Light scattering from thin smectic liquid crystal films
Quasi-elastic light scattering experiments have been performed on the tilted phases of thin smectic liquid crystal films. Films of only a few layers in thickness allow for study of phase transitions in a quasi-two-dimensional, substrate-free system. In the bend and splay geometries, it is found the scattering is well described by a simple elastic free energy describing in-plane fluctuations. The coupling of bond-orientation to the molecular tilt allows the investigation of bond-orientation order in the hexatic Smectite-I phase. This thesis discusses the elasticity and dynamics in terms of a defect-mediated theory of two-dimensional melting. In intermediate geometries, the dynamic signal reveals the presence of a previously unreported slow mode in the Smectite-C phase of 4-(2-methylbutyl)-phenyl-4[prime]-(octyloxy)-(1,1[prime])-biphenyl-4-carboxylate (8OSI). The wavevector dependence of the scattered intensity for the new mode is well described by the selection rules for out-of-plane fluctuations of the film. The overdamped dynamics reveal a non-hydrodynamic and extremely slow decay. In the Smectite-I phase of 8OSI, where the director fluctuations are strongly coupled to orientational fluctuations of the hexatic order, it is found textural defects develop in the director fields, destroying the magnetic alignment. Experiments were also performed on mixtures of 8OSI and 4-n-hexyl-phenyl-4-n-(decyloxy)-benzthiolate ([bar 1]4S6) liquid crystals. For mixtures with more than 50% [bar 1]4S6, no hexatic phase and melting proceeded via a first-order Smectic-C to Smectic-J transition. At lower concentrations of [bar 1]4S6, the author was able to change the critical properties of the Smectic-C to Smectic-I phase transition, by increasing the quantity of [bar 1]4S6 in the mixture. In the vicinity of the crystal-hexatic-liquid triple point, the Smectic-C to Smectic-I transition becomes first-order due to the influence of the crystal order-parameter.
- Research Organization:
- Massachusetts Inst. of Tech., Cambridge, MA (United States)
- OSTI ID:
- 7103265
- Country of Publication:
- United States
- Language:
- English
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