Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Ivan I. Smalyukh

Second Advisor

Rafael Piestun

Third Advisor

Noel Clark

Fourth Advisor

David M. Walba

Fifth Advisor

Christopher N. Bowman

Abstract

The study of liquid crystals has brought about many advances and innovations, not only in technology, but also in our basic understanding of the world around us. For example, recent explorations of photo-induced defects and colloidal clusters in chiral nematics reveal topologically nontrivial forms in liquid crystal director fields, such as localized field structures resembling the Hopf and Seifert fibrations. In this work, we use light to control the alignment of liquid crystal director field at surfaces, to define topologically nontrivial geometric shapes of colloidal particles, and to probe the interplay between the topology of defects, director fields and surfaces. We first explore photo-responsive, azobenzene-based surface monolayers as means to optically and dynamically control boundary conditions at liquid crystal interfaces. This enables us to induce localized regions of twisted nematic field and disclination loops, which interact elastically with particle inclusions, providing a new kind of long-range, low-power colloidal manipulation as well as a means for controlling large-scale dynamics of topological defects and colloids. Using this same surface control technique, we pattern topological defects into thin films of polymerized liquid crystals, also functionalized with azobenzene. Irradiation of these films causes internal mechanical stresses that induce changes in the films' topography, depending on the patterned topology of the internal field structure within the photoresponsive liquid crystal polymer. We then employ a two-photon photo-polymerization technique to fabricate chiral, knotted and linked polymer microstructures, which we introduce into nematic liquid crystals. Based on three-photon excitation fluorescence polarizing microscopy studies, we reconstruct the surrounding 3D director field structure, revealing induced chiral, knotted and linked defect lines and fields. Using videomicroscopy, we characterize the inter-particle elastic forces that govern long range interactions and stabilize self-assembled colloidal configurations. Since there are few examples of practical realization of topological field configurations, these knotted and inter-linked colloids and fields provide a way to gain insights into behavior of other experimentally less accessible physical systems with similar symmetry and topology. Throughout the thesis, my research demonstrated that the topology of soft condensed matter can be shaped by light, laying a groundwork for experimental exploration of low-dimensional topology of fields and surfaces as well as for practical applications of topological relations in designing new forms of self-assembly.

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