Defects and reconfiguration in nematic liquid crystal drops Charlotte G Slaughter

Format:

Book

Thesis/Dissertation

Author/Creator:

Slaughter, Charlotte G., author.

Contributor:

University of Pennsylvania. Physics and Astronomy., degree granting institution.

Language:

English

Subjects (All):

Condensed matter physics.

Physics.

Materials science.

0611.

0794.

0605.

Local Subjects:

Condensed matter physics.

Physics.

Materials science.

0611.

0794.

0605.

Genre:

Academic theses

Physical Description:

1 online resource (176 pages)

Contained In:

Dissertations Abstracts International 87-12B

Place of Publication:

Ann Arbor : ProQuest Dissertations and Theses, 2026

Language Note:

English

Summary:

This dissertation experimentally explores the behavior of thermotropic liquid crystals (LCs) in a variety of unusual environments that are driven, for example, by novel geometric confinement, applied magnetic fields, anchoring at the boundary, and mechanical stresses. Specifically, we report on (1) the coalescence of sessile and suspended radial drops in both the nematic and isotropic LC phases, (2) magnetic-field-induced inversion wall defects and defect-splitting of nematic LCs confined in toroidal and cylindrical "containers", and (3) the rheological and optical behavior of emulsions comprised of shape-changing LC oligomer drops that change morphology with temperature variation. The dissertation measures new effects and derives fundamental understanding from experiments about topological defects and director field configurations in LCs. These ideas, in turn, aid in the identification of new ways to manipulate soft materials, especially LCs, that could have relevance for technological applications of reconfigurable soft materials based on LCs. The first set of experiments investigates the coalescence of spherical and sessile LC droplets with homeotropic anchoring. We study LC in both isotropic and nematic phases. Previous theoretical studies have suggested that coalescence of two radial nematic liquid crystal (NLC) droplets should be prevented by an energy barrier due to formation of a topological defect (a ring defect) between the two droplets. In contrast, our experimental work demonstrates that there is no significant difference between coalescence of isotropic and nematic drops. However, we observe a large difference between the merging rates of sessile versus suspended drops. Nearby sessile drops merge rapidly upon contact, suggesting that droplet wetting enhances the film drainage and surfactant expulsion between drops, and thus facilitates coalescence. Using polarized optical microscopy (side- and top-views), we are able to observe droplet merging, as well as the associated defect dynamics and director field evolution which were consistent with theoretical expectations. Numerical simulations were employed to corroborate some of the features we observed and suggest that elastic energy due to defect formation is not a driving factor in merging. Instead, measurements of surface tension yield and simple theoretical modeling indicates that surface energy gains during coalescence dominate elastic energy costs. The second set of experiments investigate magnetic-field-induced director reconfiguration in NLCs confined within toroidal drops (with tangential boundary anchoring conditions) and in cylinders (with homeotropic / perpendicular anchoring). In the toroidal geometry, we observe the formation of stable alignment inversion walls, also known as Helfrich walls, in the applied magnetic field. Notably, the number and presence of point defects in the initial zero-field configuration strongly influence the type of walls that emerge. When no point defects are present, splay-bend walls form in the applied field. In contrast, when a point defect pair exists, a wall with splay-bend distortions that is present in zero-field will disappear upon field application. The cylindrically confined NLCs exhibit transitions from escaped radial configurations to planar polar and planar polar with defect configurations as the magnetic field strength increases with a critical field that depends on cylinder radius. Together, the experiments and supporting simulations quantify the mechanisms governing the formation and evolution of these defect structures and demonstrate new routes for creating stable defects. Finally, a third set of experiments explores macroscopic rheological and optical behavior of ensembles of micron-size drops containing polydisperse nematic liquid crystal oligomers (NLCOs) composed of RM82. With temperature variation, the drops reversibly evolve into morphologically distinct nematic structures ranging from spheres to branched filamentous networks. The emulsions exhibit dramatic changes in viscosity and optical scattering when the drops change shape from sphere-like to filamentous. These effects need further characterization but offer the possibility to create optical and rheological switches

Notes:

Source: Dissertations Abstracts International, Volume: 87-12, Section: B.

Advisors: Yodh, Arjun Committee members: Collings, Peter; Kamien, Randall; Blake, Cullen; Osuji, Chinedum

Ph.D. University of Pennsylvania 2026

Vendor supplied data

Local Notes:

School code: 0175

ISBN:

9798247981459

Access Restriction:

Restricted for use by site license