Showing papers by "Samo Kralj published in 2004"

Influence of finite size and wetting on nematic and smectic phase behavior of liquid crystal confined to controlled-pore matrices.

Abstract: The high-resolution calorimetric study was carried out on octylcyanobiphenyl liquid crystal (LC) confined to various controlled-pore glass (CPG) matrices with silane-treated surface. The diameter of the voids cross section ranged between 23.7 and 395 nm. The results are compared to those obtained previously on CPG voids nontreated with silane. We found a striking similarity between the shifts in the isotropic to nematic and nematic to smectic-A phase transition temperatures as a function of the void radius in which order parameter variations at the LC-void interface play the dominant role. Weaker temperature shifts are observed in silane-treated samples, where surface ordering tendency is larger. In nontreated samples, a finite-size scaling law in the maximum value of the heat capacity at the nematic to smectic-A transition was observed for void diameters larger than 20 nm. In silane-treated samples, this behavior is considerably changed by surface wetting interactions.

Annihilation of edge dislocations in smectic-A liquid crystals.

Abstract: This paper presents a theoretical study of the annihilation of edge dislocations in the same smectic plane in a bulk smectic-A phase. We use a time-dependent Landau-Ginzburg approach where the smectic ordering is described by the complex order parameter psi( r--> ,t) =eta e(iphi) . This quantity allows both the degree of layering and the position of the layers to be monitored. We are able to follow both precollision and postcollision regimes, and distinguish different early and late behaviors within these regimes. The early precollision regime is driven by changes in the phi ( r--> ) configuration. The relative velocity of the defects is approximately inversely proportional to the interdefect separation distance. In the late precollision regime the symmetry changes within the cores of defects also become influential. Following the defect collision, in the early postcollision stage, bulk layer order is approached exponentially in time. At very late times, however, there seems to be a long-time power-law tail in the order parameter fluctuation relaxation.

Random anisotropy nematic model: connection with experimental systems.

Abstract: We study theoretically the phase behavior of the continuum Random Anisotropy Nematic model. A domain-type pattern is assumed to appear in a distorted nematic liquid crystal (LC) phase. We map the model parameters to physical quantities characterizing LCs confined to Controlled-Pore Glasses and LC-aerosil dispersions. The domain size dependence on the disorder strength is obtained in accordance with the Imry-Ma prediction. The model estimates for temperature shifts of the paranematic-nematic phase transition and for the critical point, where this transition ceases to exist, are compared to the available experimental results.

Phase transitions in 8CB liquid crystal confined to a controlled-pore glass: Deuteron NMR and small angle X-ray scattering studies

Abstract: We present results of the deuteron nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) study of ordering and phase transition behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG) with nontreated and silanes-treated pore surfaces. The deuteron NMR spectra allowed to determine the degree of nematic liquid crystal ordering and also provided an indirect information on the confined 8CB smectic ordering via its influence on the nematic ordering. For the smectic phase these data are supplemented with measurements of the temperature dependence of the first-order SAXS diffraction pattern. The NMR results indicate that the average nematic and smectic order parameters of 8CB in the nontreated CPG are only weakly perturbed by the confinement. The SAXS data further suggest that in confined 8CB for both nontreated and silane-treated CPG a domainlike pattern appears in accordance with the Imry-Ma theorem.

Local elastic stability for nematic liquid crystals.

Abstract: We derive a stability criterion for nematic liquid crystals from a general study of the second variation of Frank's elastic free-energy functional. When applied to elementary director alignments compatible with the boundary conditions, such as the uniform alignment in a hybrid cell, this criterion is able to determine whether the most likely destabilizing mode is periodic or not, and to estimate the modulation length of such a mode, when it is periodic.

Interference textures of defects in a thin nematic film: an applet presentation

Abstract: We have developed an applet presentation showing different structures of a nematic liquid crystal confined to a thin transparent plan-parallel cell. The nematic structures possessing single defects or their pairs may be studied. The corresponding interference textures are shown simulating the polarization optic microscopy experiment. Several parameters defining the experimental set-up can be interactively varied enabling observation of subsequent changes in the interference pattern within a reasonable real time. The basic physics of liquid crystal defects necessary for understanding the limitations of the applet presentation is given.

Hedgehog annihilation in a confined nematic liquid crystal

Abstract: Annihilation of a pair of nematic point defects, having opposite strength is studied numerically. The defects are enclosed within an infinite cylindrical cavity whose lateral wall enforces strong homeotropic anchoring. The Lebwohl-Lasher lattice model is used. The dynamics of the system consisting of N > 105 molecules is studied using the Brownian molecular dynamics. We study pre-collision, collision and post-collision regimes. In the first regime the defects are well separated and are clearly distinguishable. Latter the core structures of defects strongly overlap and gradually decay into the defectless state.

Edge Dislocations in Smectic-A Liquid Crystals

Abstract: We study theoretically static structure and annihilation dynamics of edge dislocations in a smectic-A liquid crystal confined to a plan-parallel cell The Landau-Ginzburg type phenomenological approach is used in terms of a complex order parameter We investigate a structure of an isolated dislocation that is enforced by boundary conditions We further follow the annihilation dynamics of a pair of dislocations into a defectless state

Phase Behavior of Perturbed Liquid Crystals

Abstract: We study theoretically the combined effect of confinement and randomness on LC phase transitions in orientational (isotropic-nematic) and translational (nematic-smectic A) degrees of ordering. We focus to cases where these transitions are of (very) weakly 1st order. An adequate experimental realisation is, e.g., 8CB liquid crystal confined to a Controlled-Pore Glass matrix. Based on universal responses of “hard” and “soft” continuum fields to distortions we derive how different mechanisms influence qualitative and quantitative characteristics of phase transitions under consideration.

Nematic point defect annihilation in a cylindrical capillary

Abstract: We perform a numerical study of the annihilation of nematic radial and hyperbolic point defect. Both the pre-collision and post-collision stages are taken into account. Initially a pair of defects is induced at the axis of the cylindrical capillary. A semi-microscopic interaction potential and the Brownian molecular dynamics are used. In the early stage the cores of defects are negligibly mutually influenced. Their relative velocity is inversely proportional to the separation d between defects. As d becomes comparable to the nematic correlation length, the cores significantly deform. During the collision the defects merge and become indistinguishable. In the post-collision regime the order parameter at the collision site gradually recovers the equilibrium configuration.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.