Analysis of single-domain particle flow orientation data

Abstract

Rodlike single-domain particles tend to orient parallel to the streamlines in an extensional flow, resulting in anisotropy of the bulk magnetic permeability. The suspension is made to flow from a large chamber into a narrow tube which has an oscillator coil wound near its entrance. Particle orientation parallel to the magnetic field of the coil decreases the inductance. (Small changes in orientation can then be accurately measured with a countertimer.) Since both hydrodynamic orientation and Brownian motion depend on the shape characteristics of the particle, the orientation data provide information about the "effective-particle" shape. The gradients of the inductance with respect to flowrate, in the limits of high and low flowrate, are used to calculate relative changes in the effective-particle rotary diffusion coefficient. This method is demonstrated with a γ-Fe2O 3 suspension over a wide range of surfactant concentration. Changes in the particle magnetic properties calculated from the flow orientation data are consistent with an increase in the magnetic anisotropy coefficient by the adsorbed surfactant.