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RESULTS AND DISCUSSIONS
Figure 4 shows the measured apparent viscosity versus shear strain rate for Lord MRF-132LD fluid at
various magnetic fields for the range of 20s" 1 to 20,000s" 1 . These results are compared to those
obtained by a commercial parallel-plate MR fluid rheometer (Li, 2000). As can be seen from Figure 4,
the results of both studies agree well in'the range of 20 s* 1 to 200 s" 1 where the experimental data of
shear strain rates for these two different rheometers overlap. In addition, the experimental results show
a linear relation in the log-log scale, which implies that there is a power-law dependence (i.e.,
rj app cc y~ m ) of the apparent viscosity, rj app , on the shear strain rate, /, for MR fluids. A shear thinning
effect was observed as the apparent viscosity decreases with the increase of shear strain rate for all
three MR fluid samples. At the "on-state" (magnetic field is activated), all exponent m values are in
the range of 0.60-0.94. "m" increases with increasing the magnetic flux density. Similar phenomena
were also reported for monodispers electro-rheological fluids (Halsey, et al., 1992) and monodispers
MR fluids (Felt, et al., 1996). At the "off-state" (with no applied magnetic field), ferrofluid-based MR
fluid behaves as a Newtonian fluid, when m is close to zero. The PAO based MRPG shows a weak
shear thinning effect where its power law exponent m is much less than that of the "on-state".
Typical results of the shear stress versus shear strain rate for the Lord MRF-132LD fluid are shown in
Figure 5. The dynamic shear yield stress is obtained by extrapolating shear stress data to intercept the
shear stress axis based on the Bingham plastic model. For an increase in magnetic flux density, a
similar increase in the respective shear stress is observed.
Figure 6 shows the experimental results of shear stress dependence on the magnetic flux density for
three MR materials. The static shear yield stress was obtained by evaluating the experimental data of
the static pressure drop employing Equation (1). Note that the dynamic shear yield stress exceeds the
static case over an entire range of magnetic fields for all three samples. These results are consistent
with Kordonski et al. (1999) and Li (2000) experimental studies where they examined MR fluid in
shear mode. Volkova et al (2000) analyzed the existence of the two different shear yield stresses for
two types of magnetic particle suspensions in the presence of a magnetic field. They explained that
the dynamic shear yield stress (whic they referred to it as Bingham yield stress) is associated with the
rupture of the aggregate which reform in the presence of the magnetostatic forces, while the static yield
stress (which they referred to it as frictional yield stress) is associated with solid friction of the
particles on the plates of the rheometer. Therefore, there is no logical reason for the static and the
dynamic yield stress to have the same value.
SUMMARY AND CONCLUSIONS
A flow-mode rheometer is designed and built to evaluate the tunable rheological properties of MR
materials in the presence of a magnetic field. This instrument can be used to study the apparent
viscosity of MR fluids for a wide range of shear strain rates, from 20s" 1 to 20,000s" 1 . A Rabinowitch
correction for the slit channel flow is developed to obtain the shear strain rate at the wall as a function
of shear stress. By measuring the pressure drop as a function of flow rate, the apparent viscosity is
determined. A unique design feature for the flow-mode MR fluid rheometer used in this study allows
measurements of both the dynamic and static shear static yield stress of MR materials.