Abstract
Oil-in-water (o/w) emulsions composed of droplets of sub-micrometer size were produced by means of a high pressure wet-type jet mill, in which kerosene and liquid-paraffin of various viscosities were used as the dispersed phase. The dependencies of mean droplet size and emulsion viscosity (ηe) on operational variables were discussed.
Sauter mean diameter (d32) of droplets decreased with increasing number of passages (N), and reached a constant value at any dispersed phase viscosity (ηd) and processing pressure (P) covered here. d32 quickly reached a constant value with decreasing ηd, and the effect of P on d32 became extremely small. At high dispersed phase viscosities, d32 became independent of ηd. ηe increases with N, P and the dispersed phase fraction. However, as ηd increases, the effects of N and P on ηe became very small. Also, as d32 became smaller than 0.1µm, ηe increased sharply.
Emulsions composed of dispersed phase with low viscosity exhibited a time dependency of the droplet size distribution, presumably because of the occurrence of droplet coalescence. Such time dependency was discussed on the basis of a kinetic model. The process of the coalescence was described in terms of a second-order process consisting of a rapid coalescence process and a slow coalescence one. The slow coalescence process was only obtained at large N and Cs > 1 wt%, for any P. However, under small N and ø > 0.1, the slow coalescence process was only obtained at P > 150 MPa. The coalescence rate constant increased with increasing dispersed phase fraction, and decreased with increasing surfactant concentration.
