Ferrofluid structures: a magnetic dichroism study

Abstract

Measurements have been made of the eld strength and time dependence of the linear dichroism induced by magnetic elds in magnetite-based ferro®uid lms. When combined with optical di¬raction data, three characteristic regions, each indicative of the changing microstructure within the ferro®uid, were observed. The detection of dichroism even at low elds, indicated the presence of small,`primary' linear aggregates. Above a threshold magnetic eld strength, for which the dipolar energy of the system approximated to the thermal energy and, after a nite induction time, discrete optical spot di¬raction patterns could also be detected which indicated a parallel alignment of independently spaced linear aggregates in the ®uid. With further eld strength and time increase, the di¬raction pattern changed to strong, discrete lines, consistent with the aggregate laments associating laterally so as to form the thicker condensed bres 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 bres. As the underlying dichroism was constant, the primary aggregates maintained their integrity throughout, thereby accounting for the rapid disappearance of the di¬raction pattern upon removal of the eld. The need to appreciate the nature of these eld and time-dependent associations within ferro®uid structures is important in their design and uses for magneto-mechanical devices.

Structural properties of charge-stabilized ferrofluids under a magnetic field: A Brownian dynamics study

“…For example, spontaneous cluster formation, i.e., without applied magnetic field, has been recently reported in sterically stabilized suspensions 5 as well as field-induced cluster formation in magnetite suspensions. 6 In addition, small-angle x-ray scattering ͑SAXS͒ results coupled with x-ray photon correlation spectroscopy experiments on maghemite dispersed in silicon oil have been interpreted by the formation of dimers or trimers under field. 7 The microstructure of latter samples becomes anisotropic if an external magnetic field is applied, this anisotropy being characterized by an oscillation of the scattered intensity with the angle to the field axis at a fixed wave vector.…”

Magnetically induced optical activity and dichroism of gadolinium oxide nanoparticle–based ferrofluids

“…4. The fluids exhibit the exact intrinsic dichroism as reported earlier, 13,21 DA k ¼ À2 DA ? : Figure 4(a) represents the field-dependent LD spectra of the pristine and c-irradiated specimens.…”

“…This polarization predilection differentiates dichroism from general absorption and very often relates to the alignment of anisotropic structural elements. 21 The linear dichroism (LD) was measured using the same setup as used for the FR measurements, but with different configuration. Measurements were made with the planepolarized light parallel and perpendicular to the applied magnetic field and corresponding values of parallel and perpendicular dichroisms, DA k and DA \ , are defined as the changes in the absorbance of the sample caused by turning on the magnetic field.…”

10.1007/s11449-008-2022-9

“…The particles completely ordered by the field have the parameters Φ u = Q u and Φ p = Q p . The factors of reduction of transmitted light intensity can be determined as follows [7]: (2) where η = [1 -(1 -1/ e 2 )cos 2 ϑ 0 ] 1/2 , ϑ 0 is the angle between the spheroid symmetry axis and the light propagation direction and e is the spheroid axial ratio).…”

“…The presence of aggregates of particles in magnetic fluids significantly affects the magnetic, electric, rheological, and optical properties of the latter [1][2][3]. Two main types of aggregates are distinguished [4]: microdrop aggregates (drops of a concentrated magnetic fluid, which can be distorted under the action of a magnetic field) and quasi-solid aggregates (clusters of colloidal particles, behaving as individual large particles).…”