The purpose of the paper is to explain the linear birefringence and dichroism observed in magnetic fluids when subjected to a magnetic field. Linear aggregates of nanosize near spherical particles, which are considered to be responsible for these phenomena, have been analyzed in terms of the classical oscillating dipole model where each particle is treated as a simple dipole under the action of the electric field of a light beam. The analysis is produced in terms of the number of particles in the aggregate for both spherical and slightly nonspherical particles. Optical anisotropy factors and expressions for the magnetic-field dependence of the birefringence and dichroism due to preexisting aggregates are given. Numerical calculations have been carried out for Fe3O4 particles. Finally, the formulae are applied to experimental data where a close fit is obtained.
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.
A novel fluorescence procedure has been used to study the binding characteristics of DNA with three modern fluorochromes currently used in chromosome cytochemistry. The transient changes in the polarised components of fluorescence have been recorded for dye-tagged DNA solutions when subjected to short duration electric pulses. From these data, it has been inferred that, like ethidium bromide, berberine sulphate and quinacrine mustard both intercalate the DNA structure whilst the bi-benzimidazole derivative Hoechst 33258 binds with a distinctively different geometry, probably within the helical grooves.
By measuring the transient changes in each of the polarized components of fluorescence during the application of a pulsed electric field to a solution of macromolecules, a method has been developed for evaluating the binding characteristics of fluorescent chemotherapeutic agents to DNA. It is shown that whereas quinacrine and berberine intercalate the DNA helix, hydroxystilbamidine does not. Furthermore, measurements on both the native and the diol-epoxide forms of benzo(a)pyrene show that the former is consistent with an intercalation type of binding, whilst the latter appears to be more inclined to the major DNA axis and may possibly be associated with the external helical grooves. The significance of these findings is discussed.
A novel use for magnetic birefringence experiments is in the measurement of the birefringence of commercially important colloidal vermiculite particles. When aligned in a magnetic field, results at low field strengths show a steady state birefringence proportional to the applied field. The effects on the relaxation time of the birefringence on removal of the field are found to decrease with increasing field strength. This has been attributed to polydisperity of particle size. A theory analogous to that of electric field relaxation and used for magnetic field relaxation data has been further developed and modified to incorporate the more general case of diamagnetism. The modified theory is then used to obtain a two-parameter log-normal size distribution function for the suspended particles. The experimental results obtained are in accord with data acquired from electron micrographs. It is therefore concluded that use of the modified theory is justified and forms the basis of a valid measurement technique applicable to rigid particulate systems of any uniform, regular spheroidal morphology.
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