Dynamic Properties of Magnetic Colloidal Particles and Holes

Abstract

Our main goal is to study certain aspects of the dynamics of fluids with magnetic particles in suspension, based on their promising practical applications as new materials as welI as on its fundamental scientific interest.

In the introduction we brief the reader on the most essential properties of the system. We have characterized the monodomain magnetic particles and the time scales inherent to magnetic fluids. Having introduced the rotational diffusion equation as the most convenient tool to take into account the different mechanism inftuencing the dynamics of the particles, we have also proposed a fruitful approach for solving it in any general situation. We have also highlighted the macroscopic properties of the magnetic fluid treated now as a continuous medium and showed up the different phenomena associated with the lack of stability in the system.

In Chapter I we concentrate on two limit cases whose analysis is easier but very illustrative. The first part of the chapter is devoted to the study of a suspension of rigid dipoles, in which the magnetic moments are rigidly attached to the body of the particles themselves. In these conditions, if we apply an external magnetic field both the magnetic moment and the particle move together so that the magnetic torque acting upon it becomes zero. Thermal fluctuations tends to disrupt this order, and it turns out that, for instance, that the effective viscosity of the suspension depends on the dimensionless parameter comparing magnetic and thermal energies. In the second part we consider magnetic materials with finite anisotropy energy at high magnetic fields. For such monodomain particles the magnetic moments rapidly orient along the direction of the external field, and then as a second step the mechanical rotation of the particles takes place. In this case, the effective viscosity of the suspension is a function of the magnetic anisotropy constant of the material, of the volume of the particles as well as the thermal energy. Our results are compared to experimental measurements.

The second chapter is concerned with the determination of the viscosity and of some magnetic and optical properties of magnetic fluids in the whole range of possible experimental situations. The magnetic moments and the particles inside the liquid reorient separately but their dynamics are coupled thus giving rise to a more intricate relaxation process. We have compared part of our results with available experimental data for different ferrofluids showing quite a good agreement.

In Chapter III we joint to our discussion of magnetic fluids the presence of nonmagnetic particles of micrometer size and study their motion through the ferrofluid. The ferrofluid is considered now as a continuous medium with new transport coefficients already determined in the previous sections. Under the action of a rotating external magnetic field, we study the rotational motion of the nonmagnetic particles and compare our expressions to sorne measurements carried out in these composite systems. In this chapter we are also con cerned with the characterization of the hydrodynamic interactions among these particles in a carrier ferrofluid.

Chapter IV is intended as a brief introduction to the multiple problems which arise when one handle the aggregation phenomena which may take place in these systerns. We study the kinetics of the forrnation of the aggregates by rneans of the Smoluchowski theory of coagulation in colloids. But we account for hydrodynarnic interactions which are not usually considered when studying such process and that gives rise to sorne corrections for high concentrations of particles. In addition, the rheology of the chains that are usually observed in systerns with dipolar interactions is given for a rather simplified situation in order to elucidate the effects of the dipolar magnetic interactions.

Finally, we sum up our main conclusions and indicate some of the perspectives stimulated by the contents of this monograph and in which we plan to pursue work in the near future.

A lo largo de esta monografía nos hemos ocupado del estudio de sistemas fluidos, tanto con monodominios magnéticos como con dos tipos distintos de partículas, magnéticas y no magnéticas, en dispersión en un líquido newtoniano en situaciones fuera del equilibrio.

El comportamiento de estos sistemas se ve influenciado en gran medida por la presencia de un campo magnético externo, lo que da lugar a nuevos fenómenos que han sido el fundamento de muchas aplicaciones prácticas. Sin embargo, esta influencia depende de los diferentes procesos de relajación que tienen lugar dentro las partículas, con respecto a sus ejes cristalinos, así como fuera de ellas, con respecto al fluido portador.

Hemos descrito cuáles son estos procesos y obtenido la dependencia con respecto de los parámetros que describen las partículas y el fluido, de algunos coeficientes que caracterizan las propiedades reológicas, magnéticas y ópticas de la suspensión coloidal.