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2. Competition between local disordering and global ordering fields in nematic liquid crystalsMatej Cvetko, Milan Ambrožič, Samo Kralj, 2010, izvirni znanstveni članek Opis: We study the influence of external electric or magnetic field B on orientational ordering of nematic liquid crystals or of other rodlike objects (e.g. nanotubes immersed in a liquid) in the presence of random anisotropy field type of disorder. The LebwohlLasher lattice type of semimicroscopic approach is used at zero temperature. Therefore, results are valid well below the transition into the isotropic phase. We calculate the correlation function of systems as a function of B, concentration p of impurities imposing random anisotropy field disorder, the disorder strength W and system dimensionality (2D and 3D systems). In order to probe memory effects we calculate correlation length ▫$xi$▫ for random and homogeneous initial configurations. We determine the crossover fields Bc(p) separating roughly the ordered and disordered regime. Memory effects are apparent only in the latter case, i.e. for B < Bc. Ključne besede: disorder, ImryMa theorem, liquid crystals, memory effect, orientational order Objavljeno: 07.06.2012; Ogledov: 884; Prenosov: 196 Celotno besedilo (747,18 KB) Gradivo ima več datotek! Več...

3. Finitesize effects on order reconstruction around nematic defectsSamo Kralj, Riccardo Rosso, Epifanio G. Virga, 2010, izvirni znanstveni članek Opis: By use of the Landaude Gennes phenomenological theory, we study the texture of a nematic liquid crystal confined within a hybrid cell. Precisely, we consider cylindrically symmetric solutions containing topological defects dictated by appropriate boundary conditions. We focus our attention on cells whose dimensions are comparable with the biaxial correlation length ▫$xi_b$▫. For such severe confinements the order reconstruction (OR) configuration could be stable. Its structural details reflect the balance among boundaryenforced frustration, elastic penalties, and finitesize effects. In particular, we analyze the interplay between finitesize effects and topological defects. We show that defects are always pinned to the negatively (planar) uniaxial sheet of the OR structure. The presence of a ring defect can dramatically increase the critical threshold below which the OR structure is stable. Ključne besede: physics, liquid crystals, nematic crystals, nematic defects, structural transitions Objavljeno: 07.06.2012; Ogledov: 1084; Prenosov: 61 Povezava na celotno besedilo 
4. Memory effects in randomly perturbed systems exhibiting continuous symmetry breakingMatej Cvetko, Milan Ambrožič, Samo Kralj, 2009, izvirni znanstveni članek Opis: We studied pattern characteristics in randomly perturbed structures exhibitingcontinuous symmetry breaking. A LebwohlLashertype lattice model was used which described well the onset of orientational ordering of a system of rodlike objects. For example, such systems mimic the orientational ordering tendency in liquid crystals or in an ensemble of nanotubes. We set impurities to impose a random anisotropy type of disorder on the objects. Structural characteristics were studied as a function of concentration of impurities, interaction strength w between impurities and rodlike objects, external ordering strength and history of samples. We showed that the characteristic linear size of patterns and range of ordering strongly depend on the history of samples for weak enough strength w. The twodimensional and threedimensional simulations yielded qualitatively similar results. Ključne besede: liquid crystals, memory effect, nanotubes, structural order, percolation Objavljeno: 07.06.2012; Ogledov: 823; Prenosov: 79 Povezava na celotno besedilo 
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6. Alignment of carbon nanotubes in nematic liquid crystalsPaul van der Schoot, Vlad PopaNita, Samo Kralj, 2008, izvirni znanstveni članek Opis: The selforganizing properties of nematic liquid crystals can be used to aligncarbon nanotubes dispersed in them. Because the nanotubes are so much thinner than the elastic penetration length, the alignment is caused by the coupling of the unperturbed director field to the anisotropic interfacial tension of the nanotubes in the nematic host fluid. In order to relate the degree of alignment of the nanotubes to the properties of the nematic liquid crystal, we treat the two components on the same footing and combine LandaudeGennes free energies for the thermotropic ordering of the liquid crystal and for the lyotropic nematic ordering of carbon nanotubes caused by their mutually excluded volumes. The phase ordering of the binary mixture is analyzed as a function of the volume fraction of the carbon nanotubes, the strength of the coupling and the temperature. We find that the degree of ordering of the nanorods is enslaved by the properties of the host liquid and that it can be tuned by raising or lowering the temperature or by increasing or decreasing their concentration. By comparing the theory to recent experiments, we find the anchoring energy of multiwalled carbon nanotubes to be in the range from 1010 to 107 N m1. Ključne besede: liquid crystals, nematic crystals, molecular dynamics, stability, elasticity, carbon nanotubes Objavljeno: 07.06.2012; Ogledov: 1058; Prenosov: 46 Povezava na celotno besedilo 
7. Fingered core structure of nematic boojumsSamo Kralj, Riccardo Rosso, Epifanio G. Virga, 2008, izvirni znanstveni članek Opis: Using the Landaude Gennes phenomenological approach, we study the fine biaxial core structure of a boojum residing on the surface of a nematic liquid crystal phase. The core is formed by a negatively uniaxial finger, surrounded by a shell with maximal biaxiality. The characteristic finger's length and the shell's width are comparable to the biaxial correlation length. The finger tip is melted for topological reasons. Upon decreasing the surface anchoring strength below a critical value, the finger gradually leaves the bulk and it is expelled through the surface. Ključne besede: physics, liquid crystals, nematic crystals, line defects, surface phenomena Objavljeno: 07.06.2012; Ogledov: 910; Prenosov: 61 Povezava na celotno besedilo 
8. Annihilation of defects in liquid crystalsMilan Svetec, Milan Ambrožič, Samo Kralj, 2007, objavljeni znanstveni prispevek na konferenci Opis: The annihilation of defect is studied theoretically in liquid crystals (LCs). We consider the annihilation of point disclinations in nematic and line edge dislocations in smectic A LC phase, respectively. We stres s qualitative similarities in these processes. The whole annihilation regime is taken into account, consisting of the precollision, collision, and postcollision stage. Ključne besede: physics, liquid crystals, annihilation, defects, point defects, point disclinations, edge dislocations Objavljeno: 07.06.2012; Ogledov: 796; Prenosov: 61 Povezava na celotno besedilo 
9. Confined liquid crystaline 5CB in 2D thermodynamic space : preliminary dielectric relaxation studySebastian Pawlus, Jana Osinska, Sylwester J. Rzoska, Samo Kralj, George Cordoyiannis, 2007, objavljeni znanstveni prispevek na konferenci Opis: Results of preliminary broadband dielectric spectroscopy studies in a wide range of temperatures and pressures range for a mixture of rodlike liquid crystalline 4cyano4pentylalkylbiphenyl (5CB) and hydrophilic silica spheres (Aerosil 300) are shown. Pretransitional anomaly, observed previously in the bulk 5CB, has been found. Temperature dynamics of the mixture was investigated with via the DC conductivity ?, coupled to the reorientational relaxation. The derivative based analysis of electric conductivity showed a clear nonArrhenius dynamics and indicated the anomalous increase of the fragility strength coefficient on approaching the isotropicnematic transition. Pressure investigations of the solidification from the nematic phase showed the increase of the transition temperature on pressuring but with unusual increasing of dTNS/dP coefficient. Ključne besede: physics, liquid crystals, complex dynamics, random constraints, high pressures Objavljeno: 07.06.2012; Ogledov: 689; Prenosov: 50 Povezava na celotno besedilo 
10. Influence of a random field on particle fractionation and solidification in liquidcrystal colloid mixturesVlad PopaNita, Paul van der Schoot, Samo Kralj, 2006, izvirni znanstveni članek Opis: The influence of a randomanisotropy (RA) type disorder on the phase separation of nematogencolloid mixtures is studied theoretically by combiningthe phenomenological Landaude Gennes, CarnahanStarling, and hardsphere crystal theories. We assume that the colloids enforce the RA disorder on the surrounding thermotropic liquidcrystal (LC) molecules. We adopt the ImryMa argument according to which the lowertemperature phase exhibits a domaintype 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 domaintype 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. Ključne besede: liquid crystals, transitions, segregation, mixing, random fields, solidification, complex fluids Objavljeno: 07.06.2012; Ogledov: 784; Prenosov: 60 Povezava na celotno besedilo 