Rotational magnetic particle microrheometry: The Newtonian case

Abstract

A rotating sphere microrheometer, based on extensions of the work of Valberg [Valberg, P.A., “Magnetometry of Ingested Particles in Pulmonary Macro-Phages,” Science 224, 513–516 (1984); Valberg, P.A., and H. A. Feldman, “Magnetic Particle Motions within Living Cells: Measurement of Cytoplasmic Viscosity and Motile Activity,” Biophys. J. 52, 551–572 (1987)] and Edwards and Yeates [Edwards, P.A., and D. B. Yeates, Viscoelasticity of Biomaterials, Chap. 16, ACS Symposium Series, 489, edited by W. Glasser and H. Hatakeyama (Boston, MA, 1992)], was developed to rapidly (within 10 s) measure the rheological properties of small (∼10 μL) quantities of highly viscous (100–10 000 poise) fluids at small (10−3–10−1 Hz) rates of strain. Previous experimental work was extended by the use of MQP-B™ 1.4-μm-radius particles, which have extremely high coercivity and remanent magnetic field, and in which rotation of magnetic domains within the particle does not occur. The microrheometer was tested with a series of Newtonian viscosity standards (100–10 000 poise) and found to accurately predict viscosity (error range 3%–9%). The effects of shape, size distribution, sedimentation, particle–particle magnetic interactions, and agglomeration were investigated and found to be either negligible or easily determined. This microrheometer can be used to determine the rheological properties of minute quantities of viscous fluids and may be applicable to the measurement of zero-shear-rate viscosity of viscoelastic fluids.