Some Rheological Measurements on Magnetic Iron Oxide Suspensions in Silicone Oil

A model magnetic suspension was formulated with acicular γ‐Fe2O3 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 Rheometric 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.