Foams film elasticity

Foam films of different size, shape and spatial orientation are obtained at the approach of individual bubbles or the surfaces of a biconcave drop, or at bubble contact with the solution/air interface, or at withdrawing a frame from a solution, etc. Individual foam bubbles are usually used in the study of foam properties. They prove to be most useful in many cases, for example, in the determination of foam film elasticity, the estimation of gas diffusion from the bubble through the film, the detection of the rupture of the foam bubble films [e.g. 1], Beginning with the remarkable bubbles of Boys [2] and reaching to present day studies, single foam bubbles have since long attracted a considerable interest (see, for instance, the monograph of Dukhin, Kretzschmar and Miller [3]). 

The stabihty of a single foam film can be explained by the Gibbs elasticity E which results from the reduction ia equiUbrium surface concentration of adsorbed surfactant molecules when the film is extended (15). This produces an iacrease ia equiUbrium surface tension that acts as a restoring force. The Gibbs elasticity is given by equation 1 where O is surface tension and is surface area of the film. 

One useful method of aqueous defoaming is to add a nonfoam sta-bihzing surfac tant which is more surface-active than the stabilizing substance in the foam. Thus a foam stabilized with an ionic surfactant can be broken by the addition of a very surface-active but nonstabihzing sihcone oil. The sihcone displaces the foam stabilizer from the interface by virtue of its insolubility. However, it does not stabilize the foam because its foam films have poor elasticity and rupture easily. 

In a gas and liquid system, when gas is introduced into a culture medium, bubbles are formed. The bubbles rise rapidly through the medium and dispersion of the bubbles occurs at surface, forming froth. The froth collapses by coalescence, but in most cases the fermentation broth is viscous so this coalescence may be reduced to form stable froth. Any compounds in the broth, such as proteins, that reduce the surface tension may influence foam formation. The stability of preventing bubbles coalescing depends on the film elasticity, which is increased by the presence of peptides, proteins and soaps. On the other hand, the presence of alcohols and fatty acids will make the foam unstable. 

Regarding foam stability, it has been recognized that the surface tension under film deformation must always change in such a way as to resist the deforming forces. Thus, tension in the film where expansion takes place will increase, while it will decrease in the part where contraction takes place. A force exists that tends to restore the original condition, which is film elasticity, defined as... 

In addition to the film elasticity, other factors that may affect foam stability arc surface shear viscosity, bulk viscosity of the foaming liquid, and the presence of particulate matter. 

In concentrated emulsions and foams the thin liquid films that separate the droplets or bubbles from each other are very important in determining the overall stability of the dispersion. In order to be able to withstand deformations without rupturing, a thin liquid film must be somewhat elastic. The surface chemical explanation for thin film elasticity comes from Marangoni and Gibbs (see Ref. [199]). When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded... 

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. 

Air-water interface Surface tension Film elasticity Film viscosity Foam generation... 

The shape of foam films and border profiles in large interval of foam expansion ratio from 10 to 1500 has been experimentally studied in [83], A regular pentagonal dodecahedron made up of transparent organic glass with an elastic rubber balloon inside it which took the shape of a sphere at inflation (Fig. 1.10) was used as a model of foam cell. 

The elasticity of liquid films that plays an important role in the stability of some foam films, is characterised quantitatively by the elasticity modulus... 

Elasticity can be measured either with macroscopic vertical or horizontal foam films [94-97] or with individual foam bubbles [98]. 

Bianko and Marmur [99] have developed a new technique for the measurement of Gibbs elasticity of foam films. In order to exclude the effect of the mass transfer of the surfactant, the stretching of an isolated soap bubble is used. The surface tension needed for the calculation of the elasticity modulus is determined by the pressure in the bubble and the radius of curvature. The modulus obtained are considerably lower than those derived by the technique of Prins et al. [95]. 

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 effect of film elasticity on diffusion transfer in a foam is not studied experimentally. 

Several authors [25,29-32] believe that film elasticity is an important factor for film and foam stability. However, an immediate relation between foam stability and film elasticity has not been established but if it exists, it should not be directly proportional. Even at large values of elasticity modulus the insoluble surfactant monolayer are poor film stabilisers. The modulus of elasticity of such films does not depend on film thickness and, therefore, it can be... 

A qualitative evidence of the above are the data reported in [52]. It has been established that there is a correlation between the calculated rate of internal diffusion foam collapse and the experimentally determined rate. To obtain a stable foam from poor surfactants (alcohols, acids, etc.) under these conditions is hardly possible because of either insufficient dynamic elasticity of foam films or the lack of equilibrium elasticity (for films from insoluble surfactants). Furthermore, the n barrier for films from acid or alcohol solutions is low and the typical capillary pressures for a real foam are sufficient to induce disturbance of the film equilibrium and, respectively, foam collapse. 

The modulus of elasticity has been also determined by the amplitude of oscillations caused by the expansion of the foam column [4], The values calculated are Ey - 50-100 Pa which are about three orders of magnitude lower than those of gas compressibility. The reason for this discrepancy, however, was not treated by Sharovamikov and Kokorev [4], Probably, it is due to the fact that the oscillations along the foam column height occur without changes in the foam volume. The force regulating these oscillations proves to be the dynamic film elasticity. 

However, as discussed in Chapter 7, it is not possible to explain foam stability only with the change in film elasticity. The obscure moments in this hypothesis evolve from the... 

---