Viscosity and rheological characteristics of emulsions

The main factors which affect the viscosity of emulsions are listed in Table 8.6. The properties of the disperse phase, the continuous phase and the emulsifying agent or agents all influence the emulsion viscosity. Each factor does not act independently and the interpretation of emulsion viscosity data is complicated by this fact and the fact that particles can deform under shear depending on the nature of the interfacial film. As we have also discussed, emulsions are complex systems, often highly structured, and at phase boundaries or on the point of inversion are very sensitive to small perturbations in the system. We will deal here first with mobile emulsions and then consider briefly the semi-solid state. 

Of practical interest is how the viscosity of an emulsion changes with concentration of disperse phase. If the particles in a suspension are rigid (i.e. non-deformable) and in a dilute state ( / = 0.05) Einstein s equation gives the 

This equation reduces to Equation 8.49 when rji = rjo. Equation 8.50 was modified to take account of more concentrated systems and the following equation [185] has been found to agree with experimental data up to 0 = 0.4. 

Attempts have also been made to take into consideration the influence of the emulsifier layer around the globules. All equations show that rj increases with increase of disperse phase volume fraction, / . When 0 exceeds 0.4 to 0.5 the emulsions tend to become pseudo-plastic and the viscosity increases significantly for small changes in 0. Like all equations which deal with emulsions higher orders of / appear in the equation when l 0.05. A more general equation for concentrated systems takes the form [186, 187]  

Although large variations in the values of h have been reported, many emulsions exhibit a value of 1.3 for h [184]. When the continuous phase is immobilized, and H is a measure of this volume, the free volume of the continuous phase is thus 1 and several equations have taken the form 

---