Estudo de sistemas coloidais de partículas magnéticas conduzidas por campos oscilantes

Abstract

The self-organization of magnetic particles (rods) subjected to an oscillating external magnetic field is studied. The magnetic rods are modeled as aligned and bonded dipolar spheres, and the system dynamics are obtained through computational simulations considering the Langevin equation. Specifically, we will study two types of systems: (i) a monodisperse system of magnetic colloids with anisotropic geometry (rods) subjected to an oscillating external magnetic field, and (ii) a binary system of magnetic colloids (rods and discs) also driven by an oscillating external field. For the first type, a range of self-organized configurations, such as (1) clusters, (2) dynamic aggregation, and (3) isotropic phase, are obtained and characterized as a function of the misalignment of the rods intrinsic magnetic dipole moment relative to their axial direction, as well as in terms of the amplitude and angular frequency of the oscillating external magnetic field. In this case, there is a characteristic misalignment of the rods magnetic dipole relative to its axial axis, which is extremely relevant in determining the systems microscopic aggregation states. The resulting configurations arise from the competition between dipolar interaction and steric interaction due to the anisotropic geometry of the rods, in addition to the dipole-external magnetic field interaction. In the case of synchronization between the magnetic rods and the external magnetic field, three distinct synchronization regimes were obtained. For the second type of system, a variety of self-organized configurations are obtained, in which the structural and dynamic properties of the steady states are analyzed based on the sizes of rods and discs, and in terms of the amplitude and frequency of the oscillating external magnetic field. This thesis is organized as follows. First, we introduce soft matter systems, highlighting their main characteristics as well as some experimental and theoretical aspects. Next, the computational methods used in the simulations are presented and discussed. Then, we present the results and discussions of the studied systems in separate chapters. Finally, we describe the conclusions and perspectives of this work.