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Liquid crystal - anisotropic nanoparticles mixtures
Vlad Popa-Nita, Matej Cvetko, Samo Kralj, 2011, samostojni znanstveni sestavek ali poglavje v monografski publikaciji

Ključne besede: liquid crystals, nano particles, phase behavior
Objavljeno: 07.06.2012; Ogledov: 999; Prenosov: 79
URL Povezava na celotno besedilo

Competition between local disordering and global ordering fields in nematic liquid crystals
Matej 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 rod-like objects (e.g. nanotubes immersed in a liquid) in the presence of random anisotropy field type of disorder. The Lebwohl-Lasher lattice type of semi-microscopic 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, Imry-Ma theorem, liquid crystals, memory effect, orientational order
Objavljeno: 07.06.2012; Ogledov: 983; Prenosov: 216
.pdf Celotno besedilo (747,18 KB)
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Finite-size effects on order reconstruction around nematic defects
Samo Kralj, Riccardo Rosso, Epifanio G. Virga, 2010, izvirni znanstveni članek

Opis: By use of the Landau-de 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 boundary-enforced frustration, elastic penalties, and finite-size effects. In particular, we analyze the interplay between finite-size 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: 1178; Prenosov: 68
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Memory effects in randomly perturbed systems exhibiting continuous symmetry breaking
Matej Cvetko, Milan Ambrožič, Samo Kralj, 2009, izvirni znanstveni članek

Opis: We studied pattern characteristics in randomly perturbed structures exhibitingcontinuous symmetry breaking. A Lebwohl-Lasher-type lattice model was used which described well the onset of orientational ordering of a system of rod-like 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 rod-like 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 two-dimensional and three-dimensional simulations yielded qualitatively similar results.
Ključne besede: liquid crystals, memory effect, nanotubes, structural order, percolation
Objavljeno: 07.06.2012; Ogledov: 921; Prenosov: 87
URL Povezava na celotno besedilo

The influence of disorder on thermotropic nematic liquid crystals phase behavior
Vlad Popa-Nita, Ivan Gerlič, Samo Kralj, 2009, pregledni znanstveni članek

Opis: We review the theoretical research on the influence of disorder on structure and phase behavior of condensed matter system exhibiting continuous symmetry breaking focusing on liquid crystal phase transitions. We discuss the main properties of liquid crystals as adequate systems in which several open questions with respect to the impact of disorder on universal phase and structural behavior could be explored. Main advantages of liquid crystalline materials and different experimental realizations of random field-type disorder imposed on liquid crystal phases are described.
Ključne besede: liquid crystals, random fields, phase transition, disorder, nematic structures
Objavljeno: 07.06.2012; Ogledov: 1488; Prenosov: 220
.pdf Celotno besedilo (864,80 KB)
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Alignment of carbon nanotubes in nematic liquid crystals
Paul van der Schoot, Vlad Popa-Nita, Samo Kralj, 2008, izvirni znanstveni članek

Opis: The self-organizing 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 Landau-deGennes 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 10-10 to 10-7 N m-1.
Ključne besede: liquid crystals, nematic crystals, molecular dynamics, stability, elasticity, carbon nanotubes
Objavljeno: 07.06.2012; Ogledov: 1141; Prenosov: 56
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Fingered core structure of nematic boojums
Samo Kralj, Riccardo Rosso, Epifanio G. Virga, 2008, izvirni znanstveni članek

Opis: Using the Landau-de 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: 998; Prenosov: 86
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Annihilation of defects in liquid crystals
Milan 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 pre-collision, collision, and post-collision stage.
Ključne besede: physics, liquid crystals, annihilation, defects, point defects, point disclinations, edge dislocations
Objavljeno: 07.06.2012; Ogledov: 868; Prenosov: 70
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Confined liquid crystaline 5CB in 2D thermodynamic space : preliminary dielectric relaxation study
Sebastian 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 rod-like liquid crystalline 4-cyano-4-pentylalkylbiphenyl (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 non-Arrhenius dynamics and indicated the anomalous increase of the fragility strength coefficient on approaching the isotropic-nematic 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: 759; Prenosov: 57
URL Povezava na celotno besedilo

Influence of a random field on particle fractionation and solidification in liquid-crystal colloid mixtures
Vlad Popa-Nita, Paul van der Schoot, Samo Kralj, 2006, izvirni znanstveni članek

Opis: The influence of a random-anisotropy (RA) type disorder on the phase separation of nematogen-colloid mixtures is studied theoretically by combiningthe 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.
Ključne besede: liquid crystals, transitions, segregation, mixing, random fields, solidification, complex fluids
Objavljeno: 07.06.2012; Ogledov: 855; Prenosov: 64
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