Shape Anisotropy and Electrostatics in Self-assembly of Colloidal Particles

Wang, Ziwei

doi:https://doi.org/10.21985/n2-nydh-v957

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Shape Anisotropy and Electrostatics in Self-assembly of Colloidal Particles

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Self-assembly of colloidal particles at the nano- and microscale has been a powerful tool for producing structures with emergent properties in applications ranging from electro-mechanical systems to photonics and biomedical devices. Great success has been achieved in experiments, where a variety of exotic phases have been discovered and even reconfigurable and self-healing structures have been created by utilizing external fields. The self-assembly process results from a delicate balance between different physical interactions, thermal fluctuations, and external fields. Thus, understanding, predicting, and controlling the self-assembled structure and its dynamic process has been a challenging and central problem in materials design and engineering. In this dissertation, I employ various simulation techniques to study the self-assembly of a range of colloidal systems and specifically explore how shape anisotropy and electrostatic polarization effects play a role in the process. Chapter 2 presents a study on the crystallization process of triangular nanoprisms into a hierarchical hexagonal lattice. Large-scale Monte Carlo simulations reveal the microscopic details of the assembled superlattice which is composed of columns of randomly stacked prisms. I demonstrate that positional ordering of the superlattice indeed emerges from orientational disorder, from which the design rule is proposed that different phases can be realized by varying the ionic strength and cell height. Chapter 3 focuses on crystal growth kinetics at the nanoscale, where a prevalent layer-by-layer growth mode is discovered for a diversity of nanoparticles. Coarse-grained modeling and molecular dynamic simulations are applied to map the energy landscape involving key diffusive barriers of the nanoparticle system, which explains the thermodynamic and kinetic driving forces of the observed growth mode. By further coupling analysis of experimental imaging and kinetic Monte Carlo simulations, we show that building block size governs the crystal growth process by simultaneously controlling the ratio of surface diffusion rate to incoming flux and the interaction range. From Chapter 4, we turn our attention to explore how electrostatic polarization effects play a role in self-assembly and how it can be utilized to realize structures with controlled properties. Chapter 4 presents a general review of dielectric effects in mesoscale simulations, which provides comparison of different methods for handling polarization and highlights key physical phenomena attributed to dielectric effects at the nano- and microscale. In Chapter 5, I investigate the structural and dynamical properties of a confined dipole hard-sphere fluid near a polarizable interface. The Image Charge Method is incorporated into the Ewald summation to deal with the polarization in simulations. I demonstrate that while the global polarization only weakly depends on the substrate permittivity, the dipolar orientation in the contact layer is strongly affected by the dielectric mismatch, as is the anisotropy of the rotational dynamics. Inspired by the above observation, Chapter 6 focuses on a two-dimensional dipolar film supported by a dielectric substrate. Simulations show that the dielectric mismatch across the substrate can be utilized to achieve modulated patterns in the dipolar material. Notably, a rich phase diagram arises, where stripped and circular morphologies emerge with geometric properties that can be controlled through variation of particle shape and substrate permittivity. Chapter 7 presents a detailed comparison between the recently proposed hybrid method and the iterative boundary element method on solving the systems containing spherical dielectric interfaces. By examining the challenging case of close-packed crystal structures, we demonstrate that the hybrid method is superior to the iterative boundary element method in terms of efficiency. The effects of various parameters on efficiency, convergence, and accuracy are also explored for both methods. By applying the hybrid method in simulations, I study the self-assembly of binary suspensions of oppositely charged polarizable colloids presented in Chapter 8. A variety of anisotropic superstructures are observed, resulting from the many-body dielectric effects which impart effective directionality to interactions. Notably, both local connectivity and fractal dimension can be well controlled by varying particle size ratio and relative permittivity. Lastly, I conclude this dissertation with a brief summary of the main findings and future outlook for each chapter in Chapter 9.

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