Soap bubble elasticity

Bianco and Marmur [143] have developed a means to measure the surface elasticity of soap bubbles. Their results are well modeled by the von Szyszkowski equation (Eq. III-57) and Eq. Ill-118. They find that the elasticity increases with the size of the bubble for small bubbles but that it may go through a maximum for larger bubbles. Li and Neumann [144] have shown the effects of surface elasticity on wetting and capillary rise phenomena, with important implications for measurement of surface tension. 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

SURFACE TENSION. Fluid surfaces exhibit certain features resembhng the properties of a stretched elastic membrane hence the term surface tension. Thus, one may lay a needle or a safety-razor blade upon the surface of water, and it will lie at rest in a shallow depression caused by its weight, much as if it were on a rubber air-cushion. A soap bubble, likewise, tends to contract, and actually creates a pressure inside, somewhat after the manner of a rubber balloon. The analogy is imperfect, however, since the tension in the rubber increases with the radius of the balloon, and the pressure inside, which would otherwise decrease, remains approximately constant while the liquid film tension remains constant and the pressure in the bubble falls off as the bubble is blown. 

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]. 

Let us suppose a film sphere, a soap bubble, for example, and conceptually cut it by a plane which divides it into two hemispheres let us imagine this plane as solid, which will not damage equilibrium, and consider in particular one of the hemispheres. The film which constitutes it presses, we know, on the air imprisoned between it and the plane, and this volume of air reacts, by its elasticity, with an equal force the film hemisphere and the plane are thus pushed one in one direction, the other in the opposite direction, from which it follows that there is a force exerted by the film all along the... 

The value pjph = n - is the expansion ratio. The expansion ratio can be found experimentally by using photography of a flame propagating in a mixture filling a soap bubble or an elastic case. The expansion ratio can be computed by thermodynamic calculation of the combustion temperature and the equilibrium products composition at constant pressure. From the ideal gas state equation it follows that ... 

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. 

A foam is a dispersion of gas bubbles in a relatively small volume of a liquid or solid continuous phase. Liquid foams consist of gas bubbles separated by thin liquid films. It is not possible to make a foam from pure water the bubbles disappear as soon as they are created. However, if surface active molecules, such as soap, emulsifiers or certain proteins, are present they adsorb to the gas-liquid interfaces and stabilize the bubbles. Solid foams, e.g. bread, sponge cake or lava, have solid walls between the gas bubbles. Liquid foams have unusual macroscopic properties that arise from the physical chemistry of bubble interfaces and the structure formed by the packing of the gas bubbles. For small, gentle deformations they behave like an elastic solid and, when deformed more, they can flow like a liquid. When the pressure or temperature is changed, their volume changes approximately according to the ideal gas law (PF/r= constant). Thus, foams exhibit features of all three fundamental states of matter. In ice cream, the gas phase volume is relatively low for a foam (about 50%), so the bubbles do not come into contact, and therefore are spherical. Some foams, for example bubble bath. 

Naturally the micelles formed will not be as flat as shown schematically in Fig. 39c. In spots with a large dissociation the surface will De curved (see Fig. 36 large force A). The undissociated spots form the places where the micelles can be attached to each other. As a result a sort of network is formed and the solution acquires its elastic and strongly viscous properties. It cannot be too strongly emphasriied that we are dealing here with a dynamic system. The smallest air bubble rises slowly but surely upwards in such a soap gel. This means that the mutual attachment of the micelles is of relatively short duration. 

Just as we discussed in connection with the stability of the soap films, foams are nof thermodynamically stable, due to their large interfacial area and, thus, surface free energy. However, some foam, particularly those formed by addifion of small amoimts of foaming agent such as soaps and surfactants, can be metastable, and the bubbles may keep their stability due to Marangoni and Gibbs elasticity. 

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