Nonhomogeneities in Couette flow of ferrite suspensions

Barium ferrite particles were stabilized and suspended to 3, 5, 7, and 9 vol % in mineral oil. The particles were roughly hexagonal platelets, 100 nm wide and 17 nm thick. When sheared at rates less than 10 s^-1in Couette rheometers of several gap sizes and surface treatment these suspensions gave erratic torque readings. Cycles in the torque corresponded to cycles of alternating stick and slip in the flow, as observed through a glass outer cylinder. Optical and transmission electron micrographs of a similar sample in which the matrix was polymerized indicated flocculated structures even at very low particle volume concentrations. At shear rates greater than 10 s ^{— 1} the torque signal was steady and the flow became homogeneous in appearance. Similar behavior was observed in other flocculated suspensions, namely those made from both the magnetic and nonmagnetic forms of acicular (needle-like) shaped ferric oxide particles.