S. W. Hui

Surface-freezing transitions and novel tilted hexatic phases in smectic liquid-crystal thin films

Abstract We report the first unambiguous identification in a thermotropic liquid crystal of a hexatic smectic-L, a phase with molecular tilt direction intermediate between those of the smectic-I and smectic-F, using electron diffraction in free-standing films of 5-(4″-hexyl, 3′-fluoro-p-terphenyl-4-oxy)-pentanoic acid ethyl ester. Our structural data directly support the proposed explanation of the unusual stripe phase observed optically. The films show a rich phase diagram as a function of thickness and temperature. A novel phase transition within the surface smectic-L phase is also observed.

CHARACTERIZATION SEVERAL NOVEL PHASE TRANSITIONS IN A UNIQUE LOWER DIMENSION SYSTEM OF FREE-STANDING LIQUID-CRYSTAL FILMS

Motivated by the defect-mediated two-stage melting theory, extensive experiments have been concluded on free-standing films of a liquid-crystal sample which exhibits the isotropic-smectic-A–hexatic B-crystal-B bulk phase sequence. For a two-layer film, which is virtually two-dimensional, our calorimetric measurement shows an apparently divergent anomaly near the smectic-A-hexatic-B transition. Its anomaly has a dominant fluctuation contribution and can be fitted to a power law with an exponent α≈0.37±0.05, in disagreement with the theoretical prediction. Thicker films exhibit surface-initiated layer-by-layer (LBL) wetting transition upon cooling. Moreover, these films also exhibit interesting LBL thinning transitions above the bulk clearing temperature.

Layer-Dependent Surface-Induced Tilt, Hexatic, and Positional Orders in Free-Standing 7O.7 Films

Abstract Electron diffraction in free-standing films of 4-n-heptyloxybenzylidene-4-n-heptylaniline of up to 25 molecular layers has revealed the coexistence, as a function of distance from the surface, of three distinct phases: an outermost smectic-I layer, several layers of a middle phase, and a smectic-A interior. The middle phase is an unusual highly-correlated tilted liquid, with hexaticlike positional correlations but no long-range bond-orientational order, which transforms to the smectic-I at a lower temperature. The discovery of this unusual middle phase also represents the possibility of a novel layer-dependent two-stage tilt-induced surface-freezing transition.

Electron-diffraction studies of phase transitions in 40.8 free-standing thin films

Abstract The structural phase transitions of free-standing thin films of N-(4-n-butyloxybenzylidene)-4-n-octylaniline (40. 8), which exhibits smectic-A and crystal-B phases in the bulk, have been studied in detail using electron diffraction. When a smectic-A film is cooled, the outermost layer transforms initially to the hexatic-B phase and subsequently to the crystal-B phase at a lower temperature. The first transition is consistent with the thermal anomaly in the heat capacity and the second transition coincides with the shear response observed in mechanical measurements. Upon further cooling, the film undergoes additional layer-by-layer smectic-A-hexatic-B-crystal-B transitions in its interior.