An experimental method for the measurement of the stability of concentrated magnetic fluids

In this paper we present a device and method suited to the experimental determination of the sedimentation rate of concentrated suspensions of magnetic particles. The method is based on the measurement of the inductance of one or more sensing coils located at specified positions around a test tube containing the suspension. Such measurement is made possible by the determination of the resonant frequency of a parallel LC circuit in which L is the inductance of the sensing coil and C is the capacity of a capacitor chosen in such a way that the resonant frequency is easily measured. Upon calibration it is possible to relate the resonant frequency to the volume fraction of the particles at the coil location. The method is applied in the present work to the evaluation of the sedimentation kinetics of iron suspensions in base fluids of viscosities ranging from 0.3 to 100 mPa s and volume fractions of solids between 2.5 and 25%. Both if a single coil is used and if a set of three coils at different positions are employed, it is possible to detect the rate of accumulation of particles at the bottom of the container as well as a phenomenon of buoyancy of the largest particles brought about by the hydrostatic push of a dense fluid consisting of the smallest particles in the supporting liquid.