7.4 Viscosity of the Continuous Phase

An increase in the viscosity η of the continuous phase reduces the diffusion coefficient D of the droplets，since，for spherical droplets，

Where k=the Boltzmann constant;

T=the absolute temperature;

a=the radius of the droplets.

As the diffusion constant is reduced，the frequency of collision of the droplets and their rate of coalescence are reduced. The viscosity of the external phase is increased as the number of suspended particles increases，and this is one of the reasons that many emulsions are more stable in concentrated form than when diluted. The viscosity of the external phase in emulsions is often increased by the addition of special ingredients for this purpose，such as natural and synthetic "thickening" agents.