Wall slip and surface-induced bias in steady-shear rheometry of magnetorheological fluids

Magnetorheological (MR) fluids are commonly characterized by parallel-plate rheometry, which also reproduces the shear conditions in MR clutches and brakes. The measured response, however, depends on wall boundary conditions. This study quantifies the effect of surface condition on complete steady-shear flow curves of a commercial MR fluid measured with smooth (PP) and roughened (PPR) plates at gap heights of 0.2–0.8 mm and several magnetic field strengths. PP systematically produced lower shear stresses than PPR, by more than 60% under the most unfavorable conditions. Under the residual field, the PP–PPR difference depended strongly on shear rate, whereas under applied fields it approached a field-dependent plateau. Changing the gap also changed the magnetic flux density in the sample by up to about 12%. After this effect was accounted for, the PPR yield stress under applied fields remained within about −4% to 6% without systematic gap dependence; gap-dependent differences therefore cannot be attributed solely to wall slip or confinement. A differential PP–PPR analysis indicated slip-dominated behavior under applied fields and non-uniform deformation across the gap under the residual field. For MR devices with slip-suppressing working surfaces, designs based on smooth-plate data may underestimate the torque by about 35–50%. Quantitative MR fluid measurements require the surface condition, measuring gap, and actual magnetic conditions in the sample to be controlled and reported.