Dilution-induced instability of water based magnetic fluids

Optical observations were made of freshly made, 6-mon and 5-yr-old aqueous magnetic liquids diluted 100, 200, and 500 times with water. Dilution-induced instability of the least-concentrated fluid set in after a period of 6 mon and is completely destabilized after a time not exceeding 5 yr. The 100:1 suspension is still stable after this time period. All three freshly diluted fluids exhibit chaining in a small applied magnetic field. It is proposed that the latter, reversible phenomenon involves coalescence at a secondary minimum, whereas irreversible dilution-induced instability due to changes in surfactant entails aggregation at a primary minimum.