  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.
