Film elasticity

Figure IV-10 illustrates how F may vary with film pressure in a very complicated way although the v-a plots are relatively unstructured. The results correlated more with variations in film elasticity than with its viscosity and were explained qualitatively in terms of successive film structures with varying degrees of hydrogen bonding to the water substrate and varying degrees of structural regularity. Note the sensitivity of k to frequency a detailed study of the dispersion of k should give information about the characteristic relaxation times of various film structures.

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... 

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

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

Under dynamic conditions, where equilibrium between the surface and the film bulk cannot be realised, some specific elasticity properties are expressed. This is Marangoni s effect. Assuming that under such conditions there is an equilibrium only in some parts between the film bulk and its surface, it is possible to employ Eq. (7.6) for the material balance to calculate the modulus of elasticity. Hence, instead of the whole film volume, only the zone where equilibrium with the film surface is established, should be considered. The faster the process of film thinning, the smaller this volume is and the larger the modulus of film elasticity. In the limiting case, when it is completely impossible to achieve equilibrium between the film bulk and its surface, the elasticity of the adsorption surfactant layers takes place. 

The principle of both mechanisms of equilibrium and dynamic film elasticities is the same as a result of external disturbances (for example, local extension), a counterforce originates which returns the film to its initial state after the external action has ceased (after extension the film becomes thicker again). 

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... 

Usually the value of the component of the modulus, depending on film elasticity is much lower than that of gas elasticity, i.e. E//2o 1. Then Eq. (8.5) gives... 

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... 

Another problem in the explanation of the defoaming effect with the change in film elasticity is the absence of independent evidence concerning the relevant time scale of dynamic surface tensions over large time scales and its correspondence with the times of film stretching. 

When a membrane expands and the concentration of a surfactant at the interface decreases, there exist two mechanisms to restore the surfactant surface concentration. The first mechanism, termed the "Marangoni effect" (16), refers to the fact that the surface flow can drag with it some of the underlying layers, i.e. the surface layer can flow from areas of low surface tension, thus restoring the film thickness. It is also a source of film elasticity or resilience. 

The film elasticity was derived from tt-A isotherms as E = — A (d7r/dA). The surface dilatational modulus (E) of films with its elastic and viscous components (Ed and Ev) and loss angle tangent (tan 8) were obtained by sinusoidal periodic compressions and expansions. 

---