1998-05-14
12:00 at CAB D 18

Particles Dispersions in Liquid Crystals: Rheology, Structure, and Dynamic Interactions

Davide A. Hill

Department of Chemical Engineering, University of Notre Dame du Lac, Notre Dame, Indiana, USA

Liquid crystals (LCs) and liquid crystal polymers (LCPs) exhibit unique optical, rheological, and microstructural features, which make them suitable for a variety of applications, encompassing optical displays, high strength fibers, selective chromatographic media, membranes, and permeation barriers. These properties stem from a molecular superstructure that combines a high degree of orientational order with liquid-like fluidity. Over the past 30 years, spurred by the development of ultra-high strength fibers (DuPont's KEVLAR), substantial efforts have been devoted towards developing LCPs suitable for production of high-performance, three-dimensional structural elements. Problems remain, however, for these materials exhibit a "woody" texture, prone to easy fracture along surfaces of strong molecular alignment. It has recently been recognized that compounding LCPs with small filler particles may prove effective and economical towards improving mechanical isotropy and fracture properties. Towards this goal, several fundamental issues pertaining to the influence of the particles on the molecular orientation within the matrix, and the concomitant feedback of the matrix' Frank distortional elasticity on the microstructure of the dispersion need to be clarified. Particle-induced distortions of the orientation field in the LC can, in fact, produce dynamic interactions (forces and torques) on the suspended particles, possibly leading to their migration and even self-organization. Liquid crystallinity is ubiquitous in biological systems. Distortional elasticity may also play a role in the structural reorganization of mesomorphic, intracellular material in simple organisms. To date, particle-matrix and particle-particle interactions due to Frank elasticity are not well characterized, and experimental and theoretical studies are just beginning to emerge. In this talk we shall present experimental evidence confirming the existence of interparticle forces in LCs. The Frank and Leslie-Ericksen theories for the statics and rheology of LCs provide suitable frameworks to interpret and rationalize the experimental observations.