Ferrofluid structures: a magnetic dichroism study

Measurements have been made of the field strength and time dependence of the linear dichroism induced by magnetic fields in magnetite-based ferrofluid films. When combined with optical diffraction data, three characteristic regions, each indicative of the changing microstructure within the ferrofluid, were observed. The detection of dichroism even at low fields, indicated the presence of small, 'primary' linear aggregates. Above a threshold magnetic field strength, for which the dipolar energy of the system approximated to the thermal energy and, after a finite induction time, discrete optical spot diffraction patterns could also be detected which indicated a parallel alignment of independently spaced linear aggregates in the fluid. With further field strength and time increase, the diffraction pattern changed to strong, discrete lines, consistent with the aggregate filaments associating laterally so as to form the thicker condensed fibres often reported hitherto in the literature. Measurements in time indicated that this process involved discrete, reproducible formation times. Theoretical treatments of these processes are given and a model proposed for the structure of the fibres. As the underlying dichroism was constant, the primary aggregates maintained their integrity throughout, thereby accounting for the rapid disappearance of the diffraction pattern upon removal of the field. The need to appreciate the nature of these field and time-dependent associations within ferrofluid structures is important in their design and uses for magneto-mechanical devices.