Nematic liquid crystals are fluids which possess orientational order in one direction. The shear flow behavior of such fluids is complex, due to coupling between the average molecular orientation field, or director, and the velocity field. For most nematics, application of a shear flow tends to align the director close to the flow direction within the shear plane. These materials are classified as flow-aligning nematics. Some nematics, however, feature a sequence of instabilities with increasing shear rate that rotate the director out of the shearing plane and ultimately form secondary flow structures called roll cells. These materials are classified and tumbling nematics. The purpose of this dissertation is to quantitatively compare and contrast model flow-aligning (5CB) and tumbling (8CB) nematic liquid crystals under conditions of oscillatory and steady shear flow.

Using the optical crystallographic technique of conoscopy, we have investigated the dynamic director response to oscillatory shear for nematics initially aligned in the velocity gradient direction. We have measured the ratio of the amplitude of the director rotation to the strain amplitude (R*) and the phase difference between the director rotation and the applied strain (Δχ) as functions of frequency for both 5CB and 8CB. Comparing these results with the Leslie-Ericksen theory allows extraction of several important material parameter ratios. In particular, the high frequency limit of R* is related to λ, the material parameter which indicates either flow-aligning (λ > 1) or tumbling (λ < 1) behavior. We found that for 5CB at 32.5°C, λ ≈ 1.1, while 8CB at 35°C gives λ 0.35. Aside from confirming the flow classification of these model nematics, this new experimental method establishes a framework for the study of director dynamics in polymeric nematic liquid crystals.

Samples of both 5CB and 8CB initially aligned in the velocity gradient direction were found to produce disclinations, or line defects in orientation, when subjected to steady torsional shear flow. The shear flow of each material is characterized by a dimensionless shear rate, Er≡(η/K)̇γh², where η is a viscosity, K is an elastic constant (with units of dynes), ̇γ is the shear rate, and h is the sample thickness. We investigated the origin of disclination production in both materials by monitoring the director orientation with increasing Er using polarizing optical microscopy (POM). Additionally, we have measured the density of disclinations (length/viewing area) using image analysis techniques. It was found that shearing 8CB leads to disclination densities roughly ten times those of 5CB. The dependence of disclination density on Er followed power-law behavior of the form $ρ_Ah = αEr^β, where ρ_A is the disclination density. For 5CB, β was found to be 0.64, with a set exponent of 0.5 giving a reasonable fit. 8CB showed values of β close to 1.0 for large Er and close to 0.5 for small Er, a break occurring near Er = 2,000. Dimensional arguments suggest that for β = 0.5, the disclinations exhibit 2-dimensional behavior, while β = 1.0 indicates 3-dimensional behavior.

A strategy for future research on polymeric nematic liquid crystals using the methods developed in this dissertation is presented in conclusion.