An oscillatory slit flow type rheometer has been developed to measure the dynamic viscoelasticity in an oscillatory slit flow of an ER fluid. Using the rheometer, an ER suspension of numerously sulfonated polymer particles in an insulating oil is evaluated. The results are compared with those for the oscillatory shear flow. In the slit flow, the dynamic viscoelasticity of the ER suspension mainly depends on the fluid strain amplitude and the applied electric field strength. In the fluid strain amplitude of less than about 50 % the viscoelastic property becomes dominant, while in the fluid strain amplitude of more than that the viscoelastic plastic property dominates. As the applied field strength increases, the dynamic viscoelasticity changes from viscoelastic plastic to linear viscoelastic. In an oscillatory shear flow, on the other hand, the dynamic viscoelasticity of the ER suspension largely depends on the fluid strain and hardly on the applied field strength. The differences between the dynamic viscoelasticity in the two flow modes can be explained based on the formation and deformation of ER particle clusters which span the electrodes.
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