Surface anchoring of a nematic liquid crystal at solid and fluid interfaces

Liquid crystals (LCs) are fluid anisotropic materials that have been particularly useful in display devices including twisted nematic displays and thin-film transistor liquid crystal displays (TFT-LCDs). Their utility is attributed to a long-range orientational order that is extremely sensitive to external perturbations such as applied fields and limiting surfaces. Limiting surfaces, which may occur at either a gas, liquid, or solid interface, not only confine the LC, but also interact with it determining a molecular orientation that epitaxially transmits into the bulk.;The synergistic LC alignment arising from limiting surfaces and the LC molecules in contact with it has been a frequent research theme for investigating both physical and chemical properties of interfaces. As an elastic and anisotropic medium, anchoring of LC molecular orientations at the interface is transmitted into the bulk alignment creating distinctive textures that are easily visualized with polarized light. As the physical and/or chemical properties of the interface change, the LC molecules often respond through a reorientation of their bulk alignment.;This dissertation furthers past research efforts that investigated mesoscopic LC alignment in response to monolayer structures at interfaces. Two classes of monolayers were investigated - self-assembled monolayers (SAMs) of long-chain alkyls on glass surfaces and Langmuir monolayers of amphiphilic molecules adsorbed at fluid interfaces.;To investigate SAMs, LC cells were fabricated by sandwiching a thin layer of LCs between two surfaces; one surface anchoring the LCs with a defined orientation and the other with a SAM varying in an experimental property. The varying surface supported a long-chain alkyl SAM irradiated with UV light to create a wettability gradient that varied with light intensity. The zenithal alignment or angle between the LC molecular long-axis and the surface normal remained zero until a critical surface wettability was reached, at which point there was a substantial increase in the angle. Comparison of the measured zenithal angle with a molecular model for LC anchoring concluded a discontinuous transition in the zenithal anchoring angle along the wettability gradient. Temperature of the sample was also shown to influence the anchoring strength of the surfaces.;The fluid interface between a nematic LC and an aqueous phase allowed for the dynamic properties of Langmuir monolayers to be imaged directly by LC textures. Investigated were surfactants, saturated fatty acids, phospholipids and amphiphilic azobenzene derivatives which were all capable of coupling to LC alignment at the interface. Similar to the air/water interface, Langmuir monolayers of saturated fatty acids and their azobenzene derivatives structurally arranged in thermodynamic phases at the LC/aqueous interface with additional variability afforded by the capability of the azobenzene moieties to photoisomerize. The nematic LCs coupled to the structure of the interface and displayed textures characteristic of the adsorbed monolayer phase. This fluid interface was further used to investigate LC response to in situ bioreactions at an adsorbed monolayer. Avidin-biotin reactions at biotin-functionalized phospholipid monolayers suggested the specific recognition of avidin molecules by the monolayer induced a reorientation in the LC layer. However, the prevalence of non-specific LC response to avidin and other proteins left inconclusive the extent of reorientation due to specific biorecognition reactions. LC reorientation to specific recognition of DNA 16mers, on the other hand, proved more conclusive. Cationic surfactant monolayers created a unique environment for the hybridization of single-stranded DNA (ssDNA) target molecules to ssDNA probe molecules electrostatically bound to it. Hybridization was found to induce a structural rearrangement of the cationic surfactant monolayer generating an easily discernible optical signal from the LC. Such detection of ssDNA proved to be extraordinarily sensitive and specific to the amount and sequence of the target being detected.