EFFECT OF MAGNETIC HYSTERESIS OF THE SOLID PHASE ON THE RHEOLOGICAL PROPERTIES OF MR FLUIDS

Abstract

An experimental investigation is described concerning the effect of the existence of a remanent magnetization of the dispersed particles on the rheological properties of magnetorheological fluids (MRF). Two MRF's were used: (1) solid phase: cobalt ferrite particles + silica gel (1.5% w/w); liquid phase: silicone oil (viscosity 20 mPa·s); and (2) solid phase: carbonyl iron + silica gel; liquid phase; silicone oil. The cobalt ferrite particles were synthetized as monodisperse colloidal spheres with an average diameter of 850 nm. The dependence of the dimensionless shear stress (τ⋆/ϕ) vs. Mason number (Mn) fails to scale when a "magnetorheological hysteresis procedure" is followed, specially for the higher volume fractions used (≈ 7.5%). The yield stress (τy) is first estimated from successive rheograms obtained decreasing the external field (H0) values for different ϕ. A more precise determination can be done by applying a stress ramp in the oscillatory regime. The critical stress amplitude (τc) needed to exceed the viscoelastic linear region (VLR) is obtained. It is found that both τy and τc strongly depend on the magnetic history of the sample. As expected, the previous results were not obtained in a classical MRF of carbonyl iron particles since they do not present magnetic hysteresis. We conclude that cobalt ferrite suspensions are an other kind of MRF which works at low fields (0 – 17.8 kA/m) with the opposite effect: decrease of the yield stress with the field. This property can be improved using particles with stronger remanent magnetization.