Orientational relaxation in a discotic liquid crystal

Chakrabarti, Dwaipayan; Jana, Biman; Bagchi, Biman

doi:10.1103/physreve.75.061703

Cited by 5 publications

(4 citation statements)

References 49 publications

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“…Bagchi and coworkers showed with the Gay-Berne model that isotropic-nematic (I-N, weakly rstorder) and isotropic-columnar (I-C, strongly rst-order) phase transitions exhibit distinct orientational relaxation dynamics. 52,53 Specically, the authors observed a power-law decay of the OKE at short-to-intermediate timescales for the I-N phase transition, being absent in the I-C phase transition. Near the I-N phase transition, mesogen uids exhibit typical signatures of glass-forming liquids, including dynamical heterogeneity, non-Arrhenius temperature dependence of the orientational relaxation time, and non-exponential relaxation.…”

Section: Introductionmentioning

confidence: 97%

“…54 The Gay-Berne model has also been shown to capture a pretransitional effect, 55,56 where local pseudonematic domains strongly inuence spatial correlations of discogens in the isotropic phase, leading to an increasing correlation length scale as the I-N phase transition is approached. [52][53][54] Caprion and coworkers, in studying different aspect ratios, elucidated differences in the diffusivity of discogens in isotropic, nematic, and columnar phases by mapping timescales from their Monte Carlo study. 28 A similar time-mapping scheme was carried out by Patti and coworkers to characterize the long-time relaxation dynamics of oblate hard spherocylinders in columnar mesophases.…”

Section: Introductionmentioning

confidence: 99%

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Phase equilibria, fluid structure, and diffusivity of a discotic liquid crystal

et al. 2014

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Molecular Dynamics simulations were performed for the Gay-Berne discotic fluid parameterized by GB(0.345, 0.2, 1.0, 2.0). The volumetric phase diagram exhibits isotropic (IL), nematic (ND), and two columnar phases characterized by radial distribution functions: the transversal fluid structure varies between a hexagonal columnar (CD) phase (at higher temperatures and pressures) and a rectangular columnar (CO) phase (at lower temperatures and pressures). The slab-wise analysis of fluid dynamics suggests the formation of grain-boundary defects in the CO phase. Longitudinal fluid structure is highly periodic with narrow peaks for the CO phase, suggestive of a near-crystalline (yet diffusive) system, but is only short-ranged for the CD phase. The IL phase does not exhibit anisotropic diffusion. Transversal diffusion is more favorable in the ND phase at all times, but only favorable at short times for the columnar phases. In the columnar phases, a crossover occurs where longitudinal diffusion is favored over transversal diffusion at intermediate-to-long timescales. The anomalous diffusivity is pronounced in both columnar phases, with three identifiable contributions: (a) the rattling of discogens within a transient "interdigitation" cage, (b) the hopping of discogens across columns, and (c) the drifting motion of discogens along the orientation of the director.

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Section: Introductionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Phase equilibria, fluid structure, and diffusivity of a discotic liquid crystal

et al. 2014

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“…When a different isobar was chosen at a lower pressure for the discotic system, a direct transition from the isotropic phase to the columnar phase was observed. In this case, decay of neither the single-particle second-rank OTCF nor the OKE signal showed any power law regime . We note that the isotropic−columnar (I−C) transition is strongly first-order in nature, as reflected in the present case with a much larger change in the density corresponding to the I−C transition as compared to the I−N transition.…”

Section: Results On Analogous Dynamicsmentioning

confidence: 50%

Glassiness of Thermotropic Liquid Crystals across the Isotropic−Nematic Transition

Chakrabarti

Bagchi

2007

J. Phys. Chem. B

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The orientational dynamics of thermotropic liquid crystals across the isotropic-nematic phase transition have traditionally been investigated at long times or low frequencies using frequency domain measurements. The situation has now changed significantly with the recent report of a series of interesting transient optical Kerr effect (OKE) experiments that probed orientational relaxation of a number of calamitic liquid crystals (which consist of rod-like molecules) directly in the time domain, over a wide time window ranging from subpicoseconds to tens of microseconds. The most intriguing revelation is that the decay of the OKE signal at short to intermediate times (from a few tens of picoseconds to several hundred nanoseconds) follows multiple temporal power laws. Another remarkable feature that has emerged from these OKE measurements is the similarity in the orientational relaxation behavior between the isotropic phase of calamitic liquid crystals near the isotropic-nematic transition and supercooled molecular liquids, notwithstanding their largely different macroscopic states. In this article, we present an overview of the understanding that has emerged from recent computational and theoretical studies of calamitic liquid crystals across the isotropic-nematic transition. Topics discussed include (a) single-particle as well as collective orientational dynamics at a short-to-intermediate time window, (b) heterogeneous dynamics in orientational degrees of freedom diagnosed by a non-Gaussian parameter, (c) fragility, and (d) temperature-dependent exploration of underlying energy landscapes as calamitic liquid crystals settle into increasingly ordered mesophases upon cooling from the high-temperature isotropic phase. A comparison of our results with those of supercooled molecular liquids reveals an array of analogous features in these two important classes of soft matter systems. We further find that the onset of growth of the orientational order in the parent nematic phase induces translational order, resulting in smectic-like layers in the potential energy minima of calamitic systems if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. We discuss implications of this startling observation. We also discuss recent results on the orientational dynamics of discotic liquid crystals that are found to be rather similar to those of calamitic liquid crystals.

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“…As the I-N phase boundary is crossed upon cooling, the advent of two power law decay regimes separated by an intervening plateau at shortto-intermediate times imparts a step-like feature to the temporal behaviour of the second rank OTCF. Such power law relaxation near I-N phase boundary was an area of great interest in the recent past [11][12][13][14][15][16] and it has been investigated that the scenario is not a unique property of the model we have studied; it is a rather universal phenomenon of second rank OTCF. 17 Such a feature bears remarkable similarity to what is observed for supercooled liquids as the glass transition is approached from the above.…”

Section: Single Particle Orientational Dynamicsmentioning

confidence: 99%

Orientational dynamics and energy landscape features of thermotropic liquid crystals: An analogy with supercooled liquids

Jana

Bagchi

2007

J Chem Sci

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Recent optical kerr effect (OKE) studies have revealed that orientational relaxation of rodlike nematogens near the isotropic-nematic (I-N) phase boundary and also in the nematic phase exhibit temporal power law decay at intermediate times. Such behaviour has drawn an intriguing analogy with supercooled liquids. Here, we have investigated the single-particle and collective orientational dynamics of a family of model system of thermotropic liquid crystals using extensive computer simulations. Several remarkable features of glassy dynamics are on display including non-exponential relaxation, dynamical heterogeneity, and non-Arrhenius temperature dependence of the orientational relaxation time. Over a temperature range near the I-N phase boundary, the system behaves like a fragile glass-forming liquid. Using proper scaling, we construct the usual relaxation time versus inverse temperature plot and explicitly demonstrate that one can successfully define a density dependent fragility of liquid crystals.The fragility of liquid crystals shows a temperature and density dependence which is remarkably similar to the fragility of glass forming supercooled liquids. Energy landscape analysis of inherent structures shows that the breakdown of the Arrhenius temperature dependence of relaxation rate occurs at a temperature that marks the onset of the growth of the depth of the potential energy minima explored by the system.

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Orientational relaxation in a discotic liquid crystal

Cited by 5 publications

References 49 publications

Phase equilibria, fluid structure, and diffusivity of a discotic liquid crystal

Phase equilibria, fluid structure, and diffusivity of a discotic liquid crystal

Glassiness of Thermotropic Liquid Crystals across the Isotropic−Nematic Transition

Orientational dynamics and energy landscape features of thermotropic liquid crystals: An analogy with supercooled liquids

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