Rheology of emulsions

At an applied level, study of the rheology of emulsions is vital in many industrial applications of personal care products. It is perhaps 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, j . In very dilute emulsions ( 0.01), the relative viscosity, of the system may be related to ) using the simple Einstein equation (as for solid/liquid dispersions) [45], i.e. 

As the volume fradion of the emulsion is gradually increased, the relative viscosity becomes a more complex function of and it is convenient to use a polynomial representing the variation of with, 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 or lower than that of the dispersions medium. This problem was considered by Taylor [47], who extended Einstein s hydrodynamic treatment for suspensions to the case of droplets in a liquid medium. Taylor [47] assumed that the emulsifier film around the droplets would not prevent the transmission of tangential and normal stresses from 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 r, 

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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Borwankar R P and Case S E 1997 Rheology of emulsions, foams and gels Curr. Opin. Coiioid interface Sc/. 2 584-9... 

J.-B. Salmon, L. Becu, S. Marmeville, A. Colin 2003, (Towards local rheology of emulsions under Couette flow using dynamic light scattering), Eur. Phys. J. E 10, 209. 

G. Gillies and C.A. Prestidge Interaction Forces, Deformation and Nano-Rheology of Emulsion Droplets as Determined by Colloid Probe AFM. Adv. Colloid Interface Sci. 108-109, 197 (2004). 

L. Bressy, P. H braud, V. Schmitt, and J. Bibette Rheology of Emulsions Stabilized by Solid Interfaces. Langmuir 19, 598 (2003). 

Dickinson, E., Golding, M. (1998). Influence of calcium ions on creaming and rheology of emulsions containing sodium caseinate. Colloids and Surfaces A Physicochemical and Engineering Aspects, 144, 167-177. 

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. 

Pal, R. 1996. Rheology of emulsions containing polymeric liquids. In, Encyclopedia of Emulsion Technology, Vol. 4 (P. Becher, ed.), pp. 93-264, Marcel Dekker Inc., New York. 

Rheology of Emulsions and Immiscible Blends 413 9.3.2 Linear Viscoelasticity... 

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. 

B. W. Barry. Stmcture and rheology of emulsions stabilized by mixed emulsifiers. Rheol. Acta, 10, 96-105 (1971)... 

Barnes, H.A., Rheology of emulsions — a review, in Proc. Eirst World Congress on Emulsion, 19-22 Oct, Paris, 1993, p. 267. 

Sherman, P., in Rheology of Emulsions, P. Sherman, Ed., Pergamon Press, Oxford (1963) in Emulsion Science, P. Sherman (Ed.), Academic Press, London (1968). 

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