A model magnetic suspension was formulated with acicular y-Fe203 particles dispersed in silicone oil. Four volume fractions (2.4, 5.3, 8.7, and 12.9%) were studied in steady shearing over a wide range in rate (10-5 to 10 +3 s-1) using a Rheometrics System Four Rheometer. At medium rates instabilities in both cone and plate and parallel plate were observed. At low rates (<10-1 s-1) the stress became independent of rate indicating yield behavior. Yield stresses of 0.25, 4, 5, and 20 Pa were estimated for the four samples. On the 12.9% sample a yield stress of 20 Pa was also found using small amplitude sinusoidal oscillations and with a constant stress rheometer. Below this yield value, solid-like elastic behavior was observed.
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