Thinning of Foam Films

The period of the oscillations is close to the particle diameter [4,350,352]. In this respect, the structural forces are appropriately called the volume exclusion forces by Henderson [364], who derived an explicit (although rather complex) formula for calculating these forces. Numerical simulations [365,366] and density-functional modeling [367] of the stepwise thinning of foam films are also available. 

At higher volume fractions of the small species (polymers or micelles) the depletion may transform into oscillatory structural interaction. Experimental evidence for such interactions between solid surfaces was demonstrated by Richetti and Kekicheff and Parker et al A similar phenomenon is the stratification (stepwise thinning) of foam films. It was explained as layer by layer expulsion of micelles from the film interior. The semi-empirical approach yields the oscillatory energy density in the form... 

If the surfactant concentration is much higher than the CMC, a stepwise thinning of foam films is observed (for a history, see Refs. [752, 779]). This stratification is caused by individual layers of micelles being expelled from the thinning film. In equilibrium, an oscillatory force is observed [779]. Such structural forces will be discussed in Section 11.6. 

There appear to be two stages in the collapse of emulsions flocculation, in which some clustering of emulsion droplets takes place, and coalescence, in which the number of distinct droplets decreases (see Refs. 31-33). Coalescence rates very likely depend primarily on the film-film surface chemical repulsion and on the degree of irreversibility of film desorption, as discussed. However, if emulsions are centrifuged, a compressed polyhedral structure similar to that of foams results [32-34]—see Section XIV-8—and coalescence may now take on mechanisms more related to those operative in the thinning of foams. 

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

The conductometrical technique for the study of foam films allows to measure directly their lateral electrical conductivity. It is widely used for indirect determination of film thickness study of kinetics of film thinning and rupture as well as study of other processes. Fig. 2.16 presents a scheme of one of the devices for measurement of the lateral electrical conductivity of foam films [16]. 

The special properties of thin liquid films, in particular of foam films, involve studying various colloid-chemical aspects, such as kinetics of thinning and rupture of films, transition from CBF to NBF, isotherms of disjoining pressure, thermodynamic (equilibrium) properties, determination of the electrical parameters of surfactant adsorption layer at the liquid/gas... 

The microinterferometric method employed in the study of kinetics of foam film thinning allows to establish experimentally the liquids that form or do not form foam films. If a liquid possesses even small affinity to produce a foam, a circular film with clearly pronounced Newton rings is formed when it is drawn out of the biconcave drop. Films from aqueous surfactant solution can be obtained even at very small decrease in the surface tension (Act < 10 4 N m 1). It is sufficient to ensure a tension gradient between the film center and periphery. 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

Many theoretical and experimental data indicate that the thermodynamic and kinetic properties of the liquid in thin films differ significantly from the properties of the bulk phase of the same solution. The thermodynamics of foam films is described in Section 3.1. 

At equilibrium film thickness hi the disjoining pressure equals the external (capillary) pressure, n = p This is a common thin film and its equilibrium is described by the DLVO-theory. If h < hcr, at which the film ruptures (see Section 3.2.2), the film is common black (schematically presented in Fig. 3.42). Such a film forms via black spots (local thinnings in the initially thicker non-equilibrium film). The pressure difference nmax - pa is the barrier which hinders the transition to a film of smaller thickness. According to DLVO-theory after nmax the disjoining pressure should decrease infinitely. Results from measurements of some thermodynamic parameters of foam films [e.g. 251,252] show the existence of a second minimum in the 17(6) isotherm (in direction of thickness decrease) after which the disjoining pressure sharply ascends. 

In order to understand the nature of surface forces which characterise the thermodynamic state of black foam films as well as to establish the CBF/NBF transition, their direct experimental determination is of major importance. This has been first accomplished by Exerowa et al. [e.g. 171,172] with the especially developed Thin Liquid Film-Pressure Balance Technique, employing a porous plate measuring cell (see Section 2.1.8). This technique has been applied successfully by other authors for plotting 11(A) isotherms of foam films from various surfactants solutions [e.g. 235,260,261]. As mentioned in Chapter 2, Section 2.1.2, the Pressure Balance Technique employing the porous ring measuring cell has been first developed by Mysels and Jones [262] for foam films and a FI(A) isotherm was... 

Fig. 3.72. Schematic representation of possible surfactant structuring in thin liquid foam films that show...

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,...

Similarity of Foam Films with Emulsion and Asymmetric Thin Liquid Films... 

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. 

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

The effect of foam film type on foam stability can be studied from the tp(Apo) dependences in a wide range of pressure drops, as mentioned above, as well as from the Yl(h) dependences (disjoining pressure isotherms) for single foam films having radii close to those of films in the foam [45,46]. Fig. 7.6 depicts the Tp(Ap0) dependence of foams obtained from NaDoS aqueous solutions with different electrolyte (NaCl) concentrations, i.e. the foams are built up of different types of foam films. The surfactant concentration used in all experiments ensured maximum saturation of absorption layer. All three curves have different courses, corresponding to different film types thin films (curve 1), CBF (curve 2) and NBF (curve 3). On increasing Ap0, the foam lifetime strongly decreases compared with the time for decay in... 

The n(fc) isotherms of different types of foam films are shown in Fig. 7.8. The surfactant (NaDoS) and electrolyte (NaCl) concentrations were the same as those used in the experiments with foams. The equilibrium thickness of thin films and CBF decreased with the increase in pa = II. Films ruptured in a definite range of capillary pressure (marked with arrows on curves 1 and 2). The thickness of NBF did not change and they ruptured at a definite capillary pressure (marked with an arrow on curve 3). 

It depicts the H(i) dependence for NaDoS foams. Curve 1 refers to common thin films, curve 2 to CBF and curve 3 to NBF. It is clearly seen that the curves differ from one another. This indicates that the Ross-Miles test can be used to distinguish foams constituted of the different types of foam films. This is in agreement with the xp(Ap) dependence for NaDoS foams (seeFig. 7.6). As mentioned in the beginning of this Section a quantitative comparison of the results by the two techniques cannot be done. 

In such concentrated disperse systems three types of liquid films form foam films (G/L/G), water-emulsion films (O/W/O) and non-symmetric films (O/W/G). The kinetics of thinning of these films, their permeability as well as the energy barrier impeding the film rupture determine the stability of these systems. They might be subjected to internal collapse, i.e. coalescence of bubbles or droplets and increase in their average size, or to destruction as a whole, i.e. separation into their initial phases - gas, oil and water. 

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