A DEVICE FOR MEASURING THE CONCENTRATION AND DISPERSION QUALITY OF MAGNETIC PARTICLE SUSPENSIONS

We present a new technique (which we refer to as the rheomagnetic measurement) for studying the concentration and orientation of magnetic particles through inductance measurement. This is a novel method that may be complementary to conventional techniques, such as light scattering, photographic methods, and rheological measurements. Compared to conventional techniques, the rheomagnetic method is uniquely suitable for process control during the early stages of slurry production in manufacturing, since it requires little test time, measures the "dispersion quality" of concentrated suspensions, operates automatically and can be implemented on-line. The rheomagnetic measurement is based on the following concepts. The particles are oriented in a predominantly extensional flow field and, because they are magnetic, their orientation can be detected with a weak magnetic sensing field. Because flocs of magnetic particles orient differently in a flow field than primary particles do, this method can be useful in obtaining information about the particle flocculation aspect of "dispersion quality." A magnetic sensing field can also be used to detect the particle concentration in a quiescent flow. Experimental data on the effects of particle concentration and milling for rod-like gamma-Fe2O3 and plate-like Ba-Ferrite suspensions are discussed. This is the first observation reported on the rheomagnetic orientation measurement for Ba-Ferrite magnetic particles, and the results markedly contrasted with those of the rod-like magnetic particles but showed similarity with those of rod-like gamma-Fe2O3.