Rotor-stator homogenizers are widely used to manufacture liquid liquid emulsions, but little emphasis has been placed on understanding the processing principles of these devices. In this study, a systematic series of experiments have been performed to investigate and quantify the effects of process parameters and formulation variables on the drop size distribution (DSD) and relative sizes of oil-in-water emulsions manufactured in a Rotor-stator homogenizer. The effects of rotor speed, the dispersed phase viscosity and the dispersed phase volume fraction were studied. The DSDs by volume were predominantly bimodal, and the smaller drop size peak appeared at 1 mu m for most of the oil-in-water emulsions in our experiment. Furthermore, the relationship between the Sauter mean diameter and the maximum stable drop diameter was found to be linear in emulsions with a bimodal volume distribution, and the proportionality constant somewhat decreases as the viscosity of the dispersed phase increases. Compared with the normal and log-normal function, the Frechet function, which is usually proposed to describe the extreme value problem, can describe the monomodal number DSD very well The experiments also show that the low-viscosity droplets are stabilized by a combination of interfacial tension and viscous forces, while viscous forces primarily prevent breakage in high viscosity droplets. A modified version of Calabrese's model has been developed for combining the effect of the dispersed ! phase viscosity and volume fraction on drop size, which represents the experimental data well (C) 2013 Elsevier Ltd. All rights reserved.