Preparation of monodisperse multiple emulsions at high production rates by multi-stage premix membrane emulsification

Multiple W/O/W emulsions have been prepared by multi-stage (repeated) premix membrane emulsification using Shirasu-porous glass (SPG) membrane with a mean pore size of 10.7 μm. A coarse emulsion containing droplets with a mean particle size of about 100 μm was homogenized 5–6 times through the same membrane at a constant pressure difference of 20–300 kPa to achieve additional droplet homogenization and size reduction. The optimum conditions with regard to particle size uniformity were three homogenization cycles at a pressure difference of 100 kPa, under which the mean size of outer W/O particles was 9 μm and the span of particle size distribution was as low as 0.28. The optimum pressure difference in a single-stage process was higher, but the particle size distribution of prepared emulsions was broader than in a multi-stage process at smaller pressures. The transmembrane flux was in the range of 1.8–37 m3 m−2 h−1 and increased with increasing pressure difference and decreasing the content of W/O particles. The mean size of W/O particles in each cycle was remarkably constant over a wide range of their concentration of 1–60 vol.%.