Phase Transitions, Crystal Growth, and Dynamics of Dislocations in Colloidal Monolayers
Phase transitions represent a fascinating condensed matter physics phenomenon; however, the study of phase transitions at the microscopic level is challenging because of the difficulty in tracking individual atoms, which cannot be resolved in an optical microscope. To solve this problem, colloidal particles are often used to model these phase transitions because of the ease in tracking individual particles with optical microscopy and their ability to equilibriate at room temperature on experimentally accessible time scales (i.e., minutes to hours). However, most of the existing colloidal systems are not easily tunable, which makes it difficult to control and study phase transitions.
The goal of my thesis is to develop a magnetically tunable system for studying phase transitions using a monolayer of magnetic colloidal particles, which self-assemble under the influence of an external time-varying magnetic field. In this dissertation, I have both an engineering goal of developing experimental techniques that can grow sufficiently large colloidal crystals, and a scientific goal of studying the kinetics of phase transitions, paying particularly close attention to the early crystallization dynamics which my system is uniquely poised to probe. I have used this experimental apparatus to study the phase transitions in densely packed mono-component and bi-component colloidal monolayers. In both of these systems I have used magnetic fields to adjust the interactions between colloidal particles and image tracking algorithms to follow the system dynamics.
In the following chapters, I will describe the methods I have used to characterize crystal growth rates, and the mechanisms for how crystals heal, with the key points are summarized as follows. First, the ability to form large single crystals is fundamentally limited by impurities, such as the presence of random large or small particle contaminants, particle doublets, and particles that are randomly pinned to the substrate. When these impurities or defects are present even at concentrations as low as a few percent, it dramatically reduces the size of the attainable crystals. Second, I have showed that long-range magnetic interactions can produce complex phase diagrams that have both critical points and triple points, and that it is possible to move between the different phases on the fly by adjusting the strength of the magnetic field. This ability can be used to study the early dynamics of melting and solidification processes. Finally, I have used the system to find unique pathways that occur during the healing of colloidal crystal. One of these mechanisms involves both lattice slip and rotation, which does not appear to have been reported previously. This colloidal system thus has many potential applications both as a method to fabricate new materials and as a fundamental model for materials science.
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