Evidence of Tricritical Behavior at the Nematic Isotropic Transition
20 Jun 1998-International Journal of Modern Physics B (World Scientific Publishing Company)-Vol. 12, Iss: 15, pp 1585-1599
TL;DR: In this article, the authors derived the value of the ratio (Q*-QNI)/QNI, where Q* is the order parameter at the temperature T* of the absolute stability limit of the isotropic phase and QNI is the condition at the nematic-isotropic transition temperature TNI.
Abstract: Tricritical behavior at the nematic–isotropic phase transition is discussed by means of Landau–de Gennes phenomenological theory. The temperature difference TNI-T* is obtained near the tricritical point. We also obtained the value of the ratio (Q*-QNI)/QNI, where Q* is the order parameter at the temperature T* of the absolute stability limit of the isotropic phase and QNI is the order parameter at the nematic–isotropic transition temperature TNI. As a definite improvement over the earlier attempts utilizing Landau–de Gennes scheme the present work can reproduce low values of (Q*-QNI)/QNI and that of TNI-T* as demanded by experimental observations. The tricritical exponents are calculated from the scaling theory and compared with observed values. Using the Ginzburg criterion, we also determine the critical region of the nematic–isotropic phase transition.
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TL;DR: The Cotton-Mouton constants for three homologous series of liquid crystals were measured in the isotropic phase as mentioned in this paper, and the results were compared with theories on pretransitional phenomena.
Abstract: The Cotton-Mouton constants for three homologous series of liquid crystals were measured in the isotropic phase. Some of these measurements were performed with high temperature resolution. The results are compared with theories on pretransitional phenomena. The Landau-de Gennes theory proves to be inadequate to describe the data.
11 citations
01 Jan 2002
TL;DR: In this article, a presentation of experimental results suggesting fluidlike, spinodal and tricritical nature of the isotropic-nematic transition are given. Results presented based on broad-band dielectric spectroscopy and strong electric field induced changes of dielectrics permittivity.
Abstract: A presentation of experimental results suggesting fluidlike, spinodal and tricritical nature of the isotropic — nematic transition are given. Results presented based on broad-band dielectric spectroscopy and strong electric field induced changes of dielectric permittivity. Results above as well the obtained systematic changes of the temperature discontinuity of the isotropic — nematic transition suggest the possible existence of a tricritical point which may be experimentally reached due the application of negative pressures. The presented picture for the I-N transition correlates with the recent general model for liquid-liquid transition proposed by Tanaka [Phys. Rev. E68, 6968 (2000)].
7 citations
TL;DR: Numerical calculations show that besides the isotropic- nematic biphasic coexistence range, inclusion of attractive forces resulted in the appearance of nematic-nematic coexistence in both, disks and rods, solutions.
Abstract: Our statistical thermodynamics model of solution of stiff, platelike, biaxial particles interacting solely via repulsion on contact (athermal limit) [Phys. Rev. E 62, 5011 (2000)] is extended to incorporate dispersion interactions between the particles. Dispersion forces between anisotropic particles are accounted for using the Imura-Okano approach. Numerical calculations specialized to solutions of either rods or disks show that besides the isotropic-nematic biphasic coexistence range, inclusion of attractive forces resulted in the appearance of nematic-nematic coexistence in both, disks and rods, solutions. The critical divergence of the difference between the order parameters and concentrations of the two nematics is observed while approaching the critical temperature. The minimum aspect ratio of rods or disks for the formation of the nematic phase is also discussed.
2 citations
01 Jan 2004
TL;DR: In this article, the static nonlinear dielectric effect (NDE) in homologous series of 4-cyano-4-n-alkylbiphenyls (nCB, from n=3 to n=14), exhibiting both isotropic - nematic (I-N) and isotropicsmectic A (I -SmA) phase transitions, are presented.
Abstract: Results of studies of the static nonlinear dielectric effect (NDE) in the homologous series of 4-cyano-4-n-alkylbiphenyls (nCB, from n=3 to n=14), exhibiting both isotropic — nematic (I-N) and isotropicsmectic A (I-SmA) phase transitions, are presented. Tests were conducted in the isotropic phase where the reciprocal of NDE shows a linear dependence vs. temperature or pressure, also in the immediate vicinity of the clearing point. It was found that the discontinuity (ΔT) of the isotropic — mesophase transition increases with the length of the alkyl-chain in the nCB molecule, from 0.7 K (4CB) to ca. 11K (14CB). Regarding the influence of pressure, it continuously increases the value of ΔL(P) for the case of the I-N transitions, as shown for 5CB up to P=275MPa. For the the I-SmA transition discontinuity first drops down to ΔT(P) min≈(1/2)[ΔT(0.1MPa)] and next strongly increases with pressure rising up to P=375 MPa.
2 citations
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TL;DR: In this paper, it is argued on the basis of a reexamination of the Landau-de Gennes theory and with reference to experimental data that the isotropic-nematic transition exhibits pretransitional fluctuations characteristics of a tricritical point.
71 citations
60 citations
TL;DR: In this article, a broad set of phenomena in which the coupling of several order parameters is essential is examined from a unitary standpoint on the basis of the fluctuational theory of phase transitions.
Abstract: A broad set of phenomena in which the coupling of several order parameters is essential is examined from a unitary standpoint on the basis of the fluctuational theory of phase transitions. The coupling of an order parameter with other degrees of freedom can lead to the appearance on the phase transition lines of singular (polycritical) points, near which the character of the anomalies changes. We analyze numerous experimental studies near polycritical points, whose results as a whole confirm the conclusions of fluctuational theory. The studied objects include 3He–4He solutions, antiferromagnetics, ammonium halides, liquid crystals, and mercury near the liquid-gas critical point. The main result of studying systems with coupled order parameters is a substantial expansion of the possibility of a universal description of real phase transitions, including those that have been objects of more or less separate fields of physics, owing to their complexity.
40 citations
TL;DR: A steric, mean field, three-dimensional, simple cubic lattice model using DiMarzio lattice statistics for molecules composed of rigid cores and completely flexible pendant tails is presented in this paper.
Abstract: A steric, mean field, three‐dimensional, simple cubic lattice model using DiMarzio lattice statistics for molecules composed of rigid cores and completely flexible pendant tails is presented. The calculated trends and absolute values of the following properties at the nematic–isotropic transition for cores of length‐to‐breadth ratios of 5 and 10 with tails of varying length are discussed and compared with relevant experimental data: temperature, core order parameter, intermolecular tail order parameter, densities, relative density change, and entropy change. The results indicate that although completely flexible tails in this model are too flexible compared with hydrocarbon tails in real liquid crystals, tail flexibility of some degree should be important in the nematic–isotropic transition.
39 citations
TL;DR: In this article, the authors proved that the variations with temperature of the order parameter Q are well described by the theory based on the Landau expansion of the free energy F. The coefficients of the expansion of F are determined and the variations of F with Q are given for two nematic liquid crystals.
Abstract: The Landau approximation is known to give a good description of the pretransitional phenomena in the isotropic phase of nematic liquid crystals. We prove that, in the nematic phase too, just below the nematic-isotropic transition, the variations with temperature of the order parameter Q are well described by the theory based on the Landau expansion of the free energy F. The coefficients of the expansion of F are determined and the variations of F with Q are given for two nematic liquid crystals. The calculated values of Tc - T∗ and of the latent heats are in good agreement with the values obtained by independent way.
33 citations