Liquid Crystals of Nanoplate - Kinetics of Phase Transition and Control by External Fields

Abstract

Einstein’s seminal work on Brownian motion and Perrin’s subsequent experiments showed that colloidal particles in colloidal dispersions obey the same statistical thermodynamics as atoms. The colloid-atom analogy has paved a way to understand the physics of atomic and molecular systems. Due to their larger size than molecules and relatively slower diffusion rates, colloids show slower kinetics that enables the understanding of the physics of atomic world, for example phase transitions, on convenient length and time scales. Suspensions of non-spherical colloids such as disk shaped colloids enable the formation of novel phases, having symmetries in between that of randomly ordered liquids and perfectly ordered solid crystals, known as liquid crystals (LCs). External field induced reorientation in nematic LC phase has been exploited in electronic liquid crystal displays (LCDs), which is a multibillion dollar industry today. The topological defects in liquid crystals represent physical realizations of purely mathematical homotopy theory that also applies to cosmological phenomena and fundamental materials including superconductors, nanomagnets etc. Colloidal disks can serve as models to study liquid crystals. In this dissertation, colloidal nanoplates of zirconium phosphate having thickness of about 3 nanometers and lateral size of several hundred nanometers are used to investigate the dynamics of phase transitions in colloidal disk suspensions, phase transition kinetics and control on LC phases using electric field, temperature gradient. Experiments to study of the kinetics of liquid crystallization are conducted using polarized optical microscopy and nucleation and growth rates of the growing nematic LC phase were measured. Due to negative anisotropy of electric polarizability, a field stabilized biaxial nematic phase that was predicted in liquid crystals of platelets and a positive evidence for the same was found. Ability to control the orientations of nanoplates on microscopic level using external electric field is useful in design of functional materials. Furthermore, external temperature gradient field was proven to be effective in moving nanoplates to the hot and their increased concentration at the hot end led to isotropic to nematic phase transition. Such thermophoretic control on the growth of nematic phase can prove to be important for technological applications.