Effect of the plate surface characteristics and gap height on yield stresses of a magnetorheological fluid

Abstract

Effects of the plate material, surface roughness and measuring gap height on static and dynamic yield stresses of a magnetorheological (MR) fluid were investigated with a commercial plate–plate magnetorheometer. Magnetic and non-magnetic plates with smooth (Ra ∼ 0.3 μm) and rough (Ra ∼ 10 μm) surface finishes were used. It was shown by Hall probe measurements and finite element simulations that the use of magnetic plates or higher gap heights increases the level of magnetic flux density and changes the shape of the radial flux density profile. The yield stress increase caused by these factors was determined and subtracted from the measured values in order to examine only the effect of the wall characteristics or the gap height. Roughening of the surfaces offered a significant increase in the yield stresses for non-magnetic plates. With magnetic plates the yield stresses were higher to start with, but roughening did not increase them further. A significant part of the difference in measured stresses between rough non-magnetic and magnetic plates was caused by changes in magnetic flux density rather than by better contact of the particles to the plate surfaces. In a similar manner, an increase in gap height from 0.25 to 1.00 mm can lead to over 20% increase in measured stresses due to changes in the flux density profile. When these changes were compensated the dynamic yield stresses generally remained independent of the gap height, even in the cases where it was obvious that the wall slip was present. This suggests that with MR fluids the wall slip cannot be reliably detected by comparison of flow curves measured at different gap heights.

Dynamic and rate-dependent yielding behavior of Co0.9Ni0.1 microcluster based magnetorheological fluids

“…Generally, field-dependent magnetorheological parameters (yield stress, shear viscosity, storage modulus) exhibit linear or power-law relationship with concentration of MR fluids [17]. This also holds good for the samples as we observe systematic increment of shear stress upon application of field.…”

Unexpected shear strength change in magnetorheological fluids

“…[1][2][3] The strength of MR fluids in shearing mode is usually characterized by the static yield stress, which is measured in stress-controlled mode via tangent method, and the dynamic yield stress, in which the shear stress at zero shear rate is extrapolated by fitting the Bingham model on the experimental data. 4,5 The yield stress of MR fluids exponentially increases (exponent, 1-2) with the applied magnetic field. [6][7][8] This exponent decreases under high magnetic fields, and the shear stress tends to become saturated with increasing magnetic field because of magnetization saturation.…”

Effects of Nano-Diamond on Magnetorheological Fluid Properties

“…The wall shape change affected the magnetic¯eld strength distribution, whereas the researchers discovered that deeper the groove, higher the magnetic¯eld strength. 24 At this time, the ferromagnetic particles became a longer solid chain and because the wall was no longer smooth, the friction between the magnetic chain and the wall became higher. This led in the exhibition of a high shear yield strength.…”