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Showing papers by "Samo Kralj published in 2006"


Journal ArticleDOI
TL;DR: The annihilation of the nematic hedgehog and anti-hedgehog within an infinite cylinder of radius R is studied using the semi-microscopic lattice-type model and Brownian molecular dynamics and results agree qualitatively with the existing experimental observations and also continuum-type simulations.
Abstract: The annihilation of the nematic hedgehog and anti-hedgehog within an infinite cylinder of radius R is studied. The semi-microscopic lattice-type model and Brownian molecular dynamics are used. We distinguish among the i) early pre-collision, ii) late pre-collision, iii) early post-collision, and iv) late post-collision stages. In the pre-collision stage our results agree qualitatively with the existing experimental observations and also continuum-type simulations. The core of each defect exhibits a ring-like structure, where the ring axis is set perpendicular to the cylinder symmetry axis. For ξ(0)d/(2R) > 1 the interaction between defects is negligible, where ξ(0)d describes the initial separation of defects. Consequently, the defects annihilate within the simulation time window for ξ(0)d/(2R) < 1. For close enough defects their separation scales as ξd $ \varpropto$ (tc - t)0.4±0.1, where tc stands for the collision time. In elastically anisotropic medium the hedgehog is faster than the anti-hedgehog. In the early pre-collision stage the defects can be treated as point-like particles, possessing inherent core structure, that interact via the nematic director field. In the late pre-collision stage the cores reflect the interaction between defects. After the collision a charge-less ring structure is first formed. In the early post-collision stage the ring adopts an essentially untwisted circular structure of the radius ξr. In the late post-collision stage we observe two qualitatively different scenarios. For μ = ξr/R μc the chargeless ring triggers the nucleation growth into the planar polar structure with line defects.

29 citations


Journal ArticleDOI
TL;DR: There is a critical disorder above which both phases become identical from the orientation point of view, but have different concentrations of colloids, and another characteristic value of disorderabove which the isotropic phase can exist only in a liquid state, the crystal phase being suppressed completely.
Abstract: The influence of a random-anisotropy (RA) type disorder on the phase separation of nematogen-colloid mixtures is studied theoretically by combining the phenomenological Landau-de Gennes, Carnahan-Starling, and hard-sphere crystal theories. We assume that the colloids enforce the RA disorder on the surrounding thermotropic liquid-crystal (LC) molecules. We adopt the Imry-Ma argument according to which the lower-temperature phase exhibits a domain-type pattern. The colloids impose a finite degree of orientational ordering even in the isotropic (paranematic) phase. In the ordered phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. The RA field opposes the phase separation tendency. With increasing disorder the difference between the paranematic and speronematic ordering decreases. Consequently there is a critical disorder, above which both phases become identical from the orientation point of view, but have different concentrations of colloids. We have also estimated another characteristic value of disorder above which the isotropic phase can exist only in a liquid state, the crystal phase being suppressed completely.

17 citations


Journal ArticleDOI
TL;DR: It is shown that the onset of orientational ordering increases the phase separation tendency, and the difference between the paranematic and speronematic ordering decreases, Consequently the structure of the phase-separated pattern can be much more complex in comparison to the Lambda=0 case.
Abstract: The influence of a random-anisotropy- (RA-) type disorder on the phase separation of the nematogen\char21{}non-nematogen mixture is studied. A combination of the phenomenological Landau\char21{}de Gennes and Flory-Huggins theories is used. We assume that the non-nematogen component (i.e., impurity) enforces the RA disorder to the enclosing thermotropic liquid-crystal (LC) phase. The Imry-Ma argument is used according to which the lower-temperature phase exhibits a domain-type pattern. The disorder strength is measured in terms of the dimensionless parameter $\ensuremath{\Lambda}$. We consider the case in which the LC molecules and impurities mix in the isotropic phase for $\ensuremath{\Lambda}=0$. The impurities enforce a finite degree of orientational ordering even in the high-temperature paranematic phase. In the low-temperature phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. We show that the onset of orientational ordering increases the phase separation tendency. The RA field, however, opposes this tendency. With increasing value of $\ensuremath{\Lambda}$ the difference between the paranematic and speronematic ordering decreases. Consequently the structure of the phase-separated pattern can be much more complex in comparison to the $\ensuremath{\Lambda}=0$ case.

15 citations


Journal ArticleDOI
TL;DR: Hydrophilic and hydrophobic aerosil particles immersed in S-(+)-[4-(2-methylbutyl) phenyl 4-n-octylbiphenyl-4-carboxylate] or [p-(n-decyloxy) benzylidene-p-amino-( 2-methyl butyl) cinnamate] compound have been studied by employing x-ray diffraction.
Abstract: Hydrophilic and hydrophobic aerosil particles immersed in S-(+)-[4-(2-methylbutyl) phenyl 4-n-octylbiphenyl-4-carboxylate] or [p-(n-decyloxy) benzylidene-p-amino-(2-methylbutyl) cinnamate] compound have been studied by employing x-ray diffraction. The aerosil-induced relative layer compression Delta d has been measured in the smectic A phase as a function of aerosil concentration x. Delta d(x) is shown to sensitively monitor the intermediate-rigid transition of the adaptive aerosil network. The dominant mechanism leading to the Delta d(x) profiles is the global establishment of the aerosil-induced molecular tilt in the smectic A phase.

15 citations


Journal ArticleDOI
TL;DR: In this paper, the authors studied the nematic structure of a liquid crystal confined within a plan-parallel cell following the temperature driven isotropic-nematic phase transition quench.
Abstract: We study the nematic structure of a liquid crystal confined within a plan-parallel cell following the temperature driven isotropic-nematic phase transition quench The Lebwohl-Lasher lattice model is used The kinetics of the system is followed via Brownian molecular dynamics, enabling us to reach macroscopic time scales In the simulation we assume that at one confining plate the nematic pattern becomes imprinted and frozen in after the phase transition We study the main features of resulting nematic equilibrium structures

4 citations


Journal ArticleDOI
TL;DR: It turns out that the nonplanar minimizer has a structure more complex than that envisaged in the tensorial approach employed by McKay and Virga and that the planar configuration cannot be the absolute minimizer when the outer cylinder becomes a plane wall.
Abstract: Equilibria of a nematic liquid crystal confined between two eccentric cylinders are studied within a purely director approach. A planar equilibrium configuration competes against a three-dimensional one. A stability diagram is obtained in terms of both the ratio between the radii of the bounding cylinders and the distance between their axes. It turns out that the nonplanar minimizer has a structure more complex than that envisaged in the tensorial approach employed by McKay and Virga [Phys. Rev. E 71, 041702 (2005)] and that the planar configuration cannot be the absolute minimizer when the outer cylinder becomes a plane wall. The mechanical actions transmitted by the nematic liquid crystal on both bounding cylinders are computed and compared with other results available in the literature.

3 citations


Journal ArticleDOI
TL;DR: In this paper, the analysis of the three-dimensional polymer network distribution inside the polymer-stabilized ferroelectric liquid-crystal layer based on the laser scanning fluorescence confocal microscopy and a fluorescent dye tagging of the polymer is presented.
Abstract: The paper presents the analysis of the three-dimensional polymer network distribution inside the polymer-stabilized ferroelectric liquid-crystal layer based on the laser scanning fluorescence confocal microscopy and a fluorescent dye tagging of the polymer. The studies of polymer-stabilized ferroelectric liquid-crystal structures described in this paper are focused on the comparison of the influence of polymer network in case that the polymerization is initiated in the chevron as well as in the quasibookshelf liquid-crystal molecular orientation. In the case of the chevron structure the regular distribution of the polymer network within the layer leads to the monostability of the chevron state. On the other hand the specific distribution of the polymer in the polymer-stabilized quasibookshelf stripe textures leads to the perfect bistability, improved multiplex driving, and analog gray scale capability.

2 citations


Journal ArticleDOI
TL;DR: In this article, the influence of weak disorder on 1st and 2nd order phase transitions in which a continuous symmetry is broken is studied. Butler et al. yield an explanation for the established domain pattern and estimate the phase behaviour as a function of the disorder strength.
Abstract: We study influence of a weak disorder on 1st and 2nd order phase transitions in which a continuous symmetry is broken. As illustrative cases we choose the isotropic-nematic (I-N) and nematic-smectic A (N-SmA) liquid crystal phase transitions. We yield an explanation for the established domain pattern and estimate the phase behaviour as a function of the disorder strength.

1 citations


01 Jan 2006
TL;DR: Cordoyiannis et al. as mentioned in this paper studied phase behavior across the SmA-SmC phase transition of CE8 liquid crystal (LC) perturbed by an aerosil network.
Abstract: We study phase behavior across the SmA-SmC phase transition of CE8 liquid crystal (LC) perturbed by an aerosil network. The temperature dependence of smectic ordering is measured using X-ray scattering. We reproduce qualitatively the obtained results by employing the Landau-Ginzburg-de Gennes type phenomenological approach. The experiment reveals that with increased relative concentration x of aerosil particles the smectic layers become progressively compressed. In addition a qualitative change in temperature behavior of the smectic tilt angle is observed. We explain the compression of layers as a Correspondence/Reprint request: Dr. George Cordoyiannis, National Centre for Scientific Research “Demokritos”, 153 10 Aghia Paraskevi, Greece. E-mail: gcordo@rrp.demokritos.gr George Cordoyiannis et al. 116 consequence of attraction among aerosil particles, mediated by the intervening liquid crystal phase. This gives rise to distribution of phase transition temperatures. The compression could be also caused by decreased volume available to the LC phase. We show that for x > 0.1 the experimental results could be explained only if a finite pretilt exists in the SmA phase. 6.