Abstract This work numerically studies the flow of a Casson liquid over a flow-disturbing rib in a rectangular μ -channel. The aim is to investigate, using a previously validated CFD code, the effect of the non-Newtonian behavior on the fluid flow characteristics, i.e. the size of the reattachment length at the bottom of the channel and downstream from the rib, in a μ -channel equipped with a Flow Disturbing Rib ( FDR ). The effect of the type of fluid, the Re number and key geometrical parameters on the size of the bottom reattachment length of the primary recirculation zone is investigated by performing a parametric study based on the Design of Experiments and the Response Surface Methodology . The results show that the reattachment length is mainly affected by the Re value and the rib height. It was also found that for Re