Marangoni effect phases

U. Influence of the Marangoni Effect on the Mass Transfer BETWEEN Two PHASES... 

Fig. 11. Simulation of the physical model by means of two sources of Marangoni effect generation. The upper phase is paraffin oil and the lower phase is ethylene glycol. Through the two capillaries (the two vertical black streaks) ethyl acetate is fed.

Non-linear phenomena accompanied by periodic changes of electrochemical potential have been the subject of many research activities since Dupeyrat and Nakache [39] reported on periodic macroscopic movements of an oil/water interface and generation of electrochemical potential in 1978. These authors found such non-linear behaviour at a W/NB interface with positively charged cationic surfactants. They explained the nonlinear behaviour on the basis of formation of ion pairs between the positively charged cationic surfactants in the aqueous phase and negatively charged picrate anions dissolved in the oil phase. The ion pairs formed at a W/NB interface were assumed to be removed from the interface by a phase transfer process and oscillatory behaviour was explained in terms of the Marangoni effect. 

The reverse situation is depicted in Figure 15.1(b). When the surfactant is dissolved in the dispersed phase, the surfactants also flow along the interface with the continuous phase flowing out of the slit, but the depletion on the interface is easily countered by diffusion and adsorption of surfactants from the bulk of the dispersed phase. Therefore, there is no Marangoni effect counteracting the outflow of continuous phase, and coalescence takes place much more easily. 

Marangoni Effects in Foam Stability. To estimate the effect of interfacial tension gradients upon foam stability we used the maximum droplet pressure technique (19). The oil phases chosen were n-octane and n-dodecane and the surfactants used were 16 ... 

Of course the Marangoni effect is not the only stabilizing factor in the three phase foam. Another critical factor is droplet size. Smaller droplet size is accomplished by lower interfacial tension, wherefore it is found that C AOS yields more stable foam... 

The interactions between an oil phase and foam lamellae are extremely complex. Foam destabilization in the presence of oil may not be a simple matter of oil droplets spreading upon foam film surfaces but may often involve the migration of emulsified oil droplets from the foam film lamellae into the Plateau borders where critical factors, such as the magnitude of the Marangoni effect in the pseudoemulsion film, the pseudoemulsion film tension, the droplet size and number of droplets may all contribute to destabilizing or stabilizing the three phase foam structure. 

For all phenomena the assumption has been made that there Is no charge transfer through the particle surface. In fact, often the particles are taken as dielectric solids through which the lines of electric flow do not pass. For fluid drops, as in emulsions, this may be a poor approximation because internal flow inside the drops may occur. However, practice has shown that, due to Marangoni effects, the liquid-liquid Interface also often behaves as if it were inextenslble. Absence of ion transfer across the phase boundaiy means that there the normal component of J is zero... 

Interfacial tension gradients are very important in stabilising the thin liquid film that is located between the droplets and which is very important at the start of emulsification (films of the continuous phase may be drawn through the disperse phase and collision is very large). The magnitude of the y-gradients and of the Marangoni effect depends on the surface dilational modulus s, which for a plane... 

The Gibbs-Marangoni effect also explains the Bancroft rule, which states that the phase in which the surfactant is most soluble forms the continuous phase... 

The presence of surfactant adsorption monolayers decreases the mobility of the droplet (bubble) surfaces. This is due to the Marangoni effect (see Equation 5.282). From a general viewpoint, we may expect that the interfacial mobility will decrease with the increase of surfactant concentration until eventually the interfaces become immobile at high surfactant concentrations (see Section 5.5.2, above) therefore, a pronounced effect of surfactant concentration on the velocity of film drainage should be expected. This effect really exists (see Equation 5.286, below), but in the case of emulsions it is present only when the surfactant is predominantly soluble in the continuous phase. 

In the opposite case, when the surfactant is soluble in the continuous phase, the Marangoni effect becomes operative and the rate of film thinning becomes dependent on the surface (Gibbs) elasticity (see Equation 5.282). Moreover, the convection-driven local depletion of the surfactant monolayers in the central area of the film surfaces gives rise to fluxes of bulk and surface diffusion of surfactant molecules. The exact solution of the gives the following... 

Next, we proceed with the case when surfactant is present and the Marangoni effect becomes operative. Classical experiments carried out by Lebedev " and Silvey show that the measured velocity of sedimentation, U, of small fluid droplets in a viscous liquid (pure liquid phases assumed) does not obey the Hadamar and Rybczynski equation ... 

It is clear that the molecules present in a monolayer interact with the molecules of the underlying liquid phase and cannot be assumed merely to consist of molecules moving freely in two dimensions. Such interactions result in molecular motions energized by surface tension gradients this is called the Marangoni effect. In a glass of wine, ethyl alcohol... 

Figure 5. Gibbs—Marangoni effect in the thin-film drainage process. Surfactant is swept to the Plateau borders by flow in the film and droplet phases, and thereby create surface concentration gradients that engender surface tension gradients.

The same kind of motion can occur at the interface between two immiscible phases when a chemical reaction is taking place between components present in both phases or there is diffusion of one substance from one phase to another. This type of behavior occurs in several chemical engineering systems and is called interfacial turbulence or the Marangoni effect [3]. 

Foaming is unlikely (107) in distillation columns where the bottom product has a lower surface tension than the top product ("negative-surface-tension systems ). Here the mass transfer counteracts the Marangoni effect. Foaming is also imlikely when two liquid phases are present throughout, because one of these phases will tend to act as a defoamer (107). 

---