The viscous response of ferrofluids subjected to external magnetic field

Ferrofluids containing polydisperse magnetic nanoparticles are made stable using surfactant(s) in a compatible dispersion medium. The commercial ferrofluid APG 513 A (Ferrotec Corp) has demonstrated irreproducibility in the magneto-viscous (MV) effect from different batches. The MV effect changes from about 414% to 50% in studies from 2002-2015. This change was attributed to the removal of large Brownian particles, agglomerates and clusters resulting from dipole-dipole interactions. Even after the removal of these clusters, some residual MV effect was noted which was attributed to the preexisting aggregates produced during the fluid manufacturing process. The new APG 513 A was investigated to clarify the issue of the batch to batch irreproducibility and the existence of pre-aggregates in the fluid. The viscosity with and without a magnetic field along with magneto-granulometric, dynamic light scattering, TEM and FTIR were studied to qualify the new sample. This new APG 513 A had shown a very low MV effect (similar to 20%). The large Brownian particles were responsible for the effect and there was no evidence of dipolar interactions. The issue of preexisting aggregates was resolved by systematically diluting the fluid with the carrier. Above measurements along with the zero-field-cooled (ZFC) magnetization behavior which has never been reported in the literature before were studied on the diluted samples. The data analysis with the existing models confirms that the present APG 513 A is composed of individually stabilized particles including Brownian particles with a long sheath of dual surfactant, absence of preexisting aggregates, no evidence of dipolar interactions and a narrow size distribution.