Foams film thinning

The interaction between the two gas-liquid interfaces across a foam film can directly be measured by a thin-film balance (TFB) [571-573], A single thin foam film is formed in a hole drilled through a porous glass plate (Fig. 12.19). The plate, and therefore the liquid in the foam, is connected to a reservoir with a constant reference pressure Pr by a capillary tube. The film holder is placed into a closed cell. A constant pressure Pg is applied to the gas in the... 

The FRAP method has been applied to the measurements of molecular lateral diffusion of molecules adsorbed at the interface of equilibrium common thin foam films and of black foam films [39-43], Initially Clark et al. reported FRAP measurement of surface diffusion of the fluorescence probe 5-N(octadecanoyl)aminofluorescein incorporated into foam films stabilised with NaDoS [39]. Then followed the measurements of protein-stabilised foam films where the protein was covalently labelled with fluorescein [40,41], Studies of FRAP measurements of surface lateral diffusion in equilibrium phospholipid common thin foam films and black foam films were also reported [42,43]. 

Dynamics and stability of thin foam films have been and continue to be an object of intensive research [e.g. 28-35]. Model studies with vertical large macroscopic films with linear sizes of the order of centimeters as well as with horizontal circular microscopic films with radius of the order of millimeters were performed. The kinetics of thinning of vertical macroscopic films in described in detail in [33]. Some of the results presenting an interpretation of the dynamic properties of films and foam are considered in Chapter 7. Microscopic foam films offer certain advantages with respect to treatment of stability of foams and foam films, since the systems studied behave under strictly defined conditions. 

The lifetime At of thinning foam films can be estimated according to the relation... 

Formation and stability studies of black foam films can be summarised as follows 1) surface forces in black foam films direct measurement of disjoining pressure isotherm DLVO- and non-DLVO-forces 2) thin foam film/black foam film transition establishing the conditions for the stability of both types of black films and CBF/NBF transition 3) formation of black foam films in relation to the state of the adsorption layers at the solution/air interface 4) stability of bilayer films (NBF) theory and experimental data. 

In Section 3.3.1 it was shown that the state of thin foam films is described by the Fl(/ ) isotherm of disjoining pressure. For relatively thick films, stabilised by surfactants, this isotherm is consistent with the DLVO-theory. However, black foam films exhibit a diversion from the DLVO-theory which is expressed in the specific course of the disjoining pressure isotherm. 

Fig. 3.42. General schematic presentation of disjoining pressure isotherm of a thin foam film 1 - region...

Recently, detailed studies [484,485] on gas permeability through CBF indicated that in such particularly thin foam films the permeation is mainly determined by the monolayer permeability of the surfactant monolayer. Fig. 3.106,a depicts the values of Icml calculated using the values measured for K (Fig. 3.106,a) from Eq. (3.145), depending on the aqueous core thickness. 

Our objective in this study is to elucidate the complex phenomena occurring during the process of three phase foam thinning, to identify the interaction mechanisms between the oil droplets, the thinning foam film and the Plateau-Gibbs borders and the role of surface and interfacial tension gradients in foam stability, and to examine the implications upon crude oil displacement by foam in pourous media. 

Mechanism of rupture. Black films. The mechanism of hydrodynamic instability of thin foam films was analyzed in [278, 279, 411], The stability of ultrathin films is governed by a competition between capillary forces and the molecular component of the disjoining pressure. An instability can arise when dU/dh > 0 and the capillary pressure is not too large. This is possible if... 

The foam-dilatational viscosity, K, arises because of two primary mechanisms (37) (1) viscous flow within the thin films, and (2) interfacial tension gradients acting along the foam bubble surfaces. The effect of interfacial tension gradients is to increase the foam viscosity as they impede flow near the surfaces of the thin foam films by contributing to a larger film stress. As in the wet foam (eq 6), the foam dilatational viscosity for a dry foam, K, is inversely proportional to film thickness as well (eq 9). 

At high surfactant concentrations (i.e., much above the CMC), thin foam films were observed (48—53) to become thinner in a stepwise fashion that is, thinning foam films formed from micellar surfactant solutions exhibit a number of metastable states before attaining an equilibrium film thickness. This process can be followed in Figure 13, which shows a photocurrent (film thickness)—time interferogram of a horizontal flat film... 

Boussinesq Number A measure of the ratio of interfacial and bulk viscous effects in a thinning foam film BQ = (rf + /cs)/( f f) where if is... 

Elasticity Number. A dimensionless quantity characterizing the surface-tension gradient in a thinning foam film. 

Differences in bulk composition are possible in a thin foam film as a result of stretching of the film. If the film is sufficiently thin, then any stretching causes a depletion of the bulk phase surfactant solution between the air-liquid surfaces of the foam film as more surfactant adsorbs on those surfaces. Distances perpendicular to the film are small so that, provided the stretching occurs reasonably slowly, the equilibrium inside the film element may be always maintained. Depletion of bulk phase the surfactant concentration will therefore necessarily mean an increase of the surface tension of the film as it is stretched. This will, however, only occur if reduction of the surfactant concentration causes a concomitant increase in surface tension. In the case of a pure surfactant at concentrations above the critical micelle concentration (CMC), this may not always happen. 

The disjoining pressures in thin foam films can be as high as 30 kPa (0.3 bar). Such pressures cannot be applied using devices like that shown in Figure 2.8. They can, however, be applied directly to a foam film contained in a cylindrical cell prepared from a porous frit where the menisci in the fine holes in the latter determine the maximum capillary pressure in the Plateau border. In this approach, pressure on the film is directly applied by increasing the surrounding gas pressure. In turn, this produces an enhanced capillary pressure in the liquid trapped in the pores of the frit, presumably as the liquid moves to smaller pore radii. This technique was first used by Mysels and Jones [40] and later refined by Exerowa and Scheludko [41],... 

Kralchevsky et al. [46] suppose that stratification in thin foam films containing micelles concerns the formation of vacancies in the close-packed structure that can effectively diffuse to form thin spots, which should spontaneously grow if the spots are larger than a critical size determined by the line tension. Since the number of such vacancies is supposed proportional to the size of the film, then the smaller the film the lower the likelihood that a spot of the critical size will form. Clearly this hypothesis, although concerned with stratification in symmetrical foam films, could give an explanation for the finding that the stability of pseudoemulsion films... 

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