Phase transitions in magnetic fluids

Abstract

Experimental results concerning phase transitions in magnetic fluids are presented. Magnetic fluids are colloidal dispersions of nanometric magnetic particles which are stabilized either through electrostatic repulsions between charged particles in water, or through steric repulsions between surfactant coated particles in oil. In the both cases, fluids are synthesized in order to have repulsions which strongly dominate the interparticles interactions balance. SANS experiments performed on aqueous magnetic fluids show that the system is strongly repulsive and not sensitive to a decrease of temperature. In this case, phase transitions are not observed in a reasonable range of temperature or of applied magnetic field. In aqueous systems, SANS measurements show that repulsions are gradually screened by addition of salt, and “gas-liquid”-like transitions are observed decreasing the temperature. The temperature threshold is related to a screening parameter taking into account the ionic strength and the particles diameter. The same kind of behavior is observed for oily systems: steric repulsions have to be decreased through addition of a bad solvent of the surfactant chains in order to observe “gas-liquid” transitions if temperature is decreased.