8.5.1. Dispersed-Phase Volume Fraction
The viscosity of an emulsion increases with dispersed-phase volume fraction. At low droplet concentrations, this increase is linearly dependent on volume fraction (Equation 8.19), but it becomes steeper at higher concentrations (Equation 8.30). Above a critical dispersed-phase volume fraction (^c), the droplets are packed so closely together that they cannot easily flow past each other, and so the emulsion has gel-like properties. The precise nature of the dependence of the viscosity on volume fraction is mainly determined by the nature of the colloidal interactions between the droplets.
The viscosity of an emulsion is directly proportional to the viscosity of the continuous phase (Equations 8.19 and 8.30), and so any alteration in the rheological properties of the continuous phase has a corresponding influence on the rheology of the whole emulsion. For this reason, the presence of a thickening agent in the aqueous phase of an oil-in-water emulsion (Pettitt et al. 1995, Pal 1996) or the presence of a fat crystal network in the oil phase of a water-in-oil emulsion (Moran 1994) largely determines the overall rheological properties of the system.
The rheology of the dispersed phase has only a minor influence on the rheology of food emulsions because the droplets are covered with a fairly viscoelastic membrane, which means they have properties similar to rigid spheres (Tadros 1994, Walstra 1996a).
The influence of the droplet size and the droplet size distribution on the rheology of an emulsion depends on the dispersed-phase volume fraction and the nature of the colloidal interactions. The viscosity of dilute emulsions is independent of the droplet size when there are no long-range attractive or repulsive colloidal interactions between the droplets (Pal et al. 1992, Pal 1996). When there is a relatively long-range repulsion between the droplets (i.e., 8 ~ r), their effective volume fraction is much greater than their actual volume fraction, and so there is a large increase in the viscosity of the emulsion (Tadros 1994, Pal 1996). The droplet size also influences the degree of droplet flocculation in an emulsion (Chapters 3 and 7), which has an impact on the emulsion rheology. For example, the greater the extent of droplet flocculation, or the more open the structure of the flocs formed, the larger the emulsion viscosity.
The mean droplet size and degree of polydispersity have a particularly significant influence on the rheology of concentrated emulsions (Liu and Masliyah 1996, Pal 1996). In emulsions that contain nonflocculated droplets, the maximum packing factor (^c) depends on the poly-dispersity. Droplets are able to pack more efficiently when they are polydisperse, and therefore the viscosity of a concentrated polydisperse emulsion is less than that of a monodisperse emulsion with the same droplet volume fraction. Emulsions that contain flocculated droplets are able to form a three-dimensional gel network at lower volume fractions when the droplet size decreases.
The droplet size also alters the rheology of emulsions due to its influence on the relative importance of rotational and translation Brownian motion effects compared to the shear stress (Mewis and Macosko 1994).
The nature of the colloidal interactions between the droplets in an emulsion is one of the most important factors determining its rheological behavior. When the interactions are long range and repulsive, the effective volume fraction of the dispersed phase may be significantly greater than its actual volume fraction, ^eff = ^(1 + 8/r)3, and so the emulsion viscosity increases (Section 8.4.6). When the interactions between the droplets are sufficiently attractive, the effective volume fraction of the dispersed phase is increased due to droplet floccu-lation, which results in an increase in emulsion viscosity (Section 8.4.4). The rheological properties of an emulsion therefore depend on the relative magnitude of the attractive (mainly van der Waals, hydrophobic, and depletion interactions) and repulsive (mainly electrostatic, steric, and thermal fluctuation interactions) interactions between the droplets (Chapter 3). A food scientist can therefore control the rheological properties of food products by manipulating the colloidal interactions between the droplets. Increases in the viscosity of oil-in-water emulsions due to droplet flocculation have been induced by adding biopolymers to increase the depletion attraction (Dickinson and Golding 1997a,b), by adding biopolymers to cause bridging flocculation (Dickinson and Golding 1997a), by altering the pH or ionic strength to reduce electrostatic repulsion (Hunt and Dalgleish 1994, Demetriades et al. 1997a), and by heating protein-stabilized emulsions to increase hydrophobic attraction (Demetriades et al. 1997b).
The charge on an emulsion droplet can influence the rheological properties of an emulsion in a number of ways. First, the charge determines whether the droplets are aggregated or unaggregated and the distance of closest approach (Section 8.5.4). Second, the droplet charge influences the rheology due to the primary electroviscous effect (Pal 1996). As a
charged droplet moves through a fluid, the cloud of counterions surrounding it becomes distorted (Figure 8.18). This causes an attraction between the charge on the droplet and that associated with the cloud of counterions that lags slightly behind it. This attraction opposes the movement of the droplets and therefore increases the emulsion viscosity because more energy is needed to cause the droplets to move at the same rate as uncharged droplets.
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