T. Stoebe

An ac calorimeter for measuring heat capacity of free‐standing liquid‐crystal films

A unique ac calorimeter for measuring the heat capacity of free‐standing liquid‐crystal films has been developed. A theory of the measurement method used is presented and the experimental setup is discussed. The calorimeter has been used to study the nature of the smectic‐A‐hexatic‐B transition of several liquid‐crystal compounds. The evolution of the heat‐capacity anomaly associated with this transition has been studied for films ranging from three to a few thousand smectic layers in thickness.

PHYSICAL PROPERTIES OF THIN SUBSTRATE-FREE LIQUID-CRYSTAL FILMS

Free-standing liquid-crystal films constitute a unique and phenomenally rich system in which a number of physical phenomena may be carefully studied. In particular, they provide an ideal system in which the novel theories of defect-mediated melting in two-dimensions (2D) may be tested. Calorimetric data is presented that indicate that such theories cannot adequately account for phase transitions in effectively 2D free-standing liquid-crystal films only two molecular layers in thickness. Extensive Monte Carlo simulation studies have therefore been performed on a trial Hamiltonian that couples herringbone order to the hexatic order. In a relatively large region of parameter space, the simulation yields heat-capacity consistent with our calorimetric data. These results are discussed in the context of existing data on the structural, optical, and mechanical properties of free-standing films.

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.

Heat capacity measurements of extremely thin substrate-free liquid-crystal films

A state-of-the-art calorimetric system has been established. The system enables us to measure simultaneously heat capacity and optical reflectivity of free-standing liquid-crystal films from many hundreds down to only two molecular layers in thickness. Our experimental results on the smectic-A-hexatic-B and smectic-C-smectic-I transitions cannot be described solely in terms of the creation of bond-orientational order, indicating that additional molecular order must be present in the hexatic phases of liquid crystals.

Surface tension obtained from various smectic-a liquid-crystal free-standing films

Abstract Employing a straightforward experimental technique, we have investigated the surface tension of various liquid crystal compounds in the smectic-A and smectic-Ad phases. The results clearly demonstrate the significant role of molecular packing in determining the surface tension.

Heat capacity investigations of extremely thin liquid-crystal free-standing films

Abstract A high-resolution differential ac free-standing film calorimeter has been constructed in our laboratories. We have employed this unique calorimeter to investigate the evolution of the temperature dependence of heat-capacity anomalies through the smectic-A-hexatic-B-crystal-E transition sequence of one liquid crystal compound as the film thickness is reduced from a few hundred to just three molecular layers. The existence of free surfaces in the free-standing liquid-crystal films has a very strong effect on the development of the molecular order through the smectic-A-hexatic-B transition and other unexpected results related to the crystal-E order.

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.