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Rheological properties of a single magnetic filament. A Langevin dynamics study.

Sitges Riera, Juan (Supervisors: Sintes, T.; Cerdà, J. J.)
Master Thesis , (2015)

In this master thesis we present the results of an extensive Langevin Dynamics simulation on the behaviour of a single magnetic filament subject to different boundary conditions and external forces. Langevin dynamics allow us to recreate the solvent friction and to control the system temperature, approximating a canonical ensemble. We have analysed the rheological properties of the system as a function of the strength of the dipolar interaction, the chain length, the intensity of an external magnetic field and temperaure. Two representative cases have been considered: a) a filament placed in the bulk and b) a filament grafted onto a planar surface. In the first case, we have studied the dependence of the radius of gyration and the chain end-to-end distance with the chain length, reproducing the Flory exponent, and with the temperature. We have computed the average chain magnetisation and found the characteristic temperature at which the closure transition takes place. In the second case, we have measured the particle density profile for a magnetic filament. The results have been compared with those of a non-magnetic chain and to the theoretical predictions of the Millner, Witten and Cates [Macromolecules, 21, 2610, (1988)]. We have finally developed a simple representation of a magnetic actuator and we have found the characteristic magnetic field, as a function of the dipolar interaction and the temperature, above which the actuator is activated. We expect these results to be of interest in the design of a magnetic nano-switch controlled by an external field.