Volume fractions, emulsions, effect viscosity

Colloidal interactions between emulsion droplets play a primary role in determining emulsion rheology. If attractions predominate over repulsive forces, flocculation can occur, which leads to an increase in the effective volume fraction of the dispersed phase and thus increases viscosity (McCle-ments, 1999). Clustering of milk fat globules due to cold agglutination increases the effective volume fraction of the milk fat globules, thereby increasing viscosity (Prentice, 1992). 

Consider first the effect of a dispersed phase, of volume fraction continuous phase of viscosity D0 and dispersed particles (droplets) which do not attract. At low volume fractions the Einstein equation should apply to a suspension of solid particles at constant temperature,... 

Figure 5. Schematic diagram showing the effect of changing the volume fraction of second phase on the apparent viscosity at a fixed rate of shear of a two-phase emulsion. The different dotted lines refer to different viscosities of the pure phases A and B, The solid line suggests the viscosity that may be displayed by a system in which both the viscosities of the pure phases and the relative proportions of phases are changing continuously, as in a pyrolysis run.

Effects of Solids Size. The effect of solids size on the viscosity of the emulsion-solids mixtures is shown in Figure 19 for synthetic OAV emulsions. The oil concentration (solids-free basis) is 60% by volume, and the solids used are silica sand. The comparison is made at shear stresses of 6 and 14 Pa. The viscosity is expressed as the relative viscosity (t7ows/ 7ow)t lhat is, the viscosity of the emulsion-solids mixture divided by the viscosity of the solids-free emulsion. At low solids volume fraction (<0.1), solids size has little effect. 

Effect of Emulsion Characteristics. As discussed in Chapter 4, the rheology of emulsions is affected by several factors, including the dis-persed-phase volume fraction, droplet size distribution, viscosity of the continuous and dispersed phases, and the nature and amount of emulsifying surfactant present. All of these parameters would be expected to have some effect on flow behavior of the emulsion in porous media. However, the relationship between bulk rheological properties of an emulsion and its flow behavior in porous media is feeble at best because, in most cases, the volume... 

Effect of Aging. With increasing volume fraction of the dispersed phase, increasing droplet diameter and wider diameter distribution, the viscosity of a dispersed system increases (36). Unstable emulsions show droplet coalescence by extending the diameter distribution, accompanied by viscosity increasing, an effect, which is called "aging" (36-38). ... 

Equations proposed by Sherman for predicting viscosities from apparent volume fractions and particle diameters were useful in analyzing the effects of formulation and preparation variables and aging on emulsion viscosities. 

At lower Na+ concentration, the high negative charge (zeta potentials of salt-free emulsion droplets are 115 mv and above (5)) immobilizes a water layer around the droplets, thus effectively increasing the apparent volume fraction. Addition of Na+ reduces the net charge and some of the immobilized water is released giving a lower apparent volume fraction and lower apparent viscosity. 

Liquid droplets cannot be treated the same as solid particles in their codispersed systems. This behavior has been indicated by equation 66 or 68, in which the Einstein constant increases with increasing viscosity ratio of the dispersed phase to the continuous phase. As is shown by Yan et al. (195, 197, 198), liquid droplet and solid particle effects are additive only when the solid concentration is low, say s < 0.05, and when both solid particles and liquid droplets have comparable sizes. However, when the particle-to-droplet size ratio is large, the particles and the droplets become additive (192) for a wider solid concentration range (Figures 34 and 35). The apparent viscosity of the system may be added in terms of the two distinct model systems pure emulsion characterized by solid-free dispersed phase volume fraction and pure suspension characterized by the volume fraction of the solids. The additive rule for the ternary systems is similar to the rule for bimodal solid particle suspensions due to Farris (139) ... 

The effect of filler loading on Mooney viscosity of the rubber compound is a good indication of the immobility and hydrodynamic effect caused by the filler in the unvulcanized rubber. Figure 3.8 shows a plot of Mooney viscosity against volume fraction of filler in the vulcanizate base for data shown in Table 3.3. The theoretical value was calculated using Equation (3.12). This equation is an extension of Einstein s equation. Einstein studied colloidal suspensions and emulsions by hydrodynamic analysis. The viscosity of the... 

Polysaccharides increase the viscosity of the continuous phase of the emulsion. One of the main functions of polysaccharides in emulsions is to thicken the continuous hquid. The intended effect is usually to impart a desired texture (increase viscosity or stiffness to the system and reduce buoyancy-driven creaming or sedimentation of the emulsion droplets and other particles in the system). Because of their highly swollen molecular structure in solutions, leading to a high effective volume fraction at low concentrations, most polysaccharides are very effective in providing a high viscosity at low concentration. 

The third factor that affects emulsion rheology is the droplet size distribution. This is particularly the case at high volume fractions. When rj) > 0.6, 7 is inversely proportional to the reciprocal of the mean droplet diameter [4ff]. The above equations do not show any dependence on droplet size and an account should be made for this effect by considering the average distance between the droplets in an emulsion. At high shear rate, the droplets are completely deflocculated (i.e. all structure is destroyed) and they are equidistance from each other. At a critical separation between the droplets, which depends on droplet size, the viscosity shows a rapid increase. The average distance of separation between the droplets, hm, is related to the droplet diameter, dm, by the simple expression. 

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