Thin Films, Foams, and Emulsions

The microinterferometric method has widely been used by many authors to investigate both symmetric thin liquid films (foam and emulsion films) and asymmetric ones (wetting films). This method has been described in several papers, for example. Refs. [2, 3, 32, 33, 37] as well as in a book [1]. Here we give only a brief outline. 

We have used film interferometry to reveal a new mechanism for the stabilization of foams and emulsions due to layering inside the thinning films, as will be discussed below. 

Analysing the data presented leads to the conclusion that the behaviour of aqueous asymmetric films is similar to that of foam and emulsion films (kinetics of thinning and formation of equilibrium thin films). At certain surfactant concentration in the water phase the... 

We have seen evidence of a similar structuring of micelles in thin foam and emulsion films containing C AOS, in the form of stepwise transitions or stratification phenomena (see Figure 5). 

Kruglyakov, P.M., Equilibrium properties of free films and stabihty of foams and emulsions, in Thin Liquid Films, Ivanov, I.B., Ed., Marcel Dekker, New York, 1988, p. 161. 

Miller R, Fainerman VB, Aksenenko EV, Makievski AV, Kraegel J, Liggieri L, Ravera F, Wuestneck R, and Loglio G (2000) "Surfactant Adsorption Kinetics and Exchange of Matter for Surfactant Molecules with Changing Orientation within the Adsorption Layer" in Emulsion, Foams, and Thin Films, Mittal and Kumar Editors, Ch. 18, Marcel Dekker, pp. 313-327 Miller R, Fainerman VB, Makievski AV, Leser M, Michel M and Aksenenko EV (2004) Determination of Protein Adsorption by Comparative Drop and Bubble Profile Analysis Tensiometry. Colloids Surfaces B 36 123-126 Neumann AW and Spelt JK Eds., Applied Surface Thermodynamics, Surfactant Science Series, Vol. 63, Marcel Dekker Inc., New York, 1996 Noskov B and Logho G (1998) Dynamic surface elasticity of surfactant solutions. Colloids Surfaces A 143 167-183... 

Their presence permits the formation of interfacial tension gradients, which may be considered the most essential function. For instance, this is all that allows the formation of foams and emulsions and provides inherent stability to thin films. 

Coalescence is induced by rupture of the thin film between close emulsion droplets or gas bubbles this phenomenon of film rupture will be discussed first. However, the whole coalescence process involves a number of additional variables, and these are rather different for emulsions and foams (cf. Table 11.1), which is the reason that they are discussed separately. Our understanding of coalescence is yet unsatisfactory, because (a) so many... 

In concentrated systems with highly mobile interfaces (foams and emulsions) capillary phenomena of the first kind, related to the surface curvature in regions of film - macroscopic phase contact or in the regions where three films come into contact, may play a significant role in the energy and dynamics of film thinning. As shown in Fig. VII-2, a concave surface is formed in these types of regions. Under this surface the pressure is lowered by the amount equal to capillary pressure (see Chapter I, 3),... 

As is well known, a lot of effects of surfactants, like damping of surface waves, the rate of thinning of liquid films, foaming and stabilisation of foams and emulsions, cannot just be described by a decrease in interfacial tension or by van der Waals and electrostatic interaction forces between two interfaces. The hydrodynamic shear stress at an interface covered by a surfactant adsorption layer is a typical example for the stimulation of an important surface effect. This effect, shown schematically in Fig. 3.9., is called the Marangoni effect. 

Thin liquid films in foam and emulsion systems are usually stabilised by soluble surfactants. During the formation of such films the flow-out process of liquid disturbs the surfactant equilibrium state in the bulk and film surfaces. The situation of drainage of a surfactant containing liquid film between two oil droplets is shown in Fig. 3.15. (after Ivanov Dimitrov 1988). Here j" and are the bulk fluxes in the drops and the film, respectively, j and j are the fluxes due to surface diffusion or spreading caused by the Marangoni effect, respectively. 

The coalescence of disperse systems, such as foams and emulsions, and the contact of air bubbles with solid particles, e.g. in the process of flotation, takes place in two steps. The first is characterised by a flocculation of the system, the formation of thin liquid films with an equilibrium thickness. In the second step the film becomes thin enough for the interparticular attractions to overcome the film state so the two separated interfaces form a new interface. The situation where a small bubble attaches a liquid interface is shown in Fig. 2D. 1. 

Petrov etal. (1980) analyzed the causes for the entrapment of water between the solid substrate and the monolayer in Z-type depositions. This phenomenon has many common features with film thinning processes found during foam and emulsion breakdown and it is dependent on interfacial properties and on molecular interactions between the solid substrate and the monolayer. Petrov etal. (1980) measured the maximum speed of removal of the solid substrate before entrainment of a water layer and found it to be dependent on pH and ionic strength. There is no record in the publication of the measurement of dynamic contact angles. 

The behavior of disperse systems, such as foams and emulsions, is very complex and there have been only few attempts to derive qualitative and quantitative relationships between their stability and physicochem-ical parameters of the stabilizing adsorption layers. The starting point of most of these approaches is the hydrodynamic theory of thinning of a liquid film between two bubbles or drops according to Reynolds (1) and Levich (2). A simplified picture of the general scenario in an emulsion is the following. When two... 

Oscillatory structural forces appear in thin films of pure solvent between two smooth solid surfaces and in thin liquid films containing colloidal particles including macromolecules and surfactant micelles (Israelachvili 1992). In the first case, the oscillatory forces are called the solvation forces and they are important for the short-range interactions between solid particles and dispersions. In the second case, the structural forces affect the stability of foam and emulsion films as well as the flocculation processes in various colloids. At lower particle concentrations, the structural forces degenerate into the so-called depletion attraction, which is found to destabilize various dispersions. 

Foams and emulsions are achieved due to adsorption of foam stabilizing agents like surfactants at the interface between the dispersed and continuous phases. The foam stability is often related to the stability of thin liquid films formed between two air bubbles. All considered foam films are stabilized by ionic surfactant. 

In the opposite case, when the surfactant is soluble in the continuous phase, the Marangoni effect becomes operative and the rate of film thinning becomes dependent on the surface (Gibbs) elasticity (see Equation 4.293). Moreover, the convection-driven local depletion of the surfactant monolayers in the central area of the film surfaces gives rise to fluxes of bulk and surface diffusion of surfactant molecules. The exact solution of the problem [651,655,689,739,740,787] gives the following expression for the rate of thinning of symmetrical planar films (of both foam and emulsion type) ... 

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