A. J. Jin

Novel results of extremely thin substrate-free liquid-crystal films obtained from calorimetric and computer simulation studies

Experimental results from extremely thin free-standing liquid-crystal films have been found to exhibit divergent heat-capacity anomalies in the vicinity of the smectic-A-hexatic-B (SmA-HexB) transition. In fact, the data can be successfully fit to a power-law expression yielding the critical exponent α=0.30±0.04. The results clearly indicate that the transition cannot be described solely in terms of hexatic order and that some additional molecular order must be created in the SmA-HexB transition in these liquid crystals. This transition is therefore not the simple liquid-hexatic transition as had been suggested previously. In light of x-ray diffraction studies indicating the existence of herringbone order in some liquid-crystal samples, Bruinsma and Aeppli formulated a coupled XY hamiltonian as a possible explanation for the anomalous experimental data obtained on the SmA-HexB transition in three dimensions (i.e. bulk samples). Based on our novel calorimetric and optical reflectivity results from thin liquid-crystal films, we have conducted extensive Monte Carlo simulations to explore this model in two-dimensional lattices. These studies have resulted in a number of novel discoveries that reveal important aspects of both the SmA-HexB transition in thin liquid-crystal films and two-dimensional melting phenomena in general.

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.

Calorimetric, Optical Reflectivity and Electron-Diffraction Studies of 4O.8 Free-Standing Films

Abstract Calorimetric, optical reflectivity and electron-diffraction measurements have been employed to investigate the nature of the smectic-A-crystal-B transition in thin free-standing 4O.8 (N-4-n-butyloxybenzylidene)-4-n-octylaniline) films. The results from thin films shed an important light on previous results obtained from mechanical response investigations. Moreover, while the smectic-A-crystal-B transition of bulk 4O.8 samples shows a strongly first order transition, the corresponding transition at the outermost layers exhibits pronounced fluctuations, indicating the remarkable effects of reduced dimensionality.

Novel Layer-By-Layer Transitions Found in Free-Standing Liquid-Crystal Films

Abstract By utilizing our state-of-the-art ac differential freestanding film calorimetric system, three remarkable surface enhanced layer-by-layer transitions in several liquid-crystal compounds have been investigated. Each of these transitions can be well-described by the simple power-law expression: L = Lo t−ν. This demonstrates that these surface enhanced transitions occur on the smooth “substrate” with long-range interactions.

Layer-by-layer transitions in liquid crystals

Employing our free-standing film calorimetric system. we have identified three remarkable layer-by-layer transitions in several liquid crystal compounds. Each of these transitions can be well described by the simple power-law form:L = L0t. The layering transitions liMind near the smectic-A- hexatic-B and smectic-A crystal-B transitions are well characterized by the exponent v≈ 1 3. This value is consistent with models based on a van der Wads-like dominant intermolecular interaction. Another novel layer-by-layer thinning transition has been discovered above the hulk smectic-A isotropic transition of a perfluorinated liquid crystal compound. The value of the exponent obtained, v ≈ 3 4. cannot he easily explained using familiar models.

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.

STUDIES OF SURFACE-INDUCED LAYER-BY-LAYER TWO-STAGE PHASE TRANSFORMATIONS

Utilizing our high-resolution ac calorimeter, optical reflectivity, and electron diffraction, we have found a few very interesting phenomena associated with the free-standing films of the N-(4-n-butyloxybenzylidene)-4-n-octylaniline (4O.8) compound. Upon cooling from the smectic-A phase, the outer surface layers display a distinct hexatic-B order and subsequently transforms into crystal-B phase through a two-stage transformation. Each stage is found to be first order. The film exhibits a layer-by-layer ordering transitions developed from the film/vapor interface inwardly.