Static and dynamic studies of liquid crystals: Fundamental studies of defects in droplets and channels /

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Bibliographic Details
Author / Creator:Roberts, Tyler Franklin, author.
Ann Arbor : ProQuest Dissertations & Theses, 2015
Description:1 electronic resource (82 pages)
Format: E-Resource Dissertations
Local Note:School code: 0330
URL for this record:
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Other authors / contributors:University of Chicago. degree granting institution.
Notes:Advisors: Juan J. de Pablo Committee members: Sidney Nagel; Paul Nealey.
Dissertation Abstracts International, Volume: 77-02(E), Section: B.
Summary:The long-range molecular order within a liquid crystal (LC) has proved to be an effective environment for the self-assembly of nano- and micro-particles into highly-organized colloidal structures. These structures have numerous industrial and engineering applications. Typically aggregation takes place within bulk LC; however, recent experiments have demonstrated that the curved surface of an LC droplet can induce more complex and sophisticated colloidal structures. Alongside this research has been an effort to design LC-microfluidic devices, where colloidal aggregation could be a single process within a multi-step platform. It is now understood that the geometry of these colloidal structures and the dynamics of their aggregation are controlled through defects, areas where the LC becomes locally isotropic, or disordered. The properties of the defects present in such systems is determined by the anchoring strength between the particles and the LC, and, in the case of droplet-driven assembly, the droplet surface and the surrounding aqueous medium. Anchoring, a broad term capturing multiple molecular phenomena, determines in which direction and how strongly an LC will align with a particular surface. Unfortunately, this parameter is difficult to quantify, and numerous methods, often complex and cumbersome, have been proposed. Our understanding of defects and anchoring is incomplete, and in this work, I will use theory and simulation to explore the role of both in static and dynamic LC systems.
I will first demonstrate that aspherical nematic shells possess heretofore unseen defect structures and describe how these shells could be exploited in the construction of so-called "linker particles" with non-tetrahedral defect geometry. These aspherical shells are created by placing ellipsoidal particles within a spherical micrometer-sized LC droplet. Even for fixed droplet size and a given particle geometry, a rich variety of defect configurations are found. These findings demonstrate that ellipsoidal particles are an effective means of controlling the number and type of surface defects at the droplet interface.
Second, I propose a convenient means of determining anchoring strength within LC-microfluidic devices. This is to aid engineers and scientists where, traditionally, a separate experiment is required to measure anchoring. Knowing anchoring strength is required in predicting flow patterns and the behavior of colloidal aggregation within these devices. Through basic theory and simulation, I provide a relationship between easily-controlled device parameters and anchoring strength. I suggest that by applying a shear flow or pressure gradient and measuring the resulting angle of the LC molecules at the surface of the device, one can easily estimate anchoring strength. I conclude by demonstrating that volumetric flow rate could also be used in determining anchoring strength. Either method, through measuring surface angle or flow rate, is easier to perform over the previously purposed methods and, moreover, is more convenient for those working with microfluidic devices.