Defects, Diffusion and Fluctuations in Lyotropic Liquid Crystals.
This thesis consists of several topics: laser beam radii of submicron width, the structure of the oily streak defect in the L-(alpha) phase of lipids, the anomalous molecular diffusion and its relation to defects in the P-(beta)' phase and thermal shape fluctuations of large thin-walled lipid vesicles. Methods for direct measurement of the intensity distribution in laser beams focused by microscope optics to submicron width are described, tested and compared. Periodic undulations of paired disclinations ('oily streaks') in thin, fully hydrated liquid crystals of dimyristoyl phosphatidylcholine (DMPC) in the L-(alpha) (smectic A) phase are revealed by optical microscopy. Undulations of the lateral position of the oily streak axis appear spontaneously in all paired disclinations on formation of homeotropically aligned specimens. The diffusion of a fluorescent lipid analogue in liquid crystals of the anisotropic P-(beta)' phase of DMPC has been found to be highly variable, suggesting structural defect pathways. Fluorescence photobleaching recovery (FPR) experiments imply two effective diffusion pathways with coefficients differing by at least 100. This is consistent with fast diffusion along submicroscopic bands of disordered material ('defects') in the bilayer corrugations characteristic of this phase. The corrugations are ordinarily misaligned in a fine scale matrix but align along the paired disclinations and occasionally in an adventitious visible striped texture. Fast diffusion occurs along these linear structures. The upper limit on the effective diffusion coefficient in the ordered material is D (LESSTHEQ) 2 x 10('-16) cm('2)/sec at 22(DEGREES)C, D (LESSTHEQ) 3 x 10('-17) cm('2)/sec at 13(DEGREES)C, in contrast to D (TURN) 4 x 10('-11) cm('2)/sec at 16(DEGREES)C along aligned defect pathways. The time correlation function of the shape fluctuations of large ((GREATERTHEQ) 10(mu)m), flaccid, cylindrical and quasi-spherical hydrated phospholipid membrane vesicles consisting of one to several bimolecular layers is measured in order to obtain the curvature elastic modulus, K(,C). The mean-square amplitude of normal modes of the fluctuations yield K(,C) using the equipartition theorem. The correlation times of membrane relaxation are also found to depend on K(,C). The amplitudes and correlation times of the fundamental bending mode of cylindrical vesicles both yield K(,C) (TURN) 1-2 x 10('-12) ergs. The analysis for the quasi-spherical vesicles is more complicated but the results are consistent with theory and the cylinders.
- Pub Date:
- Physics: Condensed Matter