Rotor-stator mixers (RSMs) are available in different designs, e.g. with different number of stator slots and slot dimensions. However, the relationship between stator design and the RSM hydrodynamics is not well understood. Consequently, manufacturers still base design and stator screen recommendations on trial-and-error. This study reports experimental measurements of how the flowrate through the stator slots, and velocity profiles in the region of relevance for mixing and breakup, is influenced by the stator slot width, using particle image velocimetry. It is concluded that the flowrate can be described by a design dependent flow number for all investigated geometries and that the flow number decreases with increasing slot width. Moreover, by studying the velocity profiles at different rotor speeds and designs, it is concluded that the velocity profile, its skewness and the proportion of back-flow (fluid re-entering the slot) scales with the flow number of the design. This suggests that the flow number, in addition to rotor speed, is a highly relevant parameter for describing the effect of design on batch RSM hydrodynamics. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.