Flexoelectricity in liquid crystal materials

With the maturation of the soft matter physics-based system, liquid-crystal (LC) materials research is undergoing a resurgence. Devices based on nematic class of liquid-crystal materials are now being used beyond the field of displays with applications ranging from the field of spectroscopy, microscopy to biology. This burgeoning interest in nematic liquid crystals-based devices requires systematic understanding of both static and dynamics of nematic director under influence of various external and internal forces. This thesis reports the realisation, investigation, and elucidation of three distinct and original dynamic switching geometries within a nematic liquid crystal layer that was subject to an applied electric field. The elastic forces associated with the time-dependent nematic director distortions within the layer, and the coupling to the relative strength and direction of anchoring at the boundaries, were key critical factors in governing the detailed switching response for all of these geometries. In the first geometry, the nematic layer was subject to externally driven Poiseuille flow whilst the A.C. electric field was applied orthogonal to the layer. Switching was investigated for both planar alignment, with the easy direction parallel to the flow, as well as homeotropic alignment at the solid confining plates, in both cases as a function of the flow rate and the electric field strength. For planar alignment, flow-controlled switching between distinct nematic director distortion modes was demonstrated and elucidated, where the modes involved were the n=0;1;2 flow perturbed analogues of the sin(npz=d) director distortion modes, using nomenclature from the zero flow classic Fr´eedericksz transition perturbation analysis. In the second geometry, the nematic layer was subject to hybrid planar and homeotropic alignment respectively at the lower and upper solid confining plates. This symmetry breaking provided a topological defect line within the layer which was manipulated via a spatially periodic in-plane A.C. electric field. Precisely field controlled linear in-plane transport and motion of dispersed colloidal micro-particle was demonstrated, mediated by the A.C. electric field-induced dynamic evolution of the tortuous shape of the topological defect line. In the final geometry, the nematic layer was subject to hybrid planar and homeotropic alignment respectively at the lower and upper surface. A spatially periodic in-plane A.C. electric field creates a period stripe array of alternating mid-layer tilt orientations separated by parallel substantially linear tilt walls that contained topological defect lines. Modulating the polarity of an applied D.C. electric field in the device demonstrates the polarity dependent movement of the intervening tilt walls, as well as the accompanying topological defects as a result of electrostatic coupling to the flexoelectric polarisation. This flexoelectricity induced defect movement was studied in a positive nematic layer with a solid confined and free upper surface. The behaviour described above, in case of the free upper surface, is accompanied by a polarity dependent phase changes in the periodic wrinkling deformation at the free surface