Structure and Rheology of Lyotropic Liquid Crystals

Abstract

The aim of the present work is to probe the structure and rheology of the various lyotropic liquid crystalline substances. For this purpose, a commercial rheometer is used for studying the rheology of the substance and a custom?made polarising microscope is used to study the structure of the liquid crystals in conjunction with the rheometer. Two interesting types of behavior in lyotropic liquid crystalline phases are reported here. A thick well?aligned lamellar sample, when subjected to a high shear stress, reveals the existence of a critical shear stress above which the system becomes unstable and the shear?rate response fluctuates. The experiments seem to indicate that the critical shear stress varies as the inverse of the sample thickness. A simple analytical calculation shows that this behaviour is consistent with the onset of an undulation instability when the stress increases beyond a critical value. Visualization experiments show that there is no change in the structure of the lamellar phase when there are oscillations in the strain rate. In addition, the effect of quenching from the lamellar to the isotropic phase under shear was studied. An initially defect?ridden, disordered lamellar system was subjected to a temperature cycling experiment, where the temperature of the system is increased to isotropic and then quenched to lamellar phase with simultaneous application of shear stress. It is observed that the viscosity of the sample quenched with an applied stress is significantly lower than the viscosity of the system quenched without shear, and is also lower than the initial viscosity of the defect? ridden system. The ratio of these viscosities increases exponentially with the applied shear stress. Optical studies show that this is due to the variation in the degree of alignment of the sample. Lattice Boltzmann simulations have been carried out for generating a hexagonal phase. The range of the order parameters in the free energy functional required for generating a hexagonal phase is identified. The Lattice Boltzmann algorithm has also been parallelised.