Foams Gibbs-Marangoni effects

An absence of the Gibbs-Marangoni effect is the main reason why pure liquids do not foam. It is also interesting, in this respect, to observe that foams from moderately concentrated solutions of soaps, detergents, etc., tend to be less stable than those formed from more dilute solutions. With the more concentrated solutions, the increase in surface tension which results from local thinning is more rapidly nullified by diffusion of surfactant from the bulk solution. The opposition to fluctuations in film thickness by corresponding fluctuations in surface tension is, therefore, less effective. 

In the case where foam instability is desirable, it is essential to choose surfactants that weaken the Gibbs-Marangoni effect. A more surface-active material such as a poly(alkyl) siloxane is added to destabilize the foam. The siloxane surfactant adsorbs preferentially at the air/liquid interface, thus displacing the original surfactant that stabilizes the foam. In many cases, the siloxane surfactant is produced as an emulsion which also contains hydrophobic silica particles. This combination produces a synergetic effect for foam breaking. 

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. 

Gibbs—Marangoni Effect The effect in thin liquid films and foams when... 

Another mechanism important in foam stability is the Gibbs-Marangoni effect, and this plays a role in preventing catastrophic thinning of the fluid films and subsequent bubble collapse. Consider two adjacent air bubbles in a foam, divided by a fluid film coated with surfactant molecules. As the bubbles grow, the dividing film will increase in area and become stretched. This means that the distribution of surfactant molecules... 

A is the area of the surface. In a foam, where the surfaces are interconnected, the time-dependent Marangoni effect is important. A restoring force corresponding to the Gibbs elasticity will appear, because only a finite rate of absorption of the surface-active agent, which decreases the surface tension, can take place on the expansion and contraction of a foam. Thus the Marangoni effect is a kinetic effect. 

For maximum mechanical stability, the interfacial film resulting from the adsorbed surfactants should be condensed, with strong lateral intermolecular forces, and should exhibit high film elasticity. The liquid film between two colliding droplets in an emulsion is similar to the liquid lamella between two adjacent air sacs in a foam (Chapter 7) and shows film elasticity for the same reasons (Gibbs and Marangoni effects). 

Many surfactant solutions show dynamic surface tension behavior. That is, some time is required to establish the equilibrium surface tension. If the surface area of the solution is suddenly increased or decreased (locally), then the adsorbed surfactant layer at the interface would require some time to restore its equilibrium surface concentration by diffusion of surfactant from or to the bulk liquid. In the meantime, the original adsorbed surfactant layer is either expanded or contracted because surface tension gradients are now in effect, Gibbs—Marangoni forces arise and act in opposition to the initial disturbance. The dissipation of surface tension gradients to achieve equilibrium embodies the interface with a finite elasticity. This fact explains why some substances that lower surface tension do not stabilize foams (6) They do not have the required rate of approach to equilibrium after a surface expansion or contraction. In other words, they do not have the requisite surface elasticity. 

Here, is the so called foam parameter, and t is the viscosity m the surfactant-containing phase (Liquid 1 in Fig. 15) the influence of the transition zone film - bulk liquid is not accounted for in Eq. (76). Note that the bulk and surface diffusion fluxes (see the terms with and Z) in the latter equation), which tend to damp the surface tension gradients and to restore the uniformity of the adsorption monolayers, accelerate the film thinning (Fig. 1). Moreover, since Din Eq. (76) is divided by the film thickness h, the effect of surface diflhsion dominates that of bulk diffusion for small values of the film thickness. On the other hand, the Gibbs elasticity Eq (the Marangoni effect) decelerates the thinning. Equation (76) predicts that the rate of... 

In a foam where the films are interconnected, the related time-dependent Marangoni effect is more relevant. A similar restoring force to expansion results because of transient decreases in surface concentration (increases in surface tension) caused by the finite rate of surfactant adsorption at the surface. Such nonequilibrium surface tension effects are best described in terms of dilatational moduli. The complex dilatational modulus e of a single surface is defined in the same way as the Gibbs elasticity as in equation (2) (the factor 2 is halved as only one surface is considered). 

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