Dielectric Response of Liquid Crystals Formed by Bent-Core and Chiral Molecules

Bohdan Senyuk

Kent State University

The various homogeneous and inhomogeneous structures in liquid crystals (LCs) are often behind the interesting physical phenomena and practically useful properties. The ability to induce or control them with relatively weak electric fields makes LCs a desirable material for displays, optics, photonics and microfluidics applications. In the Dissertation, we studied the spontaneous, confinement or electric-field induced LC structures and their response to an applied electric field in nematic, cholesteric and smectic phases formed by rod-like (either achiral or chiral) and bent-core molecules. The main conclusions from this work are the following: First, we studied experimentally surface alignment, dielectric and optical properties and topological defects in thermotropic bent-core materials A131, C7 and C12, and found that observed features of these materials, such as anchoring transitions, dependence of splitting of isogyres in conoscopic images on applied electric fields or design of samples, stable isolated point defects (hedgehogs and boojums), are consistent with those of a uniaxial nematic in the entire temperature range of nematic phase. Secondly, using the fluorescence confocal polarizing microscopy for imaging the director field, we performed the first experimental studies of the scenario of layer undulations in a full three-dimensional lamellar system, represented by a short pitch cholesteric LC. We demonstrated that both qualitative and quantitative features of undulations strongly depend on the surface anchoring at the cell boundaries. We showed that the shape of undulating layers changes from sinusoidal at the onset of instabilities to zig-zag at moderate field and to the lattice of parabolic walls at high electric fields. Additionally, we demonstrated that the spatial modulation of an average refractive index resulted from layer undulations in cholesterics can be used for electric field controlled two-dimensional diffraction gratings. Finally, we described an orbiting motion of spherical colloidal particles in a smectic LC under a dc electric field. The effect is due to the field induced Quincke rotation that triggers translation of spheres through the hydrodynamic interaction with bounding walls. In the smectic A phase, the spheres can be trapped in regions with strong director distortions and forced to follow a pre-determined pathway.

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