Dissertation/Thesis Abstract

Directed assembly and dynamics of anisotropic particles
by Panczyk, Mark M., D.Eng., University of Delaware, 2013, 184; 3613049
Abstract (Summary)

Structures of nanometer and micrometer sized particles exhibit a variety of useful properties both as naturally occurring phenomena and in technological applications. In nature, colloidal crystals account for the colors in an opal and colloidal crystals can be used to create materials with photonic or phononic properties or used as coatings or templates. Creating these structures with nonspherical particles allows for a greater variety of properties. However, although the structures are dictated by thermodynamics, whether they are ultimately achievable as well as on what time scale they form, are limited by kinetics. In particular, concentrating particles slows their dynamics and reduces the rate at which they can arrange into the desired crystal. When nonspherical particles are assembled, both translational and orientational order is required to create crystals. Dicolloids, the shape of two overlapping spheres, are a particularly interesting particle shape to study due to the fact that their crystal structure can be changed with aspect ratio. The goal of this thesis is to investigate the kinetic limitations to assembly with increasing volume fraction. Specifically, we focus on concentrated monolayers of particles assembled into two-dimensional structures using oscillating electric fields.

Multiple light scattering is used to probe the dynamics of dicolloid particles with different shapes. With greater anisotropy, an increase in the diffusivity is observed. The diffusivity as a function of volume fraction could also be normalized by the random-close-packed volume fraction onto a master curve. The localization of particles was also characterized as a function of volume fraction and after accounting for repulsive interactions could be used to determine the glass transition of the dicolloids. The response of the localization length also compared well with theoretical predictions.

AC electric field induced assembly provides one potential pathway for directed self-assembly of colloidal particles. The advantage of using electric fields is that they orient and concentrate particles into a close-packed state. The structure of an assembled monolayer of dicolloids is studied both using microscopy and light scattering techniques. The scaling of the order to disorder transition was determined to be similar to that observed for spherical particles and optimal conditions for assembly were discovered. This optimum highlights the balance between creating a structure at thermal equilibrium and concentrating the particles quickly. The assembly of dicolloids is also unique in that the structure at low concentrations and in the initial phases of the assembly demonstrates end-to-end chaining which disappears for concentrated assemblies. Furthermore, orientational defects are apparent even in this low volume fraction case.

Although electric fields are chosen due to their ability to orient and concentrate nonspherical particles into structure, even at low concentration, care must be taken to ensure particles are oriented before concentrated. Otherwise they arrest into nonequilibrium structures. The concentrated particle dynamics, including the localizalization length and glass transitions are mapped out for dicolloid particles of different sizes and shapes. Values for the glass transition and diffusivity provide a metric for where the assembly is inhibited to better understand the optimal conditions for assembly processes.

Indexing (document details)
Advisor: Furst, Eric M., Wagner, Norman J.
Commitee: Lenhoff, Abraham M., Nettesheim, Florian
School: University of Delaware
Department: Chemical Engineering
School Location: United States -- Delaware
Source: DAI-B 75/06(E), Dissertation Abstracts International
Subjects: Chemical engineering
Keywords: Colloid, Dicolloids, Diffusing wave spectroscopy, Directed assembly, Dynamics, Electric field
Publication Number: 3613049
ISBN: 978-1-303-75153-0
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