Evidence of Tricritical Behavior at the Nematic Isotropic Transition

Cotton-Mouton constants and pretransitional phenomena in the isotropic phase of liquid crystals

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.

On the Tricritical Point of the Isotropic — Nematic Transition in a Rod-Like Mesogen Hidden in the Negative Pressure Region

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)].

Phase equilibria in solutions of platelike particles: systems with steric and dispersive interactions between the platelets.

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.

The Discontinuity of the Isotropic — Masophase Transition in N-Cyanobiphenyls Homologous Series from 4CB to 14CB. Nonlinear Dielectric Effect (NDE) Studies

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.

Static Phenomena Near Critical Points: Theory and Experiment

Abstract: This paper compares theory and experiment for behavior very near critical points. The primary experimental results are the "critical indices" which describe singularities in various thermodynamic derivatives and correlation functions. These indices are tabulated and compared with theory. The basic theoretical ideas are introduced via the molecular field approach, which brings in the concept of an order parameter and suggests that there are close relations among different phase transition problems. Although this theory is qualitatively correct it is quantitatively wrong, it predicts the wrong values of the critical indices. Another theoretical approach, the "scaling law" concept, which predicts relations among these indices, is described. The experimental evidence for and against the scaling laws is assessed. It is suggested that the scaling laws provide a promising approach to understanding phenomena near the critical point, but that they are by no means proved or disproved by the existing experimental data.

Phenomenology of short-range-order effects in the isotropic phase of nematic materials

Abstract: The magnetic birefringence, the intensity and the width in frequency of the Rayleigh scattering, and the flow birefringence, are discussed in terms of a small number of phenomenological parameters. A qualitative discussion of the nuclear spin-lattice relaxation times is also given.

Landau theory of the nematic-isotropic phase transition

Abstract: A review is given of the wide variety of predictions that results from a Landau-type of description of the nematic-isotropic phase transition. This includes a discussion of the nature of the order parameter and of the various types of possible phases, of the influence of external fields, and of the effect of inclusion of spatial variations of the order parameter. The various predictions are compared with the available experimental results. It is concluded that there is still no clear picture about the nature of the singularity near the nematic-isotropic phase transition. Though the assumption of classical (mean-field) critical behaviour seems to be incorrect, there is no conclusive proof which alternative applies.

Monte Carlo Study of the Isotropic-Nematic Transition in a Fluid of Thin Hard Disks

Abstract: The first numerical determination of the thermodynamic isotropic-nematic transition in a simple three-dimensional model fluid, viz., a system of infinitely thin hard platelets, is reported. Thermodynamic properties were studied with use of the constant-pressure Monte Carlo method; Widom's particle-insertion method was used to measure the chemical potential. The phase diagram is found to differ considerably from predictions of a second-virial ("Onsager") theory. Virial coefficients up to the fifth were computed; b5 is found to be negative.