Synchrotron x-ray observation of surface smectic-I hexatic layers on smectic-C liquid-crystal films

Using dislocations to probe surface reconstruction in thick freely suspended liquid crystalline films

Surface interactions can cause freely suspended thin liquid crystalline films to form phases different from the bulk material, but it is not known what happens at the surface of thick films. Edge dislocations can be used as a marker for the boundary between the bulk center and the reconstructed surface. We use noncontact mode atomic force microscopy to determine the depth of edge dislocations below the surface of freely suspended thick films of 4-n-heptyloxybenzylidene-4-n-heptylaniline (7O.7) in the crystalline B phase. Here, 3.0±0.1 nm high steps are found with a width that varies with temperature between 56 and 59°C. Using a strain model for the profile of liquid crystalline layers above an edge dislocation to estimate the depth of the dislocation, we find that the number of reconstructed surface layers increases from 4 to 50 layers as the temperature decreases from 59 to 56°C. This trend tracks the behavior of the phase boundary in the thickness dependent phase diagram of freely suspended films of 7O.7, suggesting that the surface may be reconstructed into a smectic F region.

Surface phase transitions in free-standing films of nonchiral tilted hexatic liquid crystals

Surface freezing transitions in free-standing films of achiral Schiff's-base liquid crystals 5O.6 and 7O.7 exhibiting tilted hexatic phases have been studied using optical textures. The evolution of textures with temperature in 5O.6 films is qualitatively similar to that reported earlier in the ferphenyl ester FTE1, suggesting the existence of the surface smectic-L phase, except that the stripe texture in 5O.6 consists of alternating light and dark stripes of unequal widths. No stripe texture is observed in 7O.7.

Unusual thickness-dependent heat-capacity anomalies in free-standing hexatic liquid-crystal films

High-resolution heat-capacity and optical-reflectivity measurements have been conducted near the smectic-A to hexatic-B transition in thin free-standing films of the liquid-crystal compound 64COOBC. We find an unexpected dependence on film thickness of the integrated magnitude of the heat-capacity anomalies as the films undergo layer-by-layer transitions. We measure the penetration depths of the ordering from the surface and next-to-surface layers which are pertinent to the highly correlated thermal behavior.

Direct layer-by-layer freezing of a smectic liquid-crystal surface into the crystalline phase

Electron diffraction and optical reflectivity have provided the direct confirmation of the existence of layer-by-layer surface transitions from the smectic-A immediately to the crystal-B phase in a liquid-crystal material, without going through an intermediate hexatic phase. The molecular interactions are found to be through retarded van der Waals forces. Our results suggest that a smectic-A film can transform into a crystal-B through three possible scenarios.

Structure of freely suspended chiral smectic films as determined by x-ray reflectivity and optical ellipsometry

We report on a combined x-ray reflectivity and optical ellipsometry study of freely suspended smectic (Sm) films of a chiral liquid crystalline compound with the phase sequence Sm-A-Sm-C(*)(alpha)-Sm-C*-Sm-C(*)(gamma)-Sm-C(*)(A). Using tilt magnitude profiles from x-ray reflectivity as input to model the average optical properties obtained by ellipsometry, tilt direction profiles are also obtained. In this way realistic models can be elaborated for the various types of chiral Sm-C films. We find that the surface layers are more tilted than the interior layers due to surface interactions and finite size effects. For the ferrielectric Sm-C(*)(gamma) phase the tilt direction profile corresponds to a three-layer helix, in agreement with the clock model of chiral Sm-C phases. In thin films the surface interactions suppress the bulk helix structure of the Sm-C(*)(alpha) phase and a Sm-C(*)(A)-like structure is formed with an anticlinic layer-by-layer alternation of the tilt directions.

Thermal fluctuations and x-ray scattering from free-standing smectic-A films

The present paper is devoted to theoretical investigation of thermal fluctuations and correlations between them in free-standing smectic-A films (FSSAF's) formed of liquid crystal compounds with bulk smectic-A-isotropic (SmA-I) and SmA-nematic (SmA-N) phase transitions, as well as small angle x-ray scattering from these FSSAF's. The study took into account the dependence of the bending elastic constant K and the smectic layer compressibility B on the distance from the boundary free surfaces of the films. The results of calculation are compared with those obtained in the framework of the Holyst model [Phys. Rev. Lett. 65, 2153 (1990); Phys. Rev. A. 44, 3692 (1991)] for spatially uniform FSSAF's. It has been found that, well below the temperature at which the smectic order in the bulk liquid crystal disappears, taking into account the profiles of the elastic moduli K and B does not produce noticeable differences from this model. However, at maximum temperatures of existence of FSSAF's, our results are considerably different from the predictions of the Holyst model. The results obtained are applied to calculation of specular and diffuse x-ray reflectivities of FSSAF's in the whole temperature interval of their existence. Displacement of the molecular centers within the film layers along those normal to them and the deviation of the local orientational order in FSSAF's from the ideal are also taken into account. The results of calculation are in agreement with experiments on the small angle x-ray scattering from FSSAF's.

Crystalline transitions in free-standing films of 4-n-heptyloxybenzylidene-4-n-heptylaniline

Free-standing films of 4-n-heptyloxybenzylidene-4-n-heptylaniline are known to exhibit multiple smectic-I surface layers in the presence of a smectic-A interior. The phase transitions leading to the crystallization of these films have been studied using electron diffraction. Our data are consistent with the scenario of the smectic-I surface layers first developing a crystal-B outermost layer before transforming entirely into the crystal-B phase, to be followed by the freezing of the smectic-A interior. The adjacent crystal-B and smectic-I layers show evidence of orientational epitaxy.

Layer-by-layer crystallization and the role of fluctuations in free standing smectic films

Specular x-ray reflectivity has been used to study the changes in the thermal fluctuation behavior of eight layer freely suspended N-(4-n-butoxybenzilidene)-4-n-octylaniline (4O.8) films during the process of successive layer-by-layer crystallizations. Each of these steps is preceded by the formation of an intermediate layer structure with in-plane hexatic ordering. We find an unusually large reduction of the fluctuations after the emergence of only the first hexatic toplayer. The fluctuation profiles over the film are in all cases quenched at the surfaces though less so after each crystallization step. This behavior supports theoretical arguments that hexatic order is reduced by out-of-plane smectic layer fluctuations.

Molecular theory of HexB-SmA-isotropic transitions in ultrathin liquid crystal films

A microscopic theory is developed to treat the ultrathin film of liquid crystals of molecules that have no cylindrical symmetry. The Hamiltonian is derived from the basic electrostatic interaction among electrons by considering the dipole-dipole and dipole-quadrupole interactions between nonchiral molecules. It exhibits the in-plane sixfold symmetry. From a unified model with the same interaction constants we are able to explain simultaneously the layer-thinning SmA-I transition, the anomalous multiplex heat capacity, the strong singularity in the HexB-SmA transition, and the coexistence of different phases. The theoretical calculations agree quantitatively with recent experimental results for the free-standing 54COOBC films.

Multiple-step melting in two-dimensional hexatic liquid-crystal films

An unexpected three-stage melting transition has been observed in two-dimensional (2D) free-standing liquid-crystal films by in situ electron-diffraction and optical-reflectivity measurements. These data suggest the existence of two phases between the 2D solid and liquid: a hexatic phase and, at a higher temperature, an intermediate liquid phase with hexatic-like positional correlations ( approximately 40 angstroms) but no long-range orientational order. Previous high-resolution heat-capacity measurements have revealed a divergent-like anomaly at the hexatic-liquid transition that sharply contradicts the predictions of 2D melting theories. The observation of an intermediate isotropic phase may alter our understanding of 2D melting and lead to reconciliation between current experiments and theories.

Surface Tension Measurements of Surface Freezing in Liquid Normal Alkanes

Surface tension measurements reveal surface freezing in liquid n-alkanes. A solid monolayer of molecules is found to exist up to 30 degrees C above the bulk freezing point. This surface phase exists only for carbon numbers 14 n </= 50. The measured carbon number and temperature dependence of the surface tension is interpreted within a simple thermodynamical model based on known bulk latent heat data and surface energy considerations. The vanishing of the surface phase for n </= 14 is a possible transition from surface freezing to surface melting behavior.