Foams Marangoni flows

Film Rupture. Another general mechanism by which foams evolve is the coalescence of neighboring bubbles via film mpture. This occurs if the nature of the surface-active components is such that the repulsive interactions and Marangoni flows are not sufficient to keep neighboring bubbles apart. Bubble coalescence can become more frequent as the foam drains and there is less Hquid to separate neighbors. Long-Hved foams can be easHy... 

Perturbation of the lamellae whether due to external shocks, the oil imbibition, foam flow, or lamella expansions and contractions (Marangoni flow). 

Gradients in surface (or interfacial) tension can accelerate the spreading of fluids, enhance the stability of surfactant-laden films of liquid, emulsions, and foams, and increase rates of mass transport across interfaces. The motion of fluid driven by a gradient in surface tension is referred to as a Marangoni flow . We have demonstrated that electrochemical reduction of IF to IF at an electrode that... 

There are a number of problems with this mechanism. The first problem clearly concerns the requirement that the oil spread at the air-water surface. We have already established that this is not a necessary property of antifoam oils. A second problem is conceptual—depletion of surfactant in the foam film by adsorption on particles will give rise to an increase in surface tension, which will in turn produce a Marangoni flow in the direction of the foam film. This will tend to enhance film stability. Finally it is clear that an aspect of the mechanism is an intrinsic tendency for the particle to be removed from the oil phase to the aqueous phase. However, it has been shown [43, 71] that addition of liquid paraffin to a dispersion of hydrophobed silica in sodium alkylbenzene sulfonate solution produces an enhancement of antifoam effectiveness. The duplicated results are given in Table 4.9 where the antifoam effectiveness is indicated by the ratio F (given by Equation 4.71). This is clearly not consistent with a process of removal of particles from oil having a central role in the... 

There would appear to be several types of paint film defect associated with foams and antifoams. Most are described as cratering, where crater-shaped depressions are present in films, which are attributed to a variety of causes by Kornum and Nielsen [29]. One cause concerns bubbles that burst only after the drying latex film has at least partially fused so that the shape of the bubble is largely preserved in the paint film as depicted in the sketch in Figure 9.5. Antifoams are of course included in paint formulations in order to rupture such foam films before fusion of the latex film. They may, however, themselves cause defects. Thus, Kornum and Nielsen [29] attribute some defects caused by antifoams to Marangoni flows induced by, for example, spreading from silicone oil drops. Such flows produce only transient depressions in... 

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. 

A different antifoaming mechanism was suggested by Kulkarni et al. (96). They found that surfactants adsorb on the surface of hydrophobic particles during antifoaming, and this adsorption results in deactivation of the particles. On the basis of this observation, they postulated that the adsorption of surfactants onto the hydrophobic particles is so fast that it results in surfactant depletion around the particle in a foam film, and this effect breaks the film. However, no direct proof was presented on this theory. Moreover, depletion of surfactant would cause the film liquid to flow toward the particle because of the increased surface tension (Gibbs— Marangoni effect), and thus cause a stabilizing effect. 

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