High Resolution X-Ray and Light Scattering Studies of Bilayer Smectic a Compounds

Litster, J. D.; Als‐Nielsen, J.; Birgeneau, R. J.; Dana, S. S.; Davidov, D.; García-Golding, F.; Kaplan, M. L.; Safinya, Cyrus R.; Schaetzing, Ralph

doi:10.1051/jphyscol:1979366

Cited by 91 publications

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“…In the standard formulation of smectic A hydrodynamics these two directions are parallel by construction. Only in the vicinity of phase transitions (either the nematic-smectic A or smectic A-smectic C * ) it has been shown that director fluctuations are of physical interest [30,31,32]. Neverthelessn andp differ significantly in their interaction with an applied shear flow.…”

Section: A Physical Idea Of the Modelmentioning

confidence: 99%

Shear-induced instabilities in layered liquids

2002

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Motivated by the experimentally observed shear-induced destabilization and reorientation of smectic A like systems, we consider an extended formulation of smectic A hydrodynamics. We include both, the smectic layering (via the layer displacement u and the layer normalp) and the director n of the underlying nematic order in our macroscopic hydrodynamic description and allow both directions to differ in non equilibrium situations. In an homeotropically aligned sample the nematic director does couple to an applied simple shear, whereas the smectic layering stays unchanged. This difference leads to a finite (but usually small) angle betweenn andp, which we find to be equivalent to an effective dilatation of the layers. This effective dilatation leads, above a certain threshold, to an undulation instability of the layers. We generalize our earlier approach [Rheol. Acta 39 (3), 15] and include the cross couplings with the velocity field and the order parameters for orientational and positional order and show how the order parameters interact with the undulation instability. We explore the influence of various material parameters on the instability. Comparing our results to recent experiments and molecular dynamic simulations, we find a good qualitative agreement.

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Section: A Physical Idea Of the Modelmentioning

confidence: 99%

Shear-induced instabilities in layered liquids

2002

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“…This liquid-crystal has several important differences from the closely related 8CB. The liquid-crystal 8OCB has stronger smectic and nematic interactions than 8CB as evidenced by the higher transition temperatures, the larger bare correlation lengths for smectic interactions [18], and the larger elastic constants. More specifically, 8OCB has a 17% larger bend, 36% larger twist, and 10% larger splay nematic elastic constants than 8CB (in the single elastic constant approximation, 8OCB has a ≈ 20% greater K N than 8CB with an overall uncertainty of 5%) [19].…”

Section: Introductionmentioning

confidence: 99%

Evolution of the isotropic-to-nematic phase transition in octyloxycyanobiphenyl+aerosil dispersions

et al. 2004

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High-resolution ac-calorimetry has been carried out on dispersions of aerosils in the liquid crystal octyloxycyanobiphenyl (8OCB) as a function of aerosil concentration and temperature spanning the crystal to isotropic phases. The liquid-crystal 8OCB is elastically stiffer than the previously well studied octylcyanobiphenyl (8CB)+aerosil system and so, general quenched random disorder effects and liquid-crystal specific effects can be distinguished. A double heat capacity feature is observed at the isotropic to nematic phase transition with an aerosil independent overlap of the heat capacity wings far from the transition and having a non-monotonic variation of the transition temperature. A crossover between low and high aerosil density behavior is observed for 8OCB+aerosil. These features are generally consistent with those on the 8CB+aerosil system. Differences between these two systems in the magnitude of the transition temperature shifts, heat capacity suppression, and crossover aerosil density between the two regimes of behavior indicate a liquid crystal specific effect. The low aerosil density regime is apparently more orientationally disordered than the high aerosil density regime, which is more translationally disordered. An interpretation of these results based on a temperature dependent disorder strength is discussed.Finally, a detailed thermal hysteresis study has found that crystallization of a well homogenized sample perturbs and increases the disorder for low aerosil density samples but does not influence high density samples.

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“…Finally, we note that numerous works [31][32][33][34][35][36] over the years have reported on the temperature dependence of the smectic-layer compression constant B as T → T NA (from below). However, to our knowledge, no data are yet available for the T dependence of the analogous quantity, B eff , as T → T TB [37] in the N TB phase.…”

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confidence: 99%

“…However, this is not the case in the analogous smectic-A system near the transition to the nematic phase (temperature T NA ). Here, experiment indicates D ∝ ðT NA − TÞ x with x ≈ 0.5 [31][32][33], whereas mean-field theory predicts x ¼ 1, and a more sophisticated, anisotropic scaling analysis gives x ≈ 0.67 [28].…”

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confidence: 99%

Fluctuation Modes of a Twist-Bend Nematic Liquid Crystal

et al. 2016

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We report a dynamic light-scattering study of the fluctuation modes in a thermotropic liquid crystalline mixture of monomer and dimer compounds that exhibits the twist-bend nematic (N TB ) phase. The results reveal a spectrum of overdamped fluctuations that includes two nonhydrodynamic modes and one hydrodynamic mode in the N TB phase, and a single nonhydrodynamic mode plus two hydrodynamic modes (the usual nematic optic axis or director fluctuations) in the higher temperature, uniaxial nematic phase. The properties of these fluctuations and the conditions for their observation are comprehensively explained by a Landau-de Gennes expansion of the free-energy density in terms of heliconical director and helical polarization fields that characterize the N TB structure, with the latter serving as the primary order parameter. A "coarse-graining" approximation simplifies the theoretical analysis and enables us to demonstrate quantitative agreement between the calculated and experimentally determined temperature dependence of the mode relaxation rates.

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High Resolution X-Ray and Light Scattering Studies of Bilayer Smectic a Compounds

Cited by 91 publications

References 0 publications

Shear-induced instabilities in layered liquids

Shear-induced instabilities in layered liquids

Evolution of the isotropic-to-nematic phase transition in octyloxycyanobiphenyl+aerosil dispersions

Fluctuation Modes of a Twist-Bend Nematic Liquid Crystal

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