Foam films stratified

Treatment of non-equilibrium properties of foam films requires consideration of the kinetics of expansion of a black spot in the grey film [102] as well as the formation of stratified black foam films (see Section 3.4). 

Additional data about the structure of black films are obtained by X-ray diffraction method. The first steps [336,338] have been performed with vertical foam films in a frame in a horizontal scanning diffractometer. Black films from decyltrimethyl ammonium decyl sulphate and NaBr solutions have been studied. The film thickness was calculated using a model of the mean electron density projection on the film normal. However, there was no indication whether the films were CBF or NBF. Platikanov et al. [339,340] used a new device for investigation of a horizontal black films from aqueous NaDoS solution (see Section 2.2.6). They found essentially different X-ray diffraction traces for the three types of black films CBF, NBF and stratified black films. This indicates their different structure. Precise X-ray reflectivity measurements with CBF and NBF films from NaDoS and NaCl aqueous solutions [341-343] provided more details about their structure. The data obtained for the thicknesses of the respective layers which detail the film structure are given below... 

We will consider only some more interesting aspects of stratification of microscopic horizontal foam films, giving an example of the role of stratified films in foam stability (see Chapter 7). 

Fig. 3.76 presents an analogous P(h) isotherm of foam films obtained from system n. Here stratified foam films were also observed. At constant p0 (measuring cell A), seven metastable states of the films (in the various experiments) with thicknesses ranging from 82.1 to 45.2 nm were distinguished. The latter thickness was the lowest that could be realised by a spontaneous stepwise thinning. Spontaneous and forced transitions followed upon pressure increase, similar to those shown in Fig. 3.75. The final thickness reached was about 5.6 nm, i.e. a bilayer film. Therefore, on imposing a definite pressure on the films of both systems,...

Fig. 3.75. P(h) isotherms of stratified foam films obtained at t = 23°C for system 1 curve 1 - at various...

The analysis of these P(h) isotherms emphasises that stratified foam films are formed from both systems (I and II). A phenomenon not revealed so far is that spontaneous (under constant capillary pressure) and forced (under various capillary pressures) stepwise thinning can occur in the same single foam film. A question arises as to whether the film that acquired such a thickness is in thermodynamic equilibrium or is kinetically stabilised. It should be noted that these transitions occur only in the direction of increasing pressure, i.e. the process... 

So, the basic feature of the systems studied, determining the behaviour of foam films and foams, is the formation of micellar structures (probably of lamellar type) with high oil content, which results in formation of stratified films of various thicknesses. 

Kralchevsky, P.A. Nikolov, A.D. Wasan, D.T. Ivanov, LB. Formation and expansion of dark spots in stratifying foam films. Langmuir 1990, 6 (6), 1180-1189. 

The same phenomenon was observed for ionic surfactant solutions (16, 55), such as a-olefin sulfonates (1), but here the height of the steps was close to the effective micellar diameter, which includes the electric double layer around the ionic micelles. Furthermore, foam films formed from concentrated monodisperse suspensions of polystyrene latexes (16) or silica particles (56) stratify in a similar way. 

All the experimental and theoretical data for stratifying films show that stratification is a universal phenomenon and is due to the formation of a long-range colloid ciystal-like structure within the foam film and a layer-by-layer thinning of such an ordered structure. This ordering occurs because highly charged Brownian particles interact via repulsive forces and... 

For example, the rheology of such dispersions containing stratifying films will be quite different. The bulk viscosity of the stratifying foam film was much higher than that of the pure solvent (water) (60). 

The formation of ordered microstructure in thin liquid films offers a new mechanism for the stabilization of foams. As a proof of microstructuring in real foams, Figure 17 shows a photograph of an aqueous foam system stabilized because of the stratification in the foam bubble lamellae. The practical importance of the film microstructuring is that the lifetimes of foams with stratifying films are much longer. 

FIGURE 3.9 Disjoining pressure isotherms for stratifying films., 0.1 M SDS foam film , 0.1 M SDS-dodecane pseudoemulsion film (both at 24°C). At each discontinuity in isotherm, film thins by a step of 10 nm, presumably as a result of loss of layer of micelles. (With kind permission from Springer Science+Business Media Colloid Polym. Sci., 273,1995, 165, Bergeron, V., Radke, C.J.)... 

FIGURE 9.4 Schematic representation of probable structure of stratified foam film prepared from 25.5 wt.% latex in aqueous 0.03 M AOT solution (actual free AOT concentration after depletion by adsorption on latex 2.2 mM). Film age 120 s generated using apparatus depicted in Figure 2.11 and observed in white light. (Reprinted from Colloids Surf. A, 283, Garrett, P.R., Wicks, S.P., Fowles, E., 307. Copyright 2006, with permission from Elsevier.)... 

Some authors [348,349] suggested that a possible explanation of the phenomenon can be the formation of a surfactant lamella liquid-crystal structure inside the film. Such lamellar micelles are observed to form in surfactant solutions, however, at concentrations much higher than those used in the experiments with stratifying films. The latter fact makes the explanation with a lamella liquid crystal problematic. Nikolov et al. [280,350,351] observed stratification not only with micellar surfactant solutions but also with suspensions of latex particles of micellar size. The stepwise changes in the film thickness were approximately equal to the diameter of the spherical particles contained in the foam film [280—282,352]. The experimental observations show that stratification is always observed. 

Naturally, other paraffin-chain salts with one water-soluble end group also form fairly stable films and foams. Proteins, saponins, gamboge, resinates are also capable of forming stable foams the two last named were found by Perrin to form stratified films, just like the stratified soap films. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

Effect of Stratified Layers of Polymer Latex Particles ON Foam and Pseudoemulsion Film Stability... 

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