certain value on the drainage time steeply increases in accordance with the increased surface shear viscosity and there occurs a symmetric drainage. 

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. 

The quantitative investigation of the films in metastable equilibrium under constant capillary pressure was to a considerable extent impeded by the delayed film drainage (being due to the higher viscosity of the bulk solutions [348]). In that sense the Pressure Balance Technique for investigation of microscopic single foam films allowed to detect all the metastable states of those films. Thus, a phenomenon not previously described has been... 

Temperature dependence of the critical concentration Ce of a foam bilayer formation. The Cc concentration (see Eq. (3.129)) of formation of DMPC foam bilayer was determined on the basis of observations of the final state which the foam film reached during its drainage (see Section 3.2), i.e. either rupture at a definite critical thickness without formation of black spots occurs, or formation of foam bilayer via black spots is observed. Rupture at critical thickness occurred at lower DMPC concentrations in the solution (C < Cc) and black spots were formed at higher concentrations (C > Cc). These black spots encountered the film turning it into a foam bilayer of constant radius. At each temperature a series of observations were carried out at various DMPC concentrations for the determination of Cc (the minimum DMPC concentration at which a foam bilayer is formed). This concentration is... 

Simultaneously with drainage from films into borders the liquid begins to flow out from the foam, when the pressure in the lower foam layers overweighs the external pressure. By analogy with gel syneresis Arbuzov and Grebenshchikov [ 1 ] have called the liquid outflow from the foam foam syneresis . Today both terms foam syneresis and foam drainage are used in literature. 

The rate of foam drainage is determined not only by the hydrodynamic characteristics of the foam (border shape and size, liquid phase viscosity, pressure gradient, mobility of the Iiquid/air interface, etc.) but also by the rate of internal foam (foam films and borders) collapse and the breakdown of the foam column. The decrease in the average foam dispersity (respectively the volume) leads the liberation of excess liquid which delays the establishment of hydrostatic equilibrium. However, liquid drainage causes an increase in the capillary and disjoining pressure, both of which accelerate further bubble coalescence and foam column breakdown. 

The border profile was studied in order to analyse qualitatively the influence of various foam parameters (surfactant kind and foam film type, foam column height, pressure drop, etc.) on the drainage process as well to check the validity of drainage models [12], The foam was placed in a cylindrical vessel (diameter 2.5 to 4 cm), similar to vessel 6 in Fig. 1.4. It was covered with a lid to prevent evaporation. The pressure above the foam was equal to the atmospheric pressure. The border profile was determined by simultaneous measurement of the capillary pressure at various levels of the foam column, i.e. the r(H) dependence in the direction of liquid flow was evaluated. Thus it was found that the best approximation (among the discussed in Section 5.3.3) appears to be the parabolic model of border profile. 

Eq. (5.35) describes satisfactorily the kinetics of the change in capillary pressure of a foam at the initial and final drainage stages. In t vs. 1/r2 co-ordinates, Fig. 5.11 presents the experimental data 4-5 min after drainage initiation. They are well approximated by a linear dependence. Furthermore, the time for establishing an equal capillary pressure depends considerably on the surfactant kind and foam film type. Although the cylindrical model is. 

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)... 

As already mentioned the real foam is not a simple sum of films. Its behaviour and response to different disturbances is much more complex than the behaviour of individual foam films. This is so because the films in the foam have different size and shape, and the kinetics of establishing film equilibrium is more complicated since the process of foam drainage exerts significant influence. 

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]. 

Garrett came to the conclusion that most important for the synergy action of an oil-particle antifoam seems to be the ability of the particles to facilitate the appearance of oil droplets into the air/water surface. However, the sizes of the antifoam oil/particle composites should be sufficiently small to ensure a high probability of presence in a given foam film, but not so small to slow down the film drainage and suppress antifoaming effect. It order to possess such properties the particles should be hydrophobic but not completely wetted by the oil. The contact angle 9ow at the oil/water interface should satisfy the condition [20]... 

It is necessary to bear in mind that although Eqs. (4.9) and (4.10) are rigorously fulfilled at any hydrostatically equilibrium state of the foam, the capillary pressure exerts a strong influence on the drainage and foam stability. At a certain value of the capillary pressure, depending of foam dispersity and the foam film type, the foam lifetime becomes very short and the foam breaks down instantaneously. 

Foam films and a foam from the aqueous and organic phases of an extraction system containing a 30% solution of tri-buthyl phosphate (TBP) in kerosene and nitric acid (1 mol dm 3) have been studied in a parallel mode [137]. The reasons for foaming and the effect of emulsion formation on foam stability were elucidated. Thus, a foam with a measurable lifetime was obtained when TBP was in concentration of 0.8 mol dm 3 which corresponded to the concentration of black spot formation. When the volume ratio of the organic to the aqueous phase was 1 5, the foam formed in the system was stabilised additionally by a highly disperse O/W emulsion. This was due to the reduced rate of drainage. These results are confirmed by the experimental data acquired with a specially constructed centrifugal extractor [136]. It makes it possible to perform an extraction process under conditions close to those in industry. 

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