Author: Subas Dhakal

Publisher:

Published: 2010

Total Pages: 118

ISBN-13:

This dissertation addresses three distinct problems in liquid crystal: phase transitions among different SmCP phases of bent-core liquid crystal, flexoelectric effect in nematic liquid crystal and interplay between chirality and biaxiality in cholesteric liquid crystal. These problems are studied with computer simulations and analytical calculations. The first part of this dissertation is about chiral switching in smectic phases of bent-core liquid crystals. The phenomenon is investigated with a lattice Monte Carlo simulation and a phenomenological theory. Both methods show that the antiferroelectric (AFE) ground state switches into a ferroelectric state (FE) at sufficiently large electric field. In this transition, the chirality of the phase could change or not. In the case of weak inter-layer chiral coupling, anticlinic order of molecules from layer to layer could remain through the transition -thus changing the chirality of the phase. However, for a large value of chiral coupling, the anticlinic state could change to a synclinic state in the transition -thus preserving the chirality of the phase. Another area of focus is on flexoelectric effect in nematic liquid crystal. Recent experiments have shown that bent-core liquid crystals have a surprisingly large bend flexoelectric coefficient, up to 35 nC/m, roughly three orders of magnitude larger than the typical value. With this large bend flexoelectric coefficient, bent-core liquid crystals may be practical materials for converting mechanical into electrical energy. One of the focal points of this dissertation is to explain the large flexoelectric effect found in bent-core liquid crystals. Theoretical models of splay and bend flexoelectric effect are developed that incorporates the coupling between the director distortion (either splay or bend) and polarization. Through these models the flexoelectric response as a function of interaction parameters, temperature, and applied electric field is studied. Simulations and analytical calculations show that there is a large flexoelectric effect in the nematic phase, which diverges as the system approaches the transition to the polar phase. This observation demonstrated that flexoelectricity is not just a molecular effect but could be a statistical phenomenon associated with the response of correlated volumes of molecules, which increases as one approaches the polar phase. Finally, we investigate the statistical mechanics of chirality and biaxiality in liquid crystals through a variety of theoretical approaches, including Monte Carlo simulations, lattice mean-field theory, and Landau theory. All of these calculations show that there is an important interaction between cholesteric twist and biaxial order: The twist acts as a field on the biaxial order, and conversely, the biaxial order increases the twist, i.e. reduces the pitch. We model the behavior of chiral biaxial liquid crystals as a function of temperature, and discuss how the predictions can be tested in experiments.