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Dissertation/Thesis Abstract

Advances in the Capillary Assembly of Colloids
by Shillingford, Cicely, Ph.D., New York University, 2021, 328; 28259567
Abstract (Summary)

Since the early 20th century, colloidal particles have been recognized as materials with fascinating thermodynamic and photonic properties. In modern times, their implementations in cosmetics, emulsions, food, plastics, and more are indispensible, with groundbreaking technologies continuously emerging in the field of bioinspired materials design. Academic research in this area primarily focuses on theoretical and experimental colloidal structures for catalysis, sensing, structural color, and optical devices. In the absence of building block anisotropy or intervening forces, colloids self-assemble into energetically favored closed-packed lattices with materials properties that are limited by this underlying structure. The large-scale production of useful solid-phase colloidal materials with uniform gradations, precise hierarchical construction, and long-range periodicity remains a challenge. Achieving disruptive photonic materials relies on the invention of both novel anisotropic building blocks and robust assembly strategies.

One such strategy is capillary assembly, which involves the evaporation-induced deposition of colloidal particles onto patterned substrates. Throughout the past two decades, this method has produced plethora planar and 3D nano/microarrays comprising polymer, metallic, and metal oxide colloids, with few examples of biomaterial deposition as well. The goals of capillary assembly are two-fold: clustering of colloids within discrete periodic cavities on a surface and directing crystallization of colloidal monolayers. In the former case, the clusters are often bound and then released from the template intact to give anisotropic colloidal clusters. In the latter case, colloidal lattices may be invoked as sensing microarrays, tamper-evident devices, electrical circuits, or templates for epitaxial growth for open-packed 3D structures. The work herein describes several advancements to the field of capillary colloidal assembly, and addresses needs both for anisotropic building block fabrication and directed assembly of non-trivial lattice architecture.

Chapter 1 describes the latest in colloidal science and provides a background of colloids, the forces that act upon them, categories and fabrication methods for non-spherical colloids, directed assembly strategies, and applications of colloidal structures. Capillary assembly is the fulcrum of subsequent chapters; therefore, its theoretical aspects and several exploratory experiments are detailed.

Chapter 2 chronicles the extension of capillary assembly to include that of liquid particles, comprising highly stable organosilica emulsion droplets. Upon assembly of liquid particles, coalescence and space-filling of arbitrarily shaped templates occurs, resulting in periodic pools of oil that are polymerized with heat. The hardened organosilica material was removed from the template to give faceted anisotropic colloids in the shapes of square prisms, trapezoids, and ellipsoids, with any conceivable geometry attainable. Given the capacity to accurately govern the quantity and architecture of the deposited material, this work provides a robust bottom-up and top-down method for engineering uniquely shaped colloids up to 14 ┬Ám that are not accessible using wet chemical methods.

Chapter 3 demonstrates the aforementioned structural control of liquid particle assembly by engineering chemically heterogeneous colloids through the sequential assembly and co-assembly of different materials. Janus, patchy, and multicomponent particles result, with incorporated materials including organosilica polymers, poly(styrene), and iron oxide colloids. This diversity of compositionally complex colloidal building blocks, derived using an unprecedented assembly strategy, will enable the construction of yet-unforeseen colloidal lattices with desired properties that cannot be achieved with isotropic spheres.

Chapter 4 describes the use of capillary assembly for directed crystallization of open-packed lattices with four-fold particle coordination. Templated square fences were used as guidelines to influence packing and restructuring during monolayer formation. Image recognition algorithms were deployed to track the colloidal deposition process as a function of time, revealing key mechanistic understanding of the crystallization process. The dynamic analysis elucidates that colloids undergo periodic coordination number oscillations before settling into their equilibrium structure as a result of confinement-driven nucleation and osmotic pressure-induced reconfiguration. Both the assembly strategy and insights derived from computational particle tracking will inform the use of capillary assembly as a tool for disrupting closed-packed lattice formation and enable fabrication of open-packed 3D structures.

Chapter 5 describes the impact and outlook for the advancements in capillary assembly described herein. Developing a broad library of hybrid chemically and geometrically anisotropic colloids with internally complex construction permits future engineering of open-packed photonic bandgap materials. Colloidal cages, frameworks, and superstructures that are analogous to supramolecular counterparts on the colloidal scale are envisioned, with potential uses as micron-scale heterogeneous catalysts, active particles, optical devices, and electronics. Colloidal-scale emulation of important molecular crystals is possible by fabricating shapes that mimic molecular contours. Furthermore, hybrid colloids imparted with stimuli responsive moieties such as those with dielectric and magnetic dipoles are promising as micromotors, controlled-release particles, and anti-tampering devices. Future work in the Weck research group will include electrophoretic and programmable assembly into such structures; computational modeling thereof is already in progress. The potential for incorporation of the methods herein into meaningful colloidal research is inevitable and exciting.

Indexing (document details)
Advisor: Weck, Marcus
Commitee: Ward, Michael D., Walters, Marc, Tuckerman, Mark, Woerpel, Keith
School: New York University
Department: Chemistry
School Location: United States -- New York
Source: DAI-B 82/9(E), Dissertation Abstracts International
Subjects: Materials science, Chemistry
Keywords: Capillary assembly, Colloidal emulsions, Colloids, Liquid particles, Patchy particles, Self-assembly
Publication Number: 28259567
ISBN: 9798582576143
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