Interfacial tension gradient effects

Interfacial tension gradient effects are represented as apparent dilational viscosity effects. 

The tangential bulk-phase stress component evaluated at the interface combines an elastic (interfacial tension gradient) effect, e, and an apparent viscous effect, rxj + tIj) -I- e 7o). One of the most convenient methods of measuring capillary waves is to use light scattering (29), which can yield information on both the tension and dilational modulus of the interface. 

The effectiveness of a crude oil demulsifier is correlated with the lowering of the shear viscosity and the dynamic tension gradient of the oil-water interface. The interfacial tension relaxation occurs faster with an effective demulsifier [1714]. Short relaxation times imply that interfacial tension gradients at slow film thinning are suppressed. Electron spin resonance experiments with labeled demulsifiers indicate that the demulsifiers form reverse micellelike clusters in the bulk oil [1275]. The slow unclustering of the demulsifier at the interface appears to be the rate-determining step in the tension relaxation process. 

A study on a commonly used demulsifier, namely, a phenol-formaldehyde resin, elucidated how various parameters such as interfacial tension, interfacial shear viscosity, dynamic interfacial-tension gradient, dilatational elasticity, and demulsifier clustering affect the demulsification effectiveness [1275]. 

Our goal is to develop a property-performance relationship for different types of demulsifiers. The important interfacial properties governing water-in-oil emulsion stability are shear viscosity, dynamic tension and dilational elasticity. We have studied the relative importance of these parameters in demulsification. In this paper, some of the results of our study are presented. In particular, we have found that to be effective, a demulsifier must lower the dynamic interfacial tension gradient and its ability to do so depends on the rate of unclustering of the ethylene oxide groups at the oil-water interface. 

All of these disturbances cause a many-fold increase in the rate of transfer of solute across the interface. If a chemical or thermal difference along an interface causes an interfacial tension gradient, violent flow in the direction of low a will result. This action is usually termed the Marangoni effect. 

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 ... 

Numerous studies have shown that mass transfer of solute from one phase to the other can alter the behavior of a liquid-liquid dispersion—because of interfacial tension gradients that form along the surface of a dispersed drop. For example, see Sawistowski and Goltz, Trans. Inst. Chem. Engrs., 41, p. 174 (1963) BaWcer, van Buytenen, and Beek, Chem Eng. Sci., 21(11), pp. 1039-1046 (1966) Rucken-stein and Berbente, Chem. Eng. Sci., 25(3), pp. 475—482 (1970) Lode and Heideger, Chem. Eng. Sci., 25(6), pp. 1081—1090 (1970) and Takeuchi and Numata, Int. Chem. Eng., 17(3), p. 468 (1977). These interfacial tension gradients can induce interfaci turbulence and circulation within drops. These effects, known as Marangoni instabilities, have been shown to enhance mass-transfer rates in certain cases. 

Apart from their effect on reducing y, surfactants play major roles in the deformation and break-up of droplets, and this is summarised as follows. Surfactants allow the existence of interfacial tension gradients which is cracial for the formation of stable droplets [8]. In the absence of surfactants (clean interface), the interface cannot withstand a tangential stress, and the liquid motion will be continuous (Figure 10.17a). 

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... 

Figure10.17 Interfacial tension gradients and flow nearan oil/water interface, (a) No surfactant (b) Velocity gradient causes an interfacial tension gradient (c) Interfacial tension gradient causes flow (Marangoni effect).

The droplet deformation increases with increases in the Weber number which means that, in order to produce small droplets, high stresses (i.e., high shear rates) are require. In other words, the production of nanoemulsions costs more energy than does the production of macroemulsions [4]. The role of surfactants in emulsion formation has been described in detail in Chapter 10, and the same principles apply to the formation of nanoemulsions. Thus, it is important to consider the effects of surfactants on the interfacial tension, interfacial elasticity, and interfacial tension gradients. 

It has been suggested that a spreading film of antifoam may simply displace the stabilising surfactant monolayer. In this case, as the oil lens spreads and expands on the surface, the tension will be gradually reduced to a lower uniform value. This will eliminate the stabilising effect of the interfacial tension gradients - that is, the elimination of surface elasticity. 

The Gibbs elasticity, Eq, favors the formation of emulsion 1, because it slows the film thinning. On the other hand, increased surface diffusivity, D, decreases this effect, because it helps the interfacial tension gradients to relax, thus facilitating the formation... 

C. V. Stemling and L. E. Scriven, Interfacial turbulence hydrodynamic instability and the Marangoni effect, AIChE J. 5, 514 (1959) L. E. Scriven and C. V Stemling, On cellular convection driven by surface tension gradients effects of mean surface tension and surface viscosity, J. Fluid Mech. 19, 321 (1964). 

The foam-dilatational viscosity, K, arises because of two primary mechanisms (37) (1) viscous flow within the thin films, and (2) interfacial tension gradients acting along the foam bubble surfaces. The effect of interfacial tension gradients is to increase the foam viscosity as they impede flow near the surfaces of the thin foam films by contributing to a larger film stress. As in the wet foam (eq 6), the foam dilatational viscosity for a dry foam, K, is inversely proportional to film thickness as well (eq 9). 

In order to enhance coalescence in systems stabilized by the Marangoni-Gibbs effect, the interfacial activity of the surfactant must be high enough to account for the interfacial tension gradient created [39,41,42-44]. Some researchers found that the emul-... 

Independently and almost simultaneously the effect of molecular forces near a three-phase contact line was analyzed by Miller and Ruckenstein (1974) and Jameson and Garcia del Cerro (1976). While both papers point to the presence of asymmetric force fields, mainly generated by the presence of two dense phases (solid and liquid) and a gas phase, Miller and Ruckenstein (1974) developed the concept of a resulting force to explain the movement of a contact line and Jameson and Garcia del Cerro (1976) balanced the resultant force with an interfacial tension gradient. 

Hartland and Jeelani performed a theoretical study on the effect of interfacial-tension gradients on emulsion stability (71). Dispersion stability and instability were explained in terms of a siuface mobility which is proportional to the siuface velocity. When the interfacial tension gradient is negative, the siuface mobility is negative under some conditions, which greatly reduces the drainage so that the dispersion is stable. This is a normal situation as surfactant is present at the interface. Demulsifier molecules penetrate the interface within the film, thereby lowering the interfa-... 

Lewis Pratt, in 1953, were the first to report that the observed Marangoni convection in their experimental ternary systems was beneficial to hquid-hquid extraction processes because it increased mass transfer rates. The effect of density gradients on interfacial convection was studied by several researchers including Berg Morig (1969), who investigated the interaction between buoyancy and interfacial tension driven effects in ternary systems. The combined interfacial convection was also seen to be beneficial to mass transfer processes. 

Marangoni effect is a general terra for surface flow phenomena resulting from the appearance in the interface of an interfacial tension gradient. These phenomena produce traction on the adjoining sublayers and lead to two distinct and separate effects the flux-intensive surface renewal phenomena and the area-intensive thin-film phenomena. The first phenomena predominate when the depth of the liquid is much larger than the depth of penetration of surface movement and the second are important when the two depths are comparable. In the first case the affected process parameter is the mass transfer coefficient and in the second case the interfacial area. 

Interfacial effects Interfacial tension gradient Viscosity gradient Marangoni effect (near interfacial liquid layers move from area of low interfacial tension to areas of higher interfacial tension) Interfacial turbulences caused by interfacial tension gradient increase the mass transfer to the interphase compared to the mass transfer by only diffusion [6.5]... 

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