Field-induced layer thinning transition on free-standing smectic 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.

Structural, Optical and Dynamic Properties of Thin Smectic Films

The problem of predicting structural and dynamic behavior associated with thin smectic films, both deposited on a solid surface or stretched over an opening, when the temperature is slowly increased above the bulk transition temperature towards either the nematic or isotropic phases, remains an interesting one in the physics of condensed matter. A useful route in studies of structural and optical properties of thin smectic films is provided by a combination of statistical–mechanical theories, hydrodynamics of liquid crystal phases, and optical and calorimetric techniques. We believe that this review shows some useful routes not only for the further examining of the validity of a theoretical description of thin smectic films, both deposited on a solid surface or stretched over an opening, but also for analyzing their structural, optical, and dynamic properties.

Molecular organization in freely suspended nano-thick 8CB smectic films. An atomistic simulation

Atomistic simulations of nano-thick free 8CB smectic films show the change of order across the film with temperature and thickness.

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.

Effects of fullerene addition on the thermo-optical properties of smectic liquid crystals at the vicinity of the nematic-smectic-A phase transition

The present work is devoted to the study of the thermo-optical properties of liquid crystals doped with traces of fullerene C(60) at the vicinity of the nematic-smectic-A phase transition. By using the time-resolved Z-scan technique, we measure the temperature dependence of the thermo-optical coefficient and the thermal diffusivity. Our results reveal that the critical behavior of the thermal diffusivity is strongly affected by the fullerene addition. We provide a detailed discussion under the light of the distinct mechanisms behind the thermal transport in liquid-crystal samples.

Elastic mediated force between nanoparticles adsorbed on smectic films under an external field

Within the harmonic approximation, we analytically determine the elastic-mediated interaction between colloidal nanoparticles adsorbed on the surface of smectic films under the influence of an external field. Both cases of free-standing films and films deposited over a solid substrate are considered. We show that the asymptotic decay (1/R in free-standing and exponential in deposited films) is not altered by the external field. However, the external field plays distinct roles according to the film configuration, the interparticle distance, the film thickness, and the surface tension at the film-gas interface. We provide a detailed discussion under the light of the distinct mechanisms controlling the undulations of the surface layer.

Influence of an external field on the surface tension of free-standing smectic films

We have carried out a theoretical study of the effect of the electric field E on the thermodynamical properties and surface tension γ of free-standing smectic films. Calculations, based upon the extended McMillan approach with anisotropic forces, show a stepwise reduction and increase of the values of the Helmholtz free energy f and γ, respectively, per partially fluorinated 5-n-alkyl-2-[4-n-(perfluoroalkyl-metheleneoxy)phenyl] (H10F5MOPP) molecule, as the temperature is raised above that for the bulk smectic-A-isotropic transition. Calculations show that E may not only affect the layer-thinning transition sequence, but also change the first multilayer jump in the thickness and increase the value of γ per H10F5MOPP molecule. Reasonable agreement between the theoretically predicted and the experimentally obtained data on γ of the partially fluorinated H10F5MOPP film has been obtained.

Crossover from tricritical to critical end point behavior in free-standing smectic films

We study the smectic to nematic (SmA-N) phase transition taking place at the center of a free-standing film that exhibits enhanced surface order due to the anchoring promoted by a surrounding gas. The usual McMillan mean-field approach predicts that the SmA-N transition in bulk samples can be continuous or discontinuous (first or second order) depending on the molecular geometry, with a tricritical point separating these two regimes. Here we show that the additional orientational order imposed by the surface anchoring stabilizes the surface-induced smectic and nematic phases, leading to the breakdown of the tricritical point and to the emergence of a critical end point. We report the full phase diagram, which depicts four distinct structures as the film thickness is reduced.

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.