Sedimentation and redispersion phenomena in iron-based magnetorheological fluids

In this work, the effect of three different additives (oleic acid, aluminum stearate, and silica nanoparticles) on the aggregation, sedimentation, and redispersibility of concentrated iron-based magnetorheological fluids was investigated. With this aim, the sedimentation behavior was analyzed using an electromagnetic induction method, which is suitable for studying the sedimentation of opaque magnetic suspensions. The redispersibility was studied, in a quantitative way, by means of rheological measurements, both in the presence and in the absence of external magnetic field. For this purpose, samples were subjected to a constant shear stress at different moments (steps) of the settling process. The time evolution of the corresponding shear rate was measured at each step. Interestingly, it was found that although the addition of oleic acid or aluminum stearate does not avoid particle settling, the redispersibility of the suspensions is considerably enhanced. On the contrary, silica nanoparticles behave as a gel-forming agent capable of preventing particle settling under rest conditions. Unfortunately, when iron-silica suspensions are sheared compact sediments are progressively formed, making the redispersion extremely difficult. (c) 2006 The Society of Rheology.