Date of Award

Fall 2023

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Monirosadat Sadati

Abstract

This dissertation provided detailed information on molecular orientation, phase behavior, and morphology of high chirality and blue phase liquid crystals under curved geometrical confinement. In fact, at a sufficiently high level of chirality (pitch length<500nm), the anisotropic molecules of liquid crystals arranged into three dimensional (3D) cuboidal symmetries which is so-called blue phase liquid crystal (BP). BP cubic structures are consisted of doubly twisted anisotropic molecules in the form of cylinders and an amorphous network of defect lines; wherein, their different orientations result in BPI with body-centered cubic symmetry and BPII with simple cubic symmetry. The unique molecular orientations in BPs exhibit tunable photonic band-edge properties, and their short coherence length allows for sub-millisecond response times, making them highly attractive for photonic and sensing applications. However, their narrow thermal stability range and challenges associated with their monocrystal formation make their largescale proliferation infeasible. To enhance our understanding of BPs and address the challenges associated with integrating them into micron-scale and flexible devices and applications, we investigated their properties within micron-scale curved cavities. The use of curved geometry enabled us to explore the interplay between curvature, topological confinement, surface anchoring conditions, and polymer stabilization on the molecular organization, morphologies, and phase behavior of BPs.In this research to investigate the properties in curved micron-scale geometry, a custom microfluidic device was fabricated to produce droplets and core-shell structures, which allowed us to precisely control the degree of curvature and shell thickness. My results demonstrated that the stabilizing BPs within droplets extend the thermal stability range and lead to size-dependent phase behavior. These stabilized droplets exhibited dynamic characteristics to external variations, wherein they can reconstruct their initial crystal structure afterward. The polymerization mechanism within curved geometries enables a dynamic thermal and mechanical response. The partial phase separation at the interface contributes to their dynamic attributes while stabilizing BP structure at room temperature. Additionally, in the case of core-shell structures, the combination of shell thickness, curvature, and surface boundary conditions manipulated the molecular organization of both chiral nematic and BPs, resulting in intricate molecular arrangements. In shells, our findings demonstrate that both higher degrees of curvature and strong spatial confinement destabilize BPI causing it to transition into structures and optical characteristics reminiscent of BPII. We also have found that in BPs confined within curved boundaries skyrmion lattices nucleate at greater thicknesses than those observed for a flat geometry. Furthermore, the microfluidic setup enabled us to investigate the properties of BPs in meta-states or the effects of flow-induced reorientation on their transient structural responses. The underlying principles behind these molecular organizations were explored through a combination of experimental observations and theoretical simulations in bulk conditions.

Rights

©2023, Sepideh Norouzi

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