Estudo de sistemas coloidais de partículas anisotrópicas magnéticas

Abstract

The self-assembly process of a two-dimensional ensemble of magnetic rods is studied. The rods are modelled as aligned single dipolar beads, the so-called peapod model. The system is studied by means of Molecular Dynamics and Langevin Dynamics simulations. An introduction on soft matter systems, showing their main features and some theoretical and experimental aspects is first presented. In the following, the computational methods adopted in the simulations and the mathematical treatment of the system are presented and discussed. Concerning the results of the thesis, a diversity of self-assembled configurations such as: (1) clusters, (2) percolated and (3) ordered structures are obtained and characterized with respect to the state of aggregation of the particles and ordering. By increasing the aspect ratio of the magnetic rods, it is found that in two dimensions the percolation transition is suppressed. This is opposite to what is observed in similar three dimensional systems. It is shown that such a behavior is a consequence of geometrical effects which reduce the mobility of the rods as the aspect ratio of such rods is increased. Concerning the ordering of the particles in the system, a magnetic bulk phase is found with local ferromagnetic order and an unusual non-monotonic behavior of the nematic order is also observed. Based also on extensive Langevin Dynamics simulations, the selfassembled configurations are studied for the special case where the dipole of the beads that constitute the rods are misaligned with respect to the rod axis. The misalignment is zero when the dipole is parallel to the axial axis. It is found that the density required for the formation of the percolated structure decreases with increasing misalignment of the dipole. Also, the system exhibits different aggregation states (solid or liquid) for different misalignment, even when the same density is considered. The stability of the self-assembled structures are studied with respect to temperature, and it usually increases with increasing misalignment of the dipoles.