Landau-de Gennes model with the inclusion of density change for the nematic-isotropic phase transition
10 Jul 1996-Modern Physics Letters B (World Scientific Publishing Company)-Vol. 10, Iss: 16, pp 777-778
TL;DR: In this paper, the authors examined the Landau-de-Gennes model of the nematic-isotropic phase transition with the inclusion of the density change and a higher order term (sixth-order term) in a simple way.
Abstract: The Landau-de Gennes model of the nematic-isotropic phase transition with the inclusion of the density change and a higher order term (sixth-order term) in a simple way is examined. The temperature difference TNI−T* obtained is in good agreement with the experimental one. We also obtained the value of the ratio (Q*–QNI)/QNI. The value of (Q*–QNI)/QNI is not improved over the earlier attempt, but there is a definite improvement in the value of TNI–T* which is obtained as 1.045 K.
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TL;DR: A comprehensive overview of phase transition studies can be found in this article, where the authors identify the essential key concepts and points of difficulty associated with the study of phase transitions and discuss the most widely used experimental techniques for measuring these transition properties.
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TL;DR: In this article, the authors present experimental and theoretical results for the critical behavior in the isotropic phase of nematogens and discuss the possibility of tricritical behavior at the nematic-isotropic transition.
Abstract: The low value of , where is the nematic-isotropic phase transition temperature and denotes the virtual transition temperature, is a long-standing puzzle in the physics of liquid crystals. The present review presents experimental and theoretical results on this long-standing problem. New experimental and theoretical results for the critical behaviour in the isotropic phase of nematogens are reviewed. We calculate in a unified approach the low value of , at both critical and tricritical points. The possibility of tricritical behaviour at the nematic-isotropic transition is also discussed by means of Landau theory. The various predictions are compared with the available experimental results.
62 citations
TL;DR: The isotropic-nematic phase transition in liquid crystals is described in the context of the slowing of orientational relaxation associated with divergent growth of the orientational correlation length, and the correlation length of the density fluctuations is diverging with the same temperature dependence as the pseudonematic domain correlation length.
Abstract: The isotropic phase of nematogenic liquid crystals has nanometer length scale domains with pseudonematic ordering. As the isotropic to nematic phase transition temperature (TNI) is approached from above, the orientational correlation length, ξ, of the pseudonematic domains grows as (T - T(*))(-1/2), where T(*) is 0.5-1 K below TNI. The orientational relaxation, which is a collective property of the pseudonematic domains, was measured with optical heterodyne detected-optical Kerr effect (OHD-OKE). The orientational relaxation obeys Landau-de Gennes theory, as has been shown previously. To examine the environmental evolution experienced by molecules in the pseudonematic domains, two-dimensional infrared (2D IR) vibrational echo experiments on the CN stretching mode of the non-perturbative vibrational probes 4-pentyl-4(')-selenocyanobiphenyl (5SeCB) and 4-pentyl-4(')-thiocyanobiphenyl (5SCB) in the nematogen 4-cyano-4(')-pentylbiphenyl (5CB) were performed. The 2D IR experiments measure spectral diffusion, which is caused by structural fluctuations that couple to the CN vibrational frequency. Temperature dependent studies were performed just above TNI, where the correlation length of pseudonematic domains is large and changing rapidly with temperature. These studies were compared to 2D IR experiments on 4-pentylbiphenyl (5B), a non-mesogenic liquid that is very similar in structure to 5CB. The time constants of spectral diffusion in 5CB and 5B are practically identical at temperatures ≥5 K above TNI. As the temperature is lowered, spectral diffusion in 5B slows gradually. However, the time constants for spectral diffusion in 5CB slow dramatically and diverge as T(*) is approached. This divergence has temperature dependence proportional to (T - T(*))(-1/2), precisely the same as seen for the correlation length of pseudonematic domains, but different from the observed orientational relaxation times, which are given by the Landau-de Gennes theory. The data and previous results show that spectral diffusion in 5CB has no contributions from orientational relaxation, and the structural dynamics responsible for the spectral diffusion are likely a result of density fluctuations. The results suggest that the correlation length of the density fluctuations is diverging with the same temperature dependence as the pseudonematic domain correlation length, ξ. The isotropic-nematic phase transition in liquid crystals is described in the context of the slowing of orientational relaxation associated with divergent growth of the orientational correlation length. The results presented here show that there is another divergent dynamical process, likely associated with density fluctuations.
22 citations
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.
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4 citations
TL;DR: In this paper, the authors evaluated the trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient in the Landau-de Gennes phenomenological theory for nematic liquid crystals.
Abstract: The trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient are theoretically evaluated in the Landau-de Gennes phenomenological theory for nematic liquid crystals. The cases of planar, homeotropic and tilted alignment at a planar limiting surface are considered. It is shown that the cases with surface order parameter greater than the bulk one or smaller than this can occur in function of the phenomenological coefficients in the surface free energy expression. The anchoring strength coefficient, W, diminishes when temperature approaches the nematic-isotropic point, after passing through a maximum in most of cases. The possibility of a non-zero anchoring strength at temperatures above the transition temperature is revealed. The obtained trends of W(T) describe well a large variety of experimental results from literature.
1 citations