X-ray determination of the molecular tilt and layer fluctuation profiles of freely suspended liquid-crystal films

Surface and finite-size effects on N-Sm-A-Sm-C phase transitions in free-standing films

The present study is devoted to the investigation of surface anchoring and finite-size effects on nematic-smectic-A-smectic-C (N-Sm-A-Sm-C) phase transitions in free-standing films. Using an extended version of the molecular theory for smectic-C liquid crystals, we analyze how surface anchoring and film thickness affect the thermal behavior of the order parameters in free-standing smectic films. In particular, we determine how the transition temperature depends on the surface ordering and film thickness. We show that the additional orientational order imposed by the surface anchoring may lead to a stabilization of order parameters in central layers, thus modifying the nature of the phase transitions. We compare our results with experimental findings for typical thermotropic compounds presenting a N-Sm-A-Sm-C phase sequence.

Smectic C and Nematic Phases in Strongly Adsorbed Layers of Semiflexible Polymers

Molecular dynamics simulations of semiflexible polymers in a good solvent reveal a dense adsorbed layer when the solution is exposed to an attractive planar wall. This layer exhibits both a nematic and a smectic phase (smA for short and smC for longer chains) with bond vectors aligned strictly parallel to the wall. The tilt angle of the smC phase increases strongly with the contour length of the polymers. The isotropic-nematic transition is a Kosterlitz-Thouless transition and also the nematic-smectic transition is continuous. Our finding demonstrates thus a two-dimensional realization of different liquid crystalline phases, ubiquitous in three dimensions, that occurs in a single monomolecular layer ordered at least over mesoscopic scales.

Local structural ordering in surface-confined liquid crystals

The effect of the interplay between attractive nonlocal surface interactions and attractive pair long-range intermolecular couplings on molecular structures of liquid crystals confined in thin cells with flat solid surfaces has been studied. Extending the McMillan mean field theory to include finite systems, it has been shown that confining surfaces can induce complex orientational and translational ordering of molecules. Typically, local smectic A, nematic, and isotropic phases have been shown to coexist in certain temperature ranges, provided that confining cells are sufficiently thick, albeit finite. Due to the nonlocality of surface interactions, the spatial arrangement of these local phases can display, in general, an unexpected complexity along the surface normal direction. In particular, molecules located in the vicinity of surfaces can still be organized in smectic layers, even though nematic and/or isotropic order can simultaneously appear in the interior of cells. The resulting surface freezing of smectic layers has been confirmed to occur even for rather weak surface interactions. The surface interactions cannot, however, prevent smectic layers from melting relatively close to system boundaries, even when molecules are still arranged in layers within the central region of the system. The internal interfaces, separating individual liquid-crystal phases, are demonstrated here to form fronts of local finite-size transitions that move across cells under temperature changes. Although the complex molecular ordering in surface confined liquid-crystal systems can essentially be controlled by temperature variations, specific thermal properties of these systems, especially the nature of the local transitions, are argued to be strongly conditioned to the degree of molecular packing.

The effect of surfaces on molecular ordering in thin liquid-crystal systems

A theoretical method for analyzing the interplay between pair long-range intermolecular forces and nonlocal, relatively short-range, surface interactions in liquid crystals, confined between plates of thin planar cells, is developed. It is shown that this method, as involving the concept of local orientational and translational order parameters, enables detailed investigations of the emergence of smectic A, nematic, and isotopic phases, as well as yields an insight into phase transitions between them, in cases of systems strongly affected by surfaces. The evidence of various surface effects, including the coexistence of different phases and the inward propagation of surface melting under the increase of temperature, is also given. The underlying numerical procedure, based on the algorithm of self-consistent calculations of local order parameters, is found to be very effective, allowing one to consider model systems of rather large thicknesses, corresponding to thicknesses of real sample cells.

Transition Helmholtz free energy, entropy, and heat capacity of free-standing smectic films in water: a mean-field treatment

Using the extended McMillan's mean field approach with anisotropic forces a study of both the structural and thermodynamic properties of free-standing smectic film (FSSF) in water on heating to the isotropic temperature is carried out numerically. By solving the self-consistent nonlinear equations for the order parameters, we obtained that the smectic-A-isotropic (AI) transition occurs through the series of layer-thinning transitions causing the films to thin in the stepwise manner as the temperature is increased above the bulk smectic-A-isotropic temperature TAI(bulk). With enhanced pair interactions in the bounding layers, the smectic-isotropic transition corresponds to smectic melting of the central layers. The effects of surface "enhanced" pair interactions in the bounding layers and of film thickness on the orientational and translational order parameters, the Helmholtz free energy and entropy, as well as the temperature dependence of the heat capacity of FSSFs, have also been investigated. Reasonable agreement between the theoretically predicted and the experimentally obtained - by means of optical microscopy and ellipsometry techniques - data of the temperature when the thin decylcyanobiphenyl smectic film immersing in water ruptures has been obtained.

Single-molecule diffusion in freely suspended smectic films

We present a study of the molecular diffusion in freely suspended smectic-A liquid crystal films with thicknesses ranging from 20 down to only two molecular layers. The molecular mobility is directly probed by determining the trajectories of single, fluorescent tracer molecules. We demonstrate, using several different smectic compounds, that a monotonic increase of the diffusion coefficient with decreasing film thickness is a general phenomenon. In two-layer films, the diffusion is enhanced by a factor of 3 to 5 compared to the corresponding bulk smectic phase. Molecular dynamics simulations of freely suspended smectic films are presented which support the experimental results.

Transition entropy, Helmholtz free energy, and heat capacity of free-standing smectic films above the bulk smectic-A-isotropic transition temperature: a mean-field treatment

We have carried out a numerical study of both the structural and thermodynamic properties of free-standing smectic films (FSSFs) for two cases of enhanced pair interactions in the bounding layers. Calculations, based upon the extended McMillan's approach with anisotropic forces, shows a stepwise reduction of the value of the heat capacity as the temperature is raised above the bulk smectic A-isotropic transition. The effects of surface "enhanced" pair interactions in the bounding layers and of film thickness on the orientational and translational order parameters, the Helmholtz free energy, and entropy of FSSFs have also been investigated. Reasonable agreement between the theoretically predicted and the experimentally obtained--by means of calorimetric techniques--data on the heat capacity of the partially fluorinated 5-n-alkyl-2-(4-n-(perfluoroalkyl-metheleneoxy)phenyl) (H10F5MOPP) films has been obtained.

Reversible aggregation of X-Shaped bolaamphiphiles with partially fluorinated lateral chains at the air/water interface

New compounds from the group of partially fluorinated X-shaped bolaamphiphiles are studied. These compounds exhibit an unusual reversibility of Langmuir isotherms when compressed/decompressed up to the total collapse of the film.

Surface-induced ordering effect in one antiferroelectric liquid crystal compound

Null transmission ellipsometer was employed to study the surface-induced tilt transition of an antiferroelectric liquid crystal compound in the bulk smectic- A (SmA) temperature range. The data of the surface tilt transition can be fitted with an extended mean-field model. These results indicate the transition is near a mean-field tricritical point. The surface layers' tilt directions are found to be in different planes several degrees above the bulk SmA-SmC_{alpha};{ *} transition.

Dynamic and correlation properties of solid supported smectic-A films

Dynamic properties and layer displacement-layer displacement correlation functions of the smectic-A films on a substrate are investigated. The eigenfrequencies spectrum and eigenmotions of the film are calculated within the framework of a discrete model. It was found that the static as well as dynamic properties of freely standing and solid supported smectic-A films differ significantly. In particular, the acoustic mode is absent in the films on a substrate and the surface tension is not essential for the film dynamics. The correlation length of the spatial layer displacement correlation functions is finite for films on a substrate in contrast to the free standing films. An effect of thermal fluctuations on the specular and diffuse x-ray scattering was analyzed.

Smectic membranes in motion: approaching the fast limits of x-ray photon correlation spectroscopy

The dynamics of the layer-displacement fluctuations in smectic membranes have been studied by x-ray photon correlation spectroscopy (XPCS). We report transitions from an oscillatory damping regime to simple exponential decay of the fluctuations, both as a function of membrane thickness and upon changing from specular to off-specular scattering. This behavior is in agreement with recent theories. Employing avalanche photodiode detectors and the uniform filling mode of the synchrotron storage ring, the fast limits of XPCS have been explored down to 50 ns.

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.

Temperature difference between bulk and surface transition in freely suspended smectic films

We report an ellipsometric study of freely suspended films of a chiral liquid-crystal compound possessing the phase sequence smectic-C-smectic-A-isotropic with a very broad (67 K) smectic-A phase range. We observe a smectic-C-smectic-A surface transition which is situated more than 60 K above the bulk smectic-C-smectic-A transition temperature. Possible reasons for this unusually large temperature shift are discussed.