The rheology of emulsions

Most emulsions, unless very dilute, display hoth plastic and pseudoplastic flow behaviour rather than simple Newtonian flow. The flow properties of fluid emulsions should have little influence on their biological behaviour, although the rheological characteristics of semisolid emulsions may affect their performance. The pourability, spreadability and syringeability of an emulsion will, however, be directly determined by its rheological properties. The high viscosity of w/o emulsions leads to problems with intramuscular administration of injectable formulations. Conversion to a multiple emulsion (w/o/w), in which the external oil phase is replaced by an aqueous phase, leads to a dramatic decrease in viscosity and consequent improved ease of injection. 

The influence of phase volume on the flow properties of an emulsion is shown in Fig. 7.26. In this diagram the relative viscosity of the system increases with increasing 4 , and at any given phase volume increases with decreasing mean particle size, D. These and other factors which affect emulsion viscosity are listed in Table 7.6. 

Particle size and size distribution Chemical nature 

Chemical constitution and concentration Solubility in the continuous and internal (disperse) phase 

Physical properties of the interfociol film Electroviscous effects 

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When solids are present in emulsions, the volume fractions of the particles and the droplets are usually not additive for the purpose of predicting overall dispersion viscosity. A number of different kinds of behaviours can result, depending on the sizes and shapes of the particles versus the droplets, and upon whether they exist as kinetically independent entities, whether the particles exist in adsorption layers surrounding the droplets, or dispersed within the droplets themselves, or a combination of all of the above. Pal et al. [382] and Nasr-El-Din [85] discuss the rheology of emulsions containing solids in some detail. 

Little work has been done on the rheology of emulsions with added solids, despite the fact that handling of mixtures of emulsions and solids is encoun-... 

Average Droplet Size and Droplet Size Distribution, All practical emulsions show some form of droplet size distribution with an average value representing this size distribution. The average droplet size and the droplet size distribution affect the rheology of emulsion (discussed in Chapter 4). Droplet size in relation to pore throat size affects the flow of fluids in porous media, as discussed previously (Figure 3). 

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... 

As discussed earlier, the rheology of emulsions depends on a number of factors, primary among which is the quality. Emulsions with qualities of less than 50% (oil) are considered Newtonian, whereas those having higher qualities exhibit non-Newtonian behavior. 

A number of factors affect the rheology of emulsions composition, the viscosity ratio of the dispersed-to-matrix phase (1 s 7/2/771), the droplet size and its distribution, rheology of the interphase, and so on. Often, well-stabilized emulsions follow the viscosity-concentration relationships developed for hard sphere suspensions, including the yield phenomena. In contrast, emulsions with deformable dispersed... 

At an applied level, study of the rheology of emulsions is vital in many industrial applications of personal care products. It is useful to summarize the factors that affect emulsion rheology in a qualitative way. One of the most important factors is the volume fraction of the disperse phase, ( ). In very dilute emulsions (( )< 0.01), the relative viscosity, Tir, of the system may be related to ( ) using the simple Einstein equation (as for solid/ liquid dispersions) (15) i.e.. 

Another factor that may affect the rheology of emulsions is the viscosity of the disperse droplets. This is particularly the case when the viscosity of the droplets is comparable to or lower than that of the dispersions medium. This problem was considered by Taylor (17), who extended the Einstein hydrodynamic treatment for suspensions for the case of droplets in a liquid medium. Taylor (17) assumed that the emulsifier film around the droplets would not prevent the transmission of tangential and normal stresses form the continuous phase to the disperse phase and that there was no slippage at the o/w interface. These stresses produce fluid circulation within the droplets, which reduces the flow patterns around them. Taylor derived the following expression for 11 ... 

The chapter next dealt with the rheology of emulsions stabilized by polymeric surfactants. The factors affecting the rheology of emulsions were briefly discussed. This was followed by a section on interfacial rheology and its correlation with emulsion stability. The bulk rheology of oil-in-water and water-in-oil emulsions stabilized by polymeric surfactants was described. Both steady-state and viscoelastic investigations were described. These emulsions show a transition from predominantly viscous to predominantly elastic response as the frequency of oscillation exceeds a critical value. This allows one to obtain... 

McClements, D.J. (2003) The rheology of emulsion-based food products. In B.M. McKenna (ed.), Texture in Foods, Volume 1 Semi-solid Foods. CRC Press, Boca Raton, FL, Chapter 1. 

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