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Isotropic-nematic transition in liquid crystals confined between rough walls  [PDF]
David L Cheung,Friederike Schmid
Physics , 2005, DOI: 10.1016/j.cplett.2005.11.010
Abstract: The effect of rough walls on the phase behaviour of a confined liquid crystal (LC) fluid is studied using constant pressure Monte Carlo simulations. The LC is modelled as a fluid of soft ellipsoidal molecules and the rough walls are represented as a hard wall with a number of molecules randomly embedded in them. It is found that the isotropic-nematic (IN) transition is shifted to higher pressures for rougher walls.
Isotropic-Nematic transition of long thin hard spherocylinders confined in a quasi-two-dimensional planar geometry  [PDF]
Marco Cosentino Lagomarsino,Marjolein Dijkstra,Marileen Dogterom
Physics , 2003, DOI: 10.1063/1.1588994
Abstract: We present computer simulations of long thin hard spherocylinders in a narrow planar slit. We observe a transition from the isotropic to a nematic phase with quasi-long-range orientational order upon increasing the density. This phase transition is intrinsically two dimensional and of the Kosterlitz-Thouless type. The effective two-dimensional density at which this transition occurs increases with plate separation. We qualitatively compare some of our results with experiments where microtubules are confined in a thin slit, which gave the original inspiration for this work.
Multi-Particle Collision Dynamics Algorithm for Nematic Fluids  [PDF]
Tyler N. Shendruk,Julia M. Yeomans
Physics , 2015,
Abstract: Research on transport, self-assembly and defect dynamics within confined, flowing liquid crystals requires versatile and computationally efficient mesoscopic algorithms to account for fluctuating nematohydrodynamic interactions. We present a multi-particle collision dynamics (MPCD) based algorithm to simulate liquid-crystal hydrodynamic and director fields in two and three dimensions. The nematic-MPCD method is shown to successfully reproduce the features of a nematic liquid crystal, including a nematic-isotropic phase transition with hysteresis in 3D, defect dynamics, isotropic Frank elastic coefficients, tumbling and shear alignment regimes and boundary condition dependent order parameter fields.
Diffusivity Maximum in a Reentrant Nematic Phase  [PDF]
Tillmann Stieger,Marco G. Mazza,Martin Schoen
International Journal of Molecular Sciences , 2012, DOI: 10.3390/ijms13067854
Abstract: We report molecular dynamics simulations of confined liquid crystals using the Gay–Berne–Kihara model. Upon isobaric cooling, the standard sequence of isotropic–nematic–smectic A phase transitions is found. Upon further cooling a reentrant nematic phase occurs. We investigate the temperature dependence of the self-diffusion coefficient of the fluid in the nematic, smectic and reentrant nematic phases. We find a maximum in diffusivity upon isobaric cooling. Diffusion increases dramatically in the reentrant phase due to the high orientational molecular order. As the temperature is lowered, the diffusion coefficient follows an Arrhenius behavior. The activation energy of the reentrant phase is found in reasonable agreement with the reported experimental data. We discuss how repulsive interactions may be the underlying mechanism that could explain the occurrence of reentrant nematic behavior for polar and non-polar molecules.
Nonequilibrium nematic-isotropic interface
Mesquita, Oscar Nassif de;
Brazilian Journal of Physics , 1998, DOI: 10.1590/S0103-97331998000400002
Abstract: liquid crystals have been very fruitful systems to study equilibrium phase transitions. recently, they have become an important system to study dynamics of first-order phase transitions. the moving nonequilibrium nematic-isotropic interface is a model system to study growth of stable states into metastable states and displays a myriad of dynamical instabilities that, far from equilibrium, drive the system to a scenario of spatio-temporal chaos. we present a mean-field theory for the time evolution of a planar nonequilibrium nematic-isotropic interface for pure liquid crystals using a time dependent ginzburg-landau equation, which is one of the simplest approaches to dissipative dynamics. we obtain a theoretical expression for the growth kinetics of the nematic phase into a metastable isotropic phase and compare it with our experimental results. in a directional solidification arrangement we study instabilities of the nematic-isotropic interface of the liquid crystal 8cb doped with water and hexachloroethane. the observed instabilities are similar to cellular instabilities that appear during growth of crystal-melt interfaces of binary mixtures. we then compare our results with known theories of morphological instabilities during crystal growth.
Nonequilibrium nematic-isotropic interface  [cached]
Mesquita Oscar Nassif de
Brazilian Journal of Physics , 1998,
Abstract: Liquid crystals have been very fruitful systems to study equilibrium phase transitions. Recently, they have become an important system to study dynamics of first-order phase transitions. The moving nonequilibrium nematic-isotropic interface is a model system to study growth of stable states into metastable states and displays a myriad of dynamical instabilities that, far from equilibrium, drive the system to a scenario of spatio-temporal chaos. We present a mean-field theory for the time evolution of a planar nonequilibrium nematic-isotropic interface for pure liquid crystals using a time dependent Ginzburg-Landau equation, which is one of the simplest approaches to dissipative dynamics. We obtain a theoretical expression for the growth kinetics of the nematic phase into a metastable isotropic phase and compare it with our experimental results. In a directional solidification arrangement we study instabilities of the nematic-isotropic interface of the liquid crystal 8CB doped with water and hexachloroethane. The observed instabilities are similar to cellular instabilities that appear during growth of crystal-melt interfaces of binary mixtures. We then compare our results with known theories of morphological instabilities during crystal growth.
Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels  [PDF]
Andriy V. Kityk,Matthias Wolff,Klaus Knorr,Denis Morineau,Ronan Lefort,Patrick Huber
Physics , 2008, DOI: 10.1103/PhysRevLett.101.187801
Abstract: The optical birefringence of rod-like nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 micrometer length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.
Topological Defects in Nematic Droplets of Hard Spherocylinders  [PDF]
J. Dzubiella,M. Schmidt,H. Loewen
Physics , 1999, DOI: 10.1103/PhysRevE.62.5081
Abstract: Using computer simulations we investigate the microscopic structure of the singular director field within a nematic droplet. As a theoretical model for nematic liquid crystals we take hard spherocylinders. To induce an overall topological charge, the particles are either confined to a two-dimensional circular cavity with homeotropic boundary or to the surface of a three-dimensional sphere. Both systems exhibit half-integer topological point defects. The isotropic defect core has a radius of the order of one particle length and is surrounded by free-standing density oscillations. The effective interaction between two defects is investigated. All results should be experimentally observable in thin sheets of colloidal liquid crystals.
Density functional theory study of the nematic-isotropic transition in an hybrid cell  [PDF]
I. Rodriguez-Ponce,J. M. Romero-Enrique,L. F. Rull
Physics , 2004, DOI: 10.1063/1.1829041
Abstract: We have employed the Density Functional Theory formalism to investigate the nematic-isotropic capillary transitions of a nematogen confined by walls that favor antagonist orientations to the liquid crystal molecules (hybrid cell). We analyse the behavior of the capillary transition as a function of the fluid-substrate interactions and the pore width. In addition to the usual capillary transition between isotropic-like to nematic-like states, we find that this transition can be suppressed when one substrate is wet by the isotropic phase and the other by the nematic phase. Under this condition the system presents interface-like states which allow to continuously transform the nematic-like phase to the isotropic-like phase without undergoing a phase transition. Two different mechanisms for the disappearance of the capillary transition are identified. When the director of the nematic-like state is homogeneously planar-anchored with respect to the substrates, the capillary transition ends up in a critical point. This scenario is analogous to the observed in Ising models when confined in slit pores with opposing surface fields which have critical wetting transitions. When the nematic-like state has a linearly distorted director field, the capillary transition continuously transforms in a transition between two nematic-like states.
Defect Core Structures in Twisted Nematic and Twisted Chiral Liquid Crystals  [PDF]
Xuan Zhou, Guili Zheng, Zhidong Zhang
Journal of Modern Physics (JMP) , 2013, DOI: 10.4236/jmp.2013.42037
Abstract:

The defect structures of s = ±1/2 twist disclinations in twisted nematic and twisted chiral liquid crystals have been investigated within the Landau-de Gennes theory numerically. Our results show that there exists eigenvalue exchange across the defect core of both the two models. The defect core is essentially biaxial and never isotropic. The defect centre is uniaxial and is surrounded by a strong biaxial region.

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