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rates in most MR fluid devices can reach up to 20,000s' 1 .
properties of MR fluids at high shear strain rates is warranted.
Therefore, study of the rheological
Base on these observations, a piston-driven flow type rheometer has been designed and built at the
Composite and Intelligent Materials Laboratory of the University of Nevada, Reno, to evaluate the
tunable rheological properties of MR fluids in flow mode. Three different MR fluid samples are
investigated in this study. The apparent viscosity, dynamic and static shear yield stress, under the
influence of applied magnetic fields, are studied.
METHODS AND THEORETICAL ANALYSIS
Figure 1 presents the piston-driven flow-mode rheometer system for MR fluids with a rectangular
cross section. An electromagnet provides a magnetic flux normal to the slit channel flow. The coil is
activated by a power supply in constant current mode. By using a Gauss meter, the magnetic flux
density inside the channel is measured.
Piston (connected to
Instron 882 Is)
:ro magnet
Accumulator
Figure 1. Schematic of the MR channel flow rheometer.
The MR fluid is confined in the well-sealed channel cell, and is pressurized to generate a flow through
the channel between the two parallel magnet poles. There is an external groove in the test channel in
order to align the electromagnetic field over the test section. The control of the hydraulic source piston
velocity is achieved by connecting the piston-cylinder mechanism to an Instron 882IS servo hydraulic
actuator. Two pressure transducers measure the pressure drop across the test section. The test section
has dimensions of h= 1.0mm in height, w=10mm in width and L=14mm in length (Figure 2). The
proposed MR fluid rheometer utilizes a nitrogen gas accumulator. The accumulator functions as a
sink, when the piston is positively displaced, and a source, when the piston is negatively displaced,
making it possible to measure the static and dynamic shear yield stress of MR fluids.
Pressure P
Magnetic field B
Figure 2. Geometric dimensions of the MR rheometer channel.