Emulsions composed of droplets of sub-micrometer size have widely been used as industrial products due to their peculiar characteristics. However, little research has been reported on the use of high pressure homogenizers to form emulsions composed of sub-micrometer droplets. In the present work, oil in water (o/w) emulsions composed of sub-micrometer droplets were produced by means of a high pressure wet-type jet mill, using liquid paraffin of different viscosities as a dispersed phase, and the effects of dispersed phase viscosity and operating conditions on the mean droplet size were investigated, together with the effects on emulsion viscosity to the dispersed phase viscosity, operating conditions and droplet diameter. When the number of passages (N) was unity, the volumetric drop size distribution showed a clear bimodal pattern with an increase in dispersed phase viscosity (eta(d)). For Ngreater than or equal to3, the drop size distribution showed a log-normal pattern at any eta(d) within the present experimental range. Sauter mean diameter (d(32)) decreased with increasing number of passages and reached a constant value at eta(d) and processing pressure (P) covered here. d(32) reached a constant value quickly with decreasing eta(d), and the effect of P on d(32) became extremely small. At high dispersed phase viscosities, d(32) reached a nearly constant value. For all the present experimental conditions at N = 1, the maximum droplet diameter (d(max)) based on droplet number was expressed in the form of d(max) = 1.96d(32). However, the proportionality constant increased with increasing eta(d), and decreased with increasing phi. For N = 1, the relationship between d(32) and P at the various combinations of eta(d) and phi could be expressed by a straight line on logarithmic coordinates. The slope became steeper with increasing phi and eta(d). Emulsion Viscosity (eta(e)) increased with increases in N, P and phi. However, as eta(d) increased, the effect of N and P on eta(c) became very small. Also, eta(e) increased sharply when d(32) became smaller than 0.1 mum.
