Dynamics of chiral smectic-A and twist grain boundary phases of liquid crystals

We have performed dynamic light scattering and electro-optical studies of chiral smectic-A/C and twist grain boundary phases of liquid crystals. Through chiral symmetry breaking, these phases exhibit interesting spatial modulations of the director and/or smectic layer orientation. These modulations give rise to new symmetry-restoring modes, which are detectable by light scattering and which are studied in this dissertation.; The nitro-substituted chiral smectic-A liquid crystal, KN125, exhibits a large electroclinic effect (or electric field-induced molecular tilt), and a concomitant one-dimensional modulation of the smectic layers and director parallel to the average layer plane. The domain walls of this “layer-buckling” modulation present a new phenomenological variable, whose fluctuation properties we have measured. We construct a phenomenological elastic theory of the modulated structure, which quantitatively describes our measurements. We also discuss the dynamics when a large electric field is applied to the sample, destabilizing and inducing large amplitude fluctuations in the modulation.; We also study two siloxane-substituted variants of KN125. These unusual materials are believed to have a smectic-A phase composed of randomly-oriented, tilted cores—the so-called deVries smectic- A phase. Near the chiral smectic-C phase, they also show evidence of a chiral modulation due to a “rippling” of the tilted director within the smectic planes (and with little or no accompanying layer buckling). Our electro-optical and light scattering studies, combined with a theoretical analysis of a Landau free energy proposed for the rippled phase, support these unusual features.; Finally, we explore the dynamics of the cholesteric and twist grain boundary (TGB) phase of the chiral liquid crystal, 11FBTFO1M7. The TGB phase in liquid crystals is important because it is an analog of the Abrikosov vortex lattice phase in Type II superconductors, and because it illustrates commensurate-incommensurate structural transitions in a soft system. Our preliminary results reveal coupling to grain boundary fluctuations (the symmetry restoring mode) not only in the TGB phase, but also well into the cholesteric phase. This suggests (as have other experiments) that some form of the grain boundary structure persists well into the cholesteric phase in this material. Initial results on the temperature dependence of the low frequency mode attributed to grain boundary fluctuations seem to be consistent with a series of commensurate-incommensurate transitions, where “commensurate” means that the total twist of the TGB structure in our sample is a rational fraction of π (i.e., matches the substrate anchoring conditions in our cell).