Bubbles coalescence tension

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. 

Weissenbom PK, Pugh RJ (1996) Surface tension of aqueous solutions of electrolytes relationship with hydration, oxygen solubility, and bubble coalescence. J Colloid Interface Sci 184 550-553... 

It is a well-known fact that bubble sizes in aqueous electrolyte solutions are much smaller than in pure water with equal values of viscosity, surface tension, and so on. This can be explained by the electrostatic potential of the resultant ions at the liquid surface, which reduces the rate of bubble coalescence. This fact should be remembered when planning experiments on bubble sizes or interfacial... 

Other physical properties required are viscosities, especially the viscosity of the liquid densities of the liquid and gas surface tension of the liquid, including the influence of surfactants (e.g. on bubble coalescence behaviour) and, if the gas is a mixture, the gas-phase diffusivity of the reactant A. These physical properties are needed in order to evaluate the equipment characteristics as follows. 

Under the real operating conditions of a fluidized-bed reactor, a number of interacting bubbles occur in the interior of the fluidized bed. As a rule, the interaction leads to coalescence. As detailed studies have shown, this process is quite different from that between gas bubbles in liquids because of the absence of surface-tension effects in the fluidized bed [31, 32], Werther has derived a simple empirical correlation (based on the mechanism of bubble coalescence) for the growth of the mean bubble size dy (diameter of the sphere of equal volume) with increasing height h above the grid [33, 34] ... 

The important variables that affect the bubble dynamics and flow regime in a bubble column are gas velocity, fluid properties (e.g. viscosity, surface tension etc.), nature of the gas distributor, and column diameter. Generally, at low superficial gas velocities (approximately less than 5 cm/sec) bubbles will be small and uniform though their nature will depend on the properties of the liquid. The size and uniformity of bubbles also depends on the nature of the gas distributor and the column diameter. Bubble coalescence rate along the column is small, so that if the gas is distributed uniformly at the column inlet, a homogeneous bubble column will be obtained. 

The sugars sucrose, fructose and glucose have also been found to affect bubble coalescence. On addition to water these sugars raise the surface tension and are desorbed from the air-water interface. Thus their effect on bubble coalescence equally cannot be described in terms of surfactant-like behaviour and certainly no charge effects are involved. Hence, even if an "explanation" could be found within the confines of the primitive model of electrolytes, that explanation could not accommodate this observation. The reduction in bubble coalescence achieved with increasing concentration is shown in Fig. 3.7. 

A surface tension which is low compared with water reduces the bubble coalescence. The gas bubbles remain smaller, their density in the gas/liquid flue becomes larger and this increases the pumping effect and thereby reduces the mixing time. 

Calderbank [64] already mentioned that even very small quantities of the solute (often as little as 0.05%) were sufficient, to increase the interfacial area in the G/L system many times over. This also includes substances which are so weakly surface active, that the surface tension is hardly measurably affected, but nevertheless they exercise a large influence on the rate with which the gas bubbles coalesce. 

Foaming capability relates to both foam formation and foam persistence. Surface tension lowering is necessary, but not sufficient. Other important factors include surface elasticity, surface viscosity and disjoining pressure [60]. Considering stability to aggregation, film thinning and bubble coalescence, the factors favouring foam stability can be summarized as follows ... 

Instead of resisting the break-up into smaller bubbles and thus opposing high mass-transfer rates, the surface-tension forces minimize the thickness of the film separating the bubble from the catalyst. The liquid is effectively sealed between the bubbles and cannot escape the slug that it forms. This prevents bubble coalescence, and surface tension now eliminates the need for turbulent energy to break up the bubbles. 

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