Nonequilibrium dynamics and self-organization in field-driven dipolar colloidal particles

In this thesis, we investigate the dynamics and pattern formation behavior of systems of spherical colloidal particles with permanent dipole moments in external time-dependent fields on the basis of computer simulations and theoretical considerations. The computer simulation techniques that we use are molecular dynamics simulations, Langevin dynamics simulations, Wang-Landau Monte Carlo simulations, and Brownian dynamics simulations that include hydrodynamic interactions. First, we focus on rotating fields in a three-dimensional geometry. Performing simulations at a fixed packing fraction and dipolar coupling parameter, we construct a full nonequilibrium phase diagram as function of the driving frequency and field strength. This diagram contains both synchronized states, where the individual particles follow the field with (on average) constant phase difference, and asynchronous states. The synchronization is accompanied by layer formation, which breaks down at too large driving frequencies. During the transition from layered to unlayered states, complex changes in the single-particle rotational dynamics from synchronous to asynchronous behavior appear. We investigate this dynamic behavior in detail and show that the limit frequencies can be well described as a bifurcation in the nonlinear equation of motion of a single particle rotating in a viscous medium. We also present a density functional theory, which describes the emergence of layers in perfectly synchronized states as an equilibrium phase transition. Finally, we investigate the influence of hydrodynamic interactions on these three-dimensional systems. In a next step, we consider monolayers of dipolar particles that are driven by rotating external in-plane fields. In these system, two-dimensional cluster formation can occur. We investigate at which field strengths and frequencies these clusters form and explore the influence of hydrodynamic interactions in depth. In particular, we focus on their interplay with the permanent dipolar interactions of the particles. We also examine the phase behavior of the equilibrium system resulting from a time-average of the colloidal interactions in the rotating field. In this way we demonstrate that the clustering in the driven system arises from a first-order phase transition between a vapor and a condensed phase. Lastly, we report results on the rotational ratchet effect in systems of three-dimensional dipolar particles. The ratchet effect consists of directed rotations of particles in an oscillating magnetic field, which lacks a net rotating component. We investigate the influence of both the random and deterministic contributions to the equations of motion on the ratchet effect. As a main result, we show that dipolar interactions can have an enhancing as well as a dampening effect on the ratchet behavior depending on the dipolar coupling strength of the system under consideration. The enhancement is shown to be caused by an increase in the effective field on a particle generated by neighboring magnetic particles, while the dampening is due to restricted rotational motion in the effective field. Moreover, we find a non-trivial influence of the short-range, repulsive interaction between the particles.