Study of Aqueous Dispersions of Magnetic Nanoparticles by Magnetic and   Rheological Measurements
  The observed magnetic tunability of light transmission through a ferrofluid can be effectively understood in terms of the inter-particle interaction that can be estimated from the magnetic and rheological properties of these fluids. The present study reports complementary magnetic and rheological measurements of aqueous dispersions of ferrite nanoparticles and a commercial ferrofluid. The room temperature magnetization measured in a SQUID magnetometer up to fields of 1 to 2 Tesla showed superparamagnetic behaviour of the particles and the dispersion with the background signal of the liquid showing a diamagnetic behaviour. The room temperature rheological behaviour in zero magnetic field of the fluids was investigated by measuring the viscosity as a function of shear rate from 1-100 s-1. The particle size and the nature of the carrier liquid determine the viscosity and is expected to have an effect on the inter-particle interaction. 
M Shalini, Radha Srinivasan and Dushyant C Kothari
cond-mat.soft