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Author ORCID Identifier

https://orcid.org/0000-0003-1704-1549

AccessType

Campus-Only Access for One (1) Year

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemical Engineering

Year Degree Awarded

2024

Month Degree Awarded

February

First Advisor

Maria M. Santore

Abstract

Two dimensional colloidal suspensions are a new type of ultra-thin fluid-solid composite material capable of reversibly reconforming and restructuring in response to mechanical stimuli. In contrast with classical colloidal suspensions governed by physico-chemical forces, interactions within two dimensional suspensions result from the mechanics of both the fluid and the plate-shaped colloids. This thesis uncovers foundational principles for the elasticity-dominated interactions and assembly of 2D colloids in free-standing 2D elastic fluids, in the form of giant vesicles, focusing on the intermediate concentration regime where the colloids occupy an area fraction of about 15%-17%, and comparing systems governed by pairwise and multibody interactions. The work identifies “excess area” as the key experimental variable tuning the bending energy and reveals an important distinguishing feature of these composites: with solid plate shaped colloids, interactions can be both attractive and repulsive, a result of the interplay between the bending curvature and zero shear modulus of fluid membranes and the shear rigidity of the plate-shaped domains. It then follows that subtle changes in local shape translate to large scale morphological reassembly: colloid pair separations can be tuned from contact to distances on the orders of the colloids themselves, and collections from 4-80 colloids can assume configurations including vesicle encompassing pseudo-hexagonal lattices, dynamic disordered states and closely-associated patterns. This dissertation employs giant unilamellar phospholipid vesicles (GUVs) as the platform to explore the assembly behavior of 2D colloidal suspensions. GUVs consist of a mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) that phase separate into coexisting solid DPPC membrane domains and a membrane fluid containing DOPC and DPPC at room temperature. Through the implementation of temperature- and osmotic- controlled protocols, the number of domains per vesicle can be manipulated, spanning from one, two, to many domains of relatively uniform shape and size. This research contributes a significant qualitative shift in understanding the interactions of objects in biomembranes: interactions between solid planar colloidal membrane plates in this study exhibit distinctive long-range character, with the capability to be simultaneously attractive and repulsive, thereby exhibiting preferred minimum separations between colloidal domains. This finding transcends the conventional understanding of exclusively repulsive interactions observed between fluid domains in fluid membranes and exclusively attractive interactions between membrane adhesive nanoparticles and proteins that produce local membrane bending. Furthermore, the directable assembly of solid colloidal domains is sensitive to the change of fluid membrane curvature, quantified by the value of excess membrane area, which is calculated as the actual vesicle membrane area normalized with respect to the surface area of a sphere possessing an equivalent volume. By manipulating the fluid membrane curvature through elastic adjustments and osmotic modifications, reversible colloidal assembly between domain pairs can be achieved. In conditions of multiple domains, at elevated excess area, long-range repulsive forces predominate, governing domain separation and giving rise to the formation of pseudo-hexagonal lattices. However, as the excess area is reduced, the repulsive lattice assembly undergoes a phase transition, yielding closely associated colloidal assemblies. This research unveils the shear elasticity of solid domains and excess area in the fluid membrane as a novel source of combined repulsive and attractive interactions within a model fluid-solid co-existing system. Remarkably, these interactions can be tuned via global vesicle properties, suggesting the possibility of their utilization in the controlled positioning of objects on 2D curved surfaces or pattern formation in materials. This discovery could potentially revolutionize the development of dynamic, ultra-thin materials featuring responsive functional patterns, thereby setting a new benchmark in the field, and providing a robust platform for future research applications.

DOI

https://doi.org/10.7275/36471644

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