Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Noel A. Clark

Second Advisor

Joseph E. Maclennan

Third Advisor

Matthew A. Glaser

Fourth Advisor

David M Walba

Fifth Advisor

Leo Radzihovsky

Abstract

Exfoliated monolayer zirconium phosphate sheets (1μm diameter x 2.78 nm thick) in water form colloidal suspensions exhibiting liquid crystal (LC) ordering above a critical concentration. Studies of the phase behavior and phenomenology of such suspensions show that the LC is nematic, in agreement with theory, but that the predicted lamellar smectic phase does not appear. At high concentration lamellar correlations grow in the nematic but are limited in range, either by the finite size of the sheets or by defects in the sheets. Coexistence of the nematic with an isotropic phase is observed at all concentrations, indicating a long range attraction between the sheets. At high concentration the coexisting LC forms lens-shaped nematic tactoids, analysis of which enables estimates of the nematic elasticity and surface tension. Transmission electron microscopy reveals the bispherical internal structure of the nematic director field in the tactoids. Nematic cells with initial homeotropic orientation exhibit a periodic instability in the director field, which is unstable against the formation of a bend undulation of the sheets. This phenomenon, which is also observed in lyotropic colloidal suspensions of graphene oxide sheets, may be attributed to flexopolydispersity, the slow internal rearrangement of sheets of different size to optimize their spacing and thereby maximize entropy.

Azobenzene-based molecules which form a self-assembled monolayer (SAM) tethered to a glass surface are highly photo-sensitive and dynamically orient liquid crystals in contact with them when illuminated with polarized actinic light. We probe the coupling of such monolayers to the nematic liquid crystal in a hybrid cell by studying the dynamics of liquid crystal reorientation in response to local orientational changes of the monolayer by a focused actinic laser with a rotating polarization. This locally reorients the nematic, winding up a set of nested rings of splay-bend nematic director reorientation until the required torque exceeds that of the surface coupling, after which the nematic director starts to slip. Quantitative analysis of the dynamics enables measurement of the coupling between the azo-SAM and the nematic liquid crystal.

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