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1.
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, original scientific article

Abstract: 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.
Keywords: liquid crystals, transitions, segregation, mixing, random fields, solidification, complex fluids
Published in DKUM: 07.06.2012; Views: 1164; Downloads: 82
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2.
The influence of nanoparticles on the phase and structural ordering for nematic liquid crystals
Samo Kralj, Zlatko Bradač, Vlad Popa-Nita, 2008, original scientific article

Abstract: We study the influence of nanoparticles (NPs) on liquid crystal (LC) ordering. As regards the structural ordering we consider NPs as a source of a quenched random field. Roughly such a situation is encountered in mixtures of LCs and aerosil NPs (aerosil NPs are spherular ones). Using the semi-microscopic lattice model and Brownian molecular simulation we show that after a quench from the isotropic phase a quasi-stable domain pattern forms. The characteristic size of an average domain is inversely proportional to the concentration of NPs, and domain patterns exhibit memory effects. In the study of the phase behaviour we limit consideration to NPs resembling LC molecules. A Landau-type free energy expression is derived for the mixture, originating from the Maier-Saupe molecular approach. We show that the resulting phase behaviour exhibits the slave-master behaviour as the temperature or pressure is varied.
Published in DKUM: 07.06.2012; Views: 1567; Downloads: 111
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3.
Alignment of carbon nanotubes in nematic liquid crystals
Paul van der Schoot, Vlad Popa-Nita, Samo Kralj, 2008, original scientific article

Abstract: 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.
Keywords: liquid crystals, nematic crystals, molecular dynamics, stability, elasticity, carbon nanotubes
Published in DKUM: 07.06.2012; Views: 1603; Downloads: 78
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4.
The influence of disorder on thermotropic nematic liquid crystals phase behavior
Vlad Popa-Nita, Ivan Gerlič, Samo Kralj, 2009, review article

Abstract: 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.
Keywords: liquid crystals, random fields, phase transition, disorder, nematic structures
Published in DKUM: 07.06.2012; Views: 1892; Downloads: 303
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5.
Liquid crystal-carbon nanotubes mixtures
Vlad Popa-Nita, Samo Kralj, 2010, original scientific article

Abstract: The self-organizing properties of nematic liquid crystals (LCs) can be used to align carbon nanotubes (CNTs) dispersed in them. In the previous paper [P. van der Schoot, V. Popa-Nita, and S. Kralj, J. Phys. Chem. B 112, 4512 (2008)], we have considered the weak anchoring limit of the nematic LC molecules at the nanotube's surface, where the CNT alignment is caused by the anisotropic interfacial tension of the nanotubes in the nematic host fluid. In this paper, we present the theoretical results obtained for strong enough anchoring at the CNT-LC interface for which the nematic ordering around nanotube is apparently distorted. Consequently, relatively strong long-range and anisotropic interactions can emerge within the system. In order to get insight into the impact of LC ordering on the alignment of nanotubes we treat the two mixture components on the same footing and combine Landau-de Gennes free energy for the thermotropic ordering of the liquid crystal and Doi free energy for lyotropic nematic ordering of carbon nanotubes caused by their mutually excluded volume. The phase ordering of the binary mixture is analyzedas a function of the volume fraction of the carbon nanotubes, the strength of coupling, and the temperature. We find that the degree of ordering of the nanorods can be tuned by raising or lowering the temperature orby increasing or decreasing their concentration.
Published in DKUM: 07.06.2012; Views: 1858; Downloads: 84
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6.
Liquid crystal - anisotropic nanoparticles mixtures
Vlad Popa-Nita, Matej Cvetko, Samo Kralj, 2011, independent scientific component part or a chapter in a monograph

Keywords: liquid crystals, nano particles, phase behavior
Published in DKUM: 07.06.2012; Views: 1427; Downloads: 94
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