Novel Dynamics and Structures Using Paramagnetic Colloids with Rotating Magnetic Fields
Biswal, Sibani Lisa
Doctor of Philosophy
Micron-sized colloids have long been used as model systems to study the dynamic and thermodynamic behavior of atomic systems. This is attributed to the fact that their dynamics are driven by thermal energy and they are large enough to be visualized using optical microscopy. Moreover, the interactions between particles can be tuned by surface functionalization or application of external fields. In this thesis, I will introduce the use of various rotating magnetic fields on a system of confined paramagnetic colloids to model different physical phenomena in two dimensions (2-D), whose dynamics are not easily observed at a single molecule length scale. The dynamics of a particle pair under a classic rotating magnetic field is first described with a modified Mason number, to describe the relationship between magnetic, viscous, and electrostatic interactions governing the rotational dyanmics. Next, I will describe a novel method to induce an isotropic attractive interaction between paramagnetic colloids when the frequency of the rotating field is sufficiently high. The pair interaction potential is comprised of a long-range attractive interaction induced by the external magnetic field and an electrostatic Yukawa-type repulsive interaction from the charged surfaces of the particles. This interaction potential is described by a theoretical model, which is verified by experimental measurement. Three-body effects are also measured using a three-particle system, which is the leading term of many-body effect. By solving the Laplace’s equation for magnetostatics, this three-body effect is proved to be negligible for particles far from the edges of a many-particle cluster. This validates the assumption of pair additivity in the interaction potential used in a Monte Carlo simulation. The tunable isotropic interaction provides an ideal platform to study the phase behavior of 2-D atomic systems. The melting thermodynamics and dynamics are studied in detail using simulation and experiment respectively. Thermodynamics properties, such as radial distribution function, translational order parameter, bond-orientational order parameter and Lindemann parameter of different phases are measured to show that melting transition for this system is first-order as opposed to the KTHNY theory, which states melting in 2-D should be a two-stage second-order transition. Phase coexistences are observed for the first time in 2-D system with long-range attraction, which further confirms the first-order nature of the transition. The simultaneous dislocation unbinding and disclination unbinding observed in experiment explains the inconsistency against the prediction given by the KTHNY theory. The phase diagram of this system is also constructed, which is shown to be very similar to that of atomic systems. Another novel aspect described in this thesis is the development of a novel method to achieve microscale swimming. A constant offset can be added to the rotating magnetic field if the temporal symmetry needs to be broken, and the resulting field is referred to as an eccentric rotating magnetic field. Under such a field, paramagnetic particles with mismatched sizes are shown to be able to swim in a directed manner. Swimmers consisting of multiple particles are able to fragment their arms. Stochastic forces can change the type of the fragmentation from the surrounding fluid, leading to decreased or increased swimming speed for different swimmers.
Paramagnetic Colloids; Rotating Magnetic Field; Two-dimensional Melting; Artificial Swimming