Document Type

Dissertation

Publication Date

Spring 1-1995

Abstract

Ultrathin freely suspended liquid crystal films (FSLCFs) are layered two-dimensional (2D) systems which are ideal for the study of 2D physics because of their rich phase and symmetry breaking behavior and almost exclu sively internal interactions. We have studied structural defects and instabilities in tilted smectic FSLCFs of many different materials over a wide range of layer numbers. Our observations show that the range of possible phases, structural defects and instabilities in these films is considerably broader than previously realized. Here, we report our studies of string defects, twist-bend instabilities and splay instabilities in FSLCFs.

Until now, the defects identified in tilted smectic films are point topo logical defects of unit topological charge, in which the tilt orientation changes by ±211" upon traveling once around the defect point. We have discovered a variety of new defects in 2D tilted smectic systems ( the "string" defects) in which there is discontinuity in tilt orientation along a line. We also find associ ated fractionally charged topological point defect structures. Our observations indicate the presence of additional stabilization mechanisms for 2D line defects and open the way for study of line defects in 2D systems.

Some of the most interesting structures in liquid crystals arise as a re sult of internal frustration. These are situations in which the local energetically ideal configuration cannot be extended to fill space, but must be accommodated by the appearance of defects, often in periodic arrays. We have discovered two new frustrated phases in FSLCFs: the twist-bend stripe phase and the splay IV stripe phase. The twist-bend stripe phase is formed in an achiral compound with one aliphatic and one perfluoroalkyl chain and is a novel example of spon taneous chiral-symmetry breaking. This phase transition is mainly driven by the interior twist field generated by the steric interaction of molecules in non polar films. The splay stripe phase, on the other hand, arises as a result of a polar ordering phase separation, and the instability of the ensuing domain boundaries.

Comments

Advisors: Noel A. Clark, David M. Walba

Share

COinS