Dynamics and Organization of Passive and Magnetically Driven Colloids Under Optical Confinement

Abstract

The physics of confined systems of interacting particles has been a subject of active research for more than a century. Such systems are ubiquitous in nature - examples include petroleum in porous rocks, cytoplasm in biological cells and electrons in a quantum dot. Physical properties of such systems are substantially different from those of macroscopic (bulk) systems because the fraction of particles on the surface of a small cluster is larger than that for a bulk system and this fraction increases as the size of the cluster is reduced. Also, small local changes inside a cluster, such as the presence of a single impurity or a vacancy can affect the properties of the whole cluster. For these reasons, physical processes such as crystallization, glass formation and defect dynamics in strongly confined systems can exhibit characteristics that are fundamentally different from those found in their bulk counterparts. Understanding these effects of strong confinement is also of importance in the design and assembly of novel nanomaterials.

In this thesis, we show a novel modification of a single beam optical tweezer to create strongly confined colloidal clusters. We study the potential landscape of these defocused optical traps and see how 2D colloidal clusters are formed whose sizes and phases can be controlled. We show that the crystalline phase of such clusters can be approximated as rigid discs and study their equilibrium and dynamic properties. Most importantly, we study the dynamics of foreign dopants, in the form of particles of different shapes and sizes, injected into the crystallites. The striking result obtained here is the ability of finite sized colloidal clusters to expel or internalize a foreign dopant depending on its initial position. Our simulation results suggest that that the fate of a dopant is governed to a large extent by the entropy of the system, which becomes increasingly important as the size of the crystallite is reduced. Internal energy tries to drive the system towards self-purification whereas entropic forces prefer the dopant. The competition between the two gives rise to a free energy barrier which decides the fate of the system based on the initial configuration. In the next part of the thesis we move to driven and active colloids in these optical confinements. We show that magnetic rotors fabricated through shadow evaporation can be trapped inside such optical confinements. We study the dynamics of these rotors due to the effect of magnetic actuation and optical forces, resulting in the rotors moving in circles inside the optical trap. We study their motion through different experimental parameters as well as their interactions with passive colloids and other rotors. Finally, we study magnetically actuated propellers (MAPs) in optical confinements and show how the trapping capabilities of MAPs can be used to characterize the trap stiffness. We also discuss how the confinement can be used to study the interactions between multiple MAPs.