Liquid crystals (LCs) form a class of unique phases of matter, which combine the properties of liquid and solid phases. The high responsiveness of LCs to molecules at their interface and to the geometrical shape of these interfaces make them very useful in applications. LCs are birefringent and when confined in a spherical geometry between two aqueous phases, i.e., a shell, show characteristic textures that can easily be analyzed using a polarizing optical microscope (POM). The LC shells naturally exhibit unavoidable real or virtual topological defects in the orientational field (which defines the local orientation of the optic axis). The LC configuration in a shell depends sensitively on the boundary conditions, and different configurations give different optical textures. The change in the optical texture allows us to detect the presence of a variety of analytes at the LC-water interface. This thesis explores the responses of various additives present in the aqueous phase on the LC shells. First, I demonstrate how the concentration and chemical structure of technical surfactants (e.g., sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB)) influence the optical texture of the shells. The temperature-dependent water solubility of these surfactants can lead to surfactants moving through the shell and also to LC being removed from the shell in micelles. On introducing biological lipids in the aqueous phases, we observe small spindle-shaped islands on the shells due to lipid phase separation, showing striking inhomogenous textures. Such textures on the shells allow us to detect the presence of natural lipids without requiring any labeling with, for instance, fluorescent moieties. With this, the LC shells become a prominent candidate for biosensing. Finally, I also explore the extension of the LC shell lifetime by polymerizing small fractions of chemically reactive monomers within the shell into polymer networks. The exact type of monomer and its concentration determine whether the texture is locked in permanently via a dense polymer network or retains its responsiveness to the introduction of certain analytes to the LC-water interface because the polymer network phase separates.
Chairperson: Assist. Prof. Dr. Anupam Sengupta, University of Luxembourg
Committee members: Assist. Prof. Dr. Francesca Serra, Johns Hopkins University
Prof. Dr. Nicholas L. Abbott, Cornell University
Prof. Dr. Sivashankar Krishnamoorthy, Luxembourg Institute of Science and Technology.
Prof. Dr. Jan Lagerwall, University of Luxembourg