Nematic-Isotropic Phase Transition

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Study on the Magnetic Field Dependence of the Nematic-Isotropic Phase Transition of Liquid Crystals : A Monte Carlo Study by Employing the Wang- Warner Simple Cubic Lattice Model

Study on the Magnetic Field Dependence of the Nematic-Isotropic Phase Transition of Liquid Crystals : A Monte Carlo Study by Employing the Wang- Warner Simple Cubic Lattice Model

In this paper, we want to understand the effect of magnetic field on nematic-isotropic phase transition of liquid crystal. We use Metropolis Monte Carlo technique and Wang- Warner simple cubic lattice model to calculate and plot phase diagrams of energy per site, orientational order parameter, and specific heat as function of temperature in various magnetic fields. From the plots, we can unambiguously determine the change of phase transition.

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Seeding of the nematic isotropic phase transition by an electric field

Seeding of the nematic isotropic phase transition by an electric field

In this paper we use a relatively simple continuum model to investigate the effect of dielectric inhomogeneity on the phase behaviour of a liquid crystals subject to an applied voltage. In particular, we examine the possibility of using an applied voltage to control the relative thicknesses of N and I domains by exploiting the dielectric differences between the two phases. We note that there is an analogy [16] to be drawn here with the shear- induced banding induced in colloidal liquid crystalline systems [17]. In these experiments, the system separates into N and I bands rather than adopting a continuous stress field across the entire shear cell [18]. Unlike these colloidal systems, however, the N-I interface in a single-component molecular system is characterised by a very small amplitude density step. Thus, unusually, the significant dielectric discontinuity seen at this interface has a very small associated compositional change. Consequently, formation of this particular phase boundary does not require substantial material transport.
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Drag on particles in a nematic suspension by a moving nematic isotropic interface

Drag on particles in a nematic suspension by a moving nematic isotropic interface

The spatial period of the striped structure depended on the cooling rate, as well as on the particle size. Increas- ing the particle size, as well as decreasing the cooling rate, resulted in an increase of the spatial period. We also noticed that the stripes do not appear if we have considerably larger silica particles, R > 0.5µm. Also, de- creasing the cooling rate resulted in a chaotic merging of stripes and formation of a “root”-like pattern, Fig. 1d. These results indicate that the particles are pushed by the moving nematic-isotropic phase transition front.
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Picosecond polarised fluorescence studies of oxazine 4 motion and order in nematic and isotropic phases of 5-, 6- and 7-cyanobiphenyl

Picosecond polarised fluorescence studies of oxazine 4 motion and order in nematic and isotropic phases of 5-, 6- and 7-cyanobiphenyl

IULFWLRQ LH YLVFRVLW\ H[SHULHQFHG E\ DQG φ motions of the probe. Oxazine 4 is seen to correlate strongly with the order of the liquid crystal host. Under these circumstances, the rotational diffusion of the probe should exhibit a similar asymmetry to that of the nematic host in which the bulk viscosity is highly anisotropic 19 . A slowing of the molecular spinning rate with increasing temperature has been inferred from changes in Raman band shapes for the nematic phase of pure OET 20 . This behaviour was qualitatively attributed to a breakdown in local cylindrical symmetry in the vicinity of the nematic-isotropic phase transition. The breakdown in cylindrical symmetry giving rise to so-called “biaxial fluctuations” due to the uncorrelated motion of different segments (tails) of individual nematogens whose interference acts to increase the friction (viscosity) experienced for φ diffusion. Evidence for an increase in viscosity in the approach to T NI is provided by recent measurements of translational (mass) diffusion in pure
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Mathematical descriptions of nematic polymers in the monolayer limit

Mathematical descriptions of nematic polymers in the monolayer limit

Amid all of these results, one of the classical issues of anisotropic molecular liquids, dating back to Onsager and de Gennes, is what survives from orientational degeneracy of the quiescent nematic phase just as an external field perturbation is turned on. This issue was bypassed in the continuum liquid crystal theory of Leslie and Ericksen, which did not involve an excluded-volume potential. The Leslie material parameters produced either a shear flow-aligning or a tumbling liquid crystalline fluid, with no freedom to transition between them, by varying experimental conditions such as volume fraction of the nematic polymer solvent mixture or the shear rate. Furthermore, there was no dynamics without an external field (hydrodynamic, electric or magnetic). For liquid crystalline polymers, however, the experimental evidence was mounting in the 1980’s for not only an equilibrium 1 st -order isotropic-nematic phase transition, but a shear rate-
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Transient and stationary flow behaviour of side chain liquid-crystalline polymers: Evidence of a shear-induced isotropic-to-nematic phase transition

Transient and stationary flow behaviour of side chain liquid-crystalline polymers: Evidence of a shear-induced isotropic-to-nematic phase transition

Introduction. – Shear flow dramatically influences the microstructural order in com- plex fluids, and can sometimes induce phase transitions. Examples include the shear-induced isotropic-to-nematic (IN) phase transition in liquid-crystalline systems [1, 2] and in colloidal suspensions [3]. The possibility of inducing an isotropic-to-nematic (IN) transition by shear flow is appealing, and has been reported experimentally in semiflexible wormlike micelles [2]. Although this system is well studied, polymeric-like entanglements, the annealed length dis- tribution, and solvent greatly complicate the physics. Mather et al. [4] have recently studied a linear thermotropic polymer and shown that it can undergo a flow-induced IN transition. A similar transition was then discovered in side chain liquid-crystalline polymers (SCLCPs) [5], based on birefringence measurements. In one SCLCP system, Small-Angle Neutron Scattering (SANS) demonstrated that, while the mesogenic side group is perpendicular to the polymeric backbone in the equilibrium nematic phase, flow induces parallel alignment, with backbone and mesogen distribution functions prolate and oriented generally in the flow direction [5]. The aim of this paper is to continue this work with a study of the non-linear rheology of this system.
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Phase separation effects and the nematic-isotropic transition in polymer and low molecular weight liquid crystals doped with nanoparticles

Phase separation effects and the nematic-isotropic transition in polymer and low molecular weight liquid crystals doped with nanoparticles

It should be noted that the molecular theory developed in this paper yields a number of results which are in contrast to common expectations and assumptions. Firstly, the phase separation does not necessarily occur at all concentrations of NPs, but typically the coexisting nematic and isotropic phases are stable only above a certain threshold value of the NP volume fraction which depends on the parameters of the system (see the phase diagrams in Fig. 1 and 2). There is no phase separa- tion at very low volume fractions. Moreover, in the case illus- trated by Fig. 2, the phase separation takes place only within a  nite range of NP concentration, i.e. it disappears when the volume fraction of NPs exceeds the second threshold value. Secondly, the phase separation in composites does not occur only in a narrow temperature interval (as in conventional nematic mixtures) but may be present over a broad temperature range, even as broad as the nematic phase itself. Finally, in some cases (e.g. for f > 0.02 in Fig. 2) the system may rst undergo a transition into the homogeneous nematic phase and then separate into the nematic and the isotropic phase with different concentrations of NPs at some temperature below the N – I transition point.
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A NOVEL MESOMORPHIC HOMOLOGOUS SERIES OF ESTER DERIVATIVES P (P’ N ALKOXY BENZOYLOXY) N HEPTYL BENZOATES

A NOVEL MESOMORPHIC HOMOLOGOUS SERIES OF ESTER DERIVATIVES P (P’ N ALKOXY BENZOYLOXY) N HEPTYL BENZOATES

p-Hydroxy n-Heptyl Benzoate is nonmesomorphic substance but on linking it with dimerized p-n-alkoxy Benzoic acids through p-n-alkoxy acid chlorides give rise to corresponding p-(p’- n-alkoxy benzoyloxy) n-Heptyl Benzoate esters with spatial configuration. Six, out of elevan homologues are mesomorphic and remaining five viz; methyl to n-Pentyl homologues is nonmesomorphic. i.e. n-octyl, n-decyl, n-dodecyl, n-tetradecyl and n-hexadecyl homologues are not smectogenic even in the monotropic condition. All the six mesomorphic homologues are only and entirely nematogenic in enantiotropic manner. Transition temperatures are plotted versus the number of carbon atoms in n-alkyl chain of left n-alkoxy terminal end group. Like points are joined to obtain solid-isotropic or solid-nematogenic transition curve and nematic-isotropic transition curve. The resulted phase diagram obtained is shown in the figure-I from table -1. Solid -nematic/isotropic transition curve rise from methyl to ethyl and Falls from n-propyl to n-hexyl homologue and then rises to n- octyl by ten degree and again falls up to decyl homologue. Finally it falls by ten to two degrees to hexadecyl homologue through tetradecyl homologue. Thus, it follows zigzag path of rising and falling as series is ascended in normal manner. Nematic-isotropic transition curve follows rise tendency from n- hexyl to n-octyl homologue in normal manner, then it passes through minima [14 to 21] at decyl homologue and falls abnormally from decyl to hexadecyl homologue as series is ascended. Odd-even effect is not observed for nematic isotropic transition curve. Mesomorphic properties like average thermal stability, commencement of mesophase, mesophase length, degree of mesomorphism etc. are compared with structurally similar homologous series. p-n- Alkoxy benzoic acids are dimeric, in which two phenyl rings are bonded through hydrogen bonding. Hydrogen bonding breaks on esterification and resulted esters consist of two phenyl rings bridged through carboxyl -COO- unit with right terminal end group –COOC 7 H 15 (n) .
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Statistical Parameters and Phase Transition Temperatures of Nematic Liquid Crystals Using Image Analysis Technique

Statistical Parameters and Phase Transition Temperatures of Nematic Liquid Crystals Using Image Analysis Technique

Liquid crystal 5CB was supplied by Frinton Laboratories, Inc., USA. The experiment involved a Meopta polarising optical microscope attached with a hot stage [7] and a Canon high-resolution colour camera. A commercially available glass slide is used for sample preparation. The slide is of 75 mm length and 25 mm width, a thickness of 1.45 mm, is ground on all sides, cleaned with acetone and is dried at 40◦C. This glass slide is simply rubbed in order to make unidirectional alignment in one direction. A small amount of liquid crystalline compound under investigation is placed on the glass slide, which is heated to become isotropic. A small cover slip of thickness of 0.16 mm is placed above the sample and allowed to cool to achieve homogeneous and uniform distribution of the compound. The colour image recorded by the camera has a resolution of 1936 × 1288 pixels, which represents the 24bit true colour pixel tone that ranges from intensity values 0 to 255 in Red, Green and Blue colours. The translated greyscale image is analysed using MATLAB software [8,9]. Theory related to the computation of statistical parameters from the textures of liquid crystals is explained briefly in [10-14].
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Structure, Photochromism and Liquid Crystal  Properties of 1 Alkyl 2 (Arylazo)Imidazoles  (Raai CnH2n+1, n (Even) = 10   22)

Structure, Photochromism and Liquid Crystal Properties of 1 Alkyl 2 (Arylazo)Imidazoles (Raai CnH2n+1, n (Even) = 10 22)

at 58˚C (5a) and 65˚C (7a) respectively. The crystal and nematic phases are under thermal equilibrium. During cooling from isotropic phase, 5a shows strongly fluctu- ating textures of the N phase and has been transformed to textures in which “Schlieren” regions can be seen (Fig- ure 7). Homeotropic and homogenous oriented domains cause this coexistence, which permits the assignment of the smectic phase in compound 7a. The transition tem- perature and associated enthalpies are Cr → N 37.4˚C (15.9 KJ·mol −1 ) → Iso 58˚C (0.5 KJ·mol −1 /heating cycle)
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Chirality of an achiral bent core nematic mesogen observed in planar and homeotropic cells under certain boundary conditions

Chirality of an achiral bent core nematic mesogen observed in planar and homeotropic cells under certain boundary conditions

domains are observed while the sample is cooled from the isotropic to the nematic phase. In a part of the cell with a smaller cell gap, we may get homochiral domains that can be stabilized over a shorter period of time. Meanwhile in a thicker part of the cell, it takes a considerable time for the domains to get stabilized. Besides, as the energy difference between the domains of oppo- site handedness is small, it is extremely difficult to distinguish one domain from the other with opposite handedness by rotation of an analyzer using the method of color comparisons. The color change brought about by a rotation of the analyzer is too insensitive to confirm the handedness. A 2 p twisted cell was made with a cell gap of 8.2 m m. Fig. 7(a) shows a few domains immediately after the transition to the nematic phase. With the passage of time, the two domains merge into a single one as seen in Fig. 7(b). Two of the domains have opposite handedness. This is confirmed by decrossing the polarizers in the opposite direction as shown in Fig. 7(c) and (d). If the twist of the directors were to be induced by a chiral impurity, the domain with one chirality can only be observed on the transition to the nematic phase. Therefore the presence of the domain with opposite handedness is incompatible with an explanation of a sample contamination. Recently, Kim et al. reported a nematic phase (N * T ) 25 to appear
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Self-assembling, macroscopically oriented, polymer filaments; a doubly nematic organogel

Self-assembling, macroscopically oriented, polymer filaments; a doubly nematic organogel

The behaviour of peaks A and B on heating at ! 2℃ ! ��� ! is shown in Fig. 7 (bottom). Both disappear at the I+F to I phase transition, with little change in position of either as a function of temperature. A small, but significant change in the periodicity of A is observed on increasing temperature, varying from !! 43 ! �� at room temperature to !! 30 ! �� just below the transition to the isotropic phase. The periodicity of B remains approximately constant throughout ( !! 17 ! �� at room temperature and !!14! �� at the phase transition). Interestingly, a periodicity of !!!! �� is close to the molecular length calculated for one conformer of Compound 1 and is identical to the !!!! �� ! periodicity measured at low temperatures in the DC phase of the material. 9, 15, 16 The diffuse nature of peak A is also reminiscent of the scattering characteristic of the DC phase. The periodicity of peak B, ~2.1nm, is much closer to that found for 5CB !!!!!! �� !, but not sufficiently close to offer a robust explanation for it. 17 Despite the fact that the phase transition from N+F to I+F around ! 38℃ is clearly observable via polarizing microscopy and DSC, there is no change in the SAXS pattern until the mixture becomes completely isotropic.
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Self-assembling, macroscopically oriented, polymer filaments; a doubly nematic organogel

Self-assembling, macroscopically oriented, polymer filaments; a doubly nematic organogel

We first consider what structures could be associated with these periodicities. They are are rather difficult to understand in detail, but we note the following. Peaks Aʹ !!! 85 ! �� ! and A’’ ( !! 54 ! �� ) are quite close to the values determined in the nematic phase of the pure bent-core compound ( !!!!! �� ) and therefore must originate from the BC material. However, the occurrence of two distinct peaks is a surprise. Fig. 8c shows the temperature dependence of the periodicity of the peaks Aʹ, Aʹʹ, and B on cooling from the isotropic phase; the experiment was performed immediately following the heating cycle to just above the I+F to I transition. Although a hint of peak Aʹʹ comes into existence at the same temperature as others (just into the I+F phase), the scattering is initially far too weak and diffuse to fully analyse and it only becomes distinct a few degrees into the N+F phase at 41! 9℃ and fully defined at 36 ! 9℃ . Peak B cannot readily be associated either with 5CB or with the BC compound alone. The fact that the peaks are observable at all temperatures in the I+F phase region, under circumstances where there is no optical evidence of the fibres, suggests that
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Self assembling, macroscopically oriented, polymer filaments: a doubly nematic organogel

Self assembling, macroscopically oriented, polymer filaments: a doubly nematic organogel

We first consider what structures could be associated with these periodicities. They are are rather difficult to understand in detail, but we note the following. Peaks A′ (3.85 𝑛𝑚) and A’’ (4.54 𝑛𝑚) are quite close to the values determined in the nematic phase of the pure bent-core compound (~4.3 𝑛𝑚) and therefore must originate from the BC material. However, the occurrence of two distinct peaks is a surprise. Fig. 8c shows the temperature dependence of the periodicity of the peaks A′, A′′, and B on cooling from the isotropic phase; the experiment was performed immediately following the heating cycle to just above the I+F to I transition. Although a hint of peak A′′ comes into existence at the same temperature as others (just into the I+F phase), the scattering is initially far too weak and diffuse to fully analyse and it only becomes distinct a few degrees into the N+F phase at 41.9℃ and fully defined at 36.9℃. Peak B cannot readily be associated either with 5CB or with the BC compound alone. The fact that the peaks are observable at all temperatures in the I+F phase region, under circumstances where there is no optical evidence of the fibres, suggests that the polymer-like structure that is retained to temperatures much higher than the I+F transition begins to
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Dual gauge field theory of quantum liquid crystals in three dimensions

Dual gauge field theory of quantum liquid crystals in three dimensions

topological melting ideas to the quantum realms, initially in two space dimensions. The emphasis has been here all along on the fundamental, theoretical side based on the symmetries and associated defects. The main restriction is that it only deals with matter formed from bosons: the constructions rest on the machinery of statistical physics being mobilized in the D + 1-dimensional Euclidean space-time, turning into the quantum physics of bosons after Wick rotation. This matter lives in the Galilean continuum and the point of departure is the spontaneous breaking of space translations and rotations into a crystal. The KTNHY transition is just one particular example of a Kramers-Wannier (or weak-strong) duality and it was found out in the 1980s how to extend this to three dimensions when dealing with Abelian symmetries. In the context of crystalline elasticity one can rest on strain-stress duality, where phonon degrees of freedom are mapped to dual stress gauge fields. This amounts to a generalization of the famous vortex-boson or Abelian-Higgs duality, as pioneered by Kleinert [30]. Using the well-known mapping of a D-dimensional quantum to a D + 1-dimensional classical system, the 2 + 1D quantum liquid crystals were investigated by strain-stress duality starting with Ref. [31]. The procedure is essentially the same as the KTNHY case in two dimensions. One first establishes the structure of the weak-strong duality by focusing on the minimal U(1) case associated with vortex melting, to then extend it to the richer theater of the space groups underlying the crystalline symmetry breaking, profiting from the fact that the restoration of translational invariance by dislocations is associated with an Abelian symmetry.
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Lattice-Boltzmann simulation of free nematic-isotropic interfaces

Lattice-Boltzmann simulation of free nematic-isotropic interfaces

We examined the applicability of the hybrid method of lattice Boltzmann and finite di ff er- ences for the simulation of nematic-isotropic interfaces. The shape and width of the interface is correctly reproduced and the free interface is static at the coexistence temperature. At tem- peratures slightly different from the coexistence, the interface moves with constant velocity that depends on the temperature. The interface exists only for a certain range of temperatures: for temperatures much lower or higher than the coexistence, one of the two phases becomes unstable and the interface disappears without propagation. The circular interfaces are not static at the coexistence temperature. Thus, we stabilize them by a temperature shift in a way that the interface velocity is comparable to the spurious velocities. This allows us to visualize the profile of theses spurious currents and to estimate their magnitude.
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Synthesis , Characterization and Theortical Study For
Azo mesogenic compounds

Synthesis , Characterization and Theortical Study For Azo mesogenic compounds

structure were were characterized by 1 H-NMR and IR as well as elemental analysis . The mesomorphic properties of these compounds were investigated via differential scanning calometry and polarizing microscopy . . The thermal data indicate that all of these compounds exhibit mesomorphic properties (Nematic phase).The group efficiency order for the Nematic phase thermal stability can be derived for the compounds as : NO 2 >OCH 3 >CH 3 >Cl, With

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Lattice Boltzmann method for Q tensor nemato dynamics in liquid crystal display devices

Lattice Boltzmann method for Q tensor nemato dynamics in liquid crystal display devices

The temperature behaviour of the order parameter is shown in figure 2.2 as predicted in equation 2.18, it gives the equilibrium order parameter value of an undistorted nematic for a given set of Landau deGennes coefficients, the disconti­ nuity clearly being visible. It should be noted in this approach that S is unbounded for lower and lower temperatures and from the initial assumptions the expansion should really be applied for small S. However from figure 2.2 we see the behaviour of the order parameter in the nematic region is not unreasonable. In this thesis we will work mainly well into the nematic phase (S ~ 0.6) applying this theory without reservation. We also point out that expansions in term of Qap are unbounded in that there exist an infinite number of orders of Qap but to make practical use of the Landau deGennes theory we only keep terms that allow a one to one mapping of the current experimentally used parameters which are based upon an ha expansion that is bounded due to its unit vector property.
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Evolution of the low temperature Fermi surface of superconducting FeSe1−xSx across a nematic phase transition

Evolution of the low temperature Fermi surface of superconducting FeSe1−xSx across a nematic phase transition

a prominent low frequency oscillation associated with a small, but highly mobile band, which disappears at the nematic phase boundary near x ~ 0.17, indicative of a topological Lifshitz transition. The quasiparticle masses are larger inside the nematic phase, indicative of a strongly correlated state, but they become suppressed outside it. The experimentally observed changes in the Fermi surface topology, together with the varying degree of electronic correlations, will change the balance of electronic interactions in the multi-band system FeSe 1 − x S x and promote different k z -dependent superconducting pairing channels inside and outside the
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