Thermorheological behavior of magnetic dispersions

We examine the temperature dependence of the steady and oscillatory shear f low properties of magnetic dispersions. The dispersions-characteristic of t hose used in the production of flexible magnetic data storage media-consist of acicular ferromagnetic metal oxide particles suspended in a solution of low molecular weight polymer and organic solvent. Although the viscosity o f the suspending fluid is independent of shear rate and displays traditiona l temperature dependence, the viscosity of the dispersion is dramatically s hear thinning and effectively independent of temperature, The small-amplitu de oscillatory shear flow material functions are also nearly independent of temperature. By examining dispersions wherein part of the magnetic particl es are replaced with nonmagnetic-but otherwise identical-particles we confi rm that the temperature independence is a consequence of the rheological be havior being governed by temperature-independent interparticle magnetic for ces. (C) 2001 The Society of Rheology.