Elastic constants and orientational viscosities of a bent-core nematic liquid crystal

Majumdar, Madhabi; Szűcs, Péter; Jákli, Antal; Gleeson, J. T.; Sprunt, Samuel

doi:10.1103/physreve.83.031701

Cited by 105 publications

(90 citation statements)

References 33 publications

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“…20 The material studied had dielectric and elastic coefficients comparable to conventional nematic LCs. 27 Inequality (5b) More generally, important relationships for the design and development of flexoelectric materials and devices have been rigorously derived herein, in terms of Oseen-Frank continuum theory. For liquid crystals composed of small elongated organic molecules, it seems unlikely that flexoelectric coefficients with magnitude greater than a few tens of pC/m are achievable.…”

confidence: 99%

The limits of flexoelectricity in liquid crystals

2011

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The flexoelectric conversion of mechanical to electrical energy in nematic liquid crystals is investigated using continuum theory. Since the electrical energy produced cannot exceed the mechanical energy supplied, and vice-versa, upper bounds are imposed on the magnitudes of the flexoelectric coefficients in terms of the elastic and dielectric coefficients. For conventional values of the elastic and dielectric coefficients, it is shown that the flexoelectric coefficients may not be larger than a few tens of pC/m. This has important consequences for the future use of such flexoelectric materials in devices and the related energetics of distorted equilibrium structures

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confidence: 99%

The limits of flexoelectricity in liquid crystals

2011

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“…Extraordinarily large values for the bend flexoelectric coefficient |e 3 | for BCN by a factor of 10 3 greater than calamatics observed by Harden et al [9] were suggested to arise from the smecticlike cluster formation in the nematic phase of these compounds [24,25]. Discrepancies in the results between the elastic and flexoelectric coefficients given in the literature prompted us to investigate properties of three BCN nematic LCs that belong to a homologous series.…”

Section: Flexoelectricitymentioning

confidence: 95%

“…They found that closer to the nematic-smectic phase transition temperature both K 11 and K 33 first decrease and then increase slightly with a reduction in temperature, in contrast to both of these elastic constants increasing significantly closer to the nematicsmectic phase transition temperature in the host 8OCB. In recent investigations, measurement of these constants in pure BCNs in nematic phase has also yielded intriguing results, K 33 is found in most cases to be lower than K 11 [24,25,[32][33][34].…”

Section: Flexoelectricitymentioning

confidence: 99%

“…Some of these include intrinsic biaxiality [19] and the optical biaxiality measured directly [20] and induced biaxiality [21], a reversal in the sign of the elastic and dielectric anisotropies [22][23][24][25], and the observed chiral domains of the opposite polarity in C5 [26] and C9 [26]. For a widely studied conventional calamitic such as 4-cyano-4 -pentylbiphenyl (5CB), K 33 is greater than K 11 and the ratio of K 33 /K 11 varies from 2 to 3 [27].…”

Section: Flexoelectricitymentioning

confidence: 99%

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Effect of cybotactic clusters on the elastic and flexoelectric properties of bent-core liquid crystals belonging to the same homologous series

et al. 2013

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We report results of the splay (K 11 ) and bend (K 33 ) elastic constants and the effective flexoelectric coefficient of three bent-core liquid crystals belonging to a homologous series of 4-cyanoresorcinol bisbenzoates with varying chain lengths. Based on the results of x-ray scattering studies, one of the three compounds with a shorter chain length (C4) has few, if any, clusters present in its nematic phase and behaves quite normally, whereas the others two with longer chain lengths (C6 and C7) show the presence of cybotactic nematic phase with smectic C type clusters. These grow in size with a reduction in temperature. K 33 is found to be the least for C7, whereas it is weakly dependent on temperature. K 33 is somewhat higher for C4 and C6 and is almost independent of temperature. K 11 for C6 and C7 is higher by 20% to 50% than C4 depending on the temperature. K 11 increases linearly with a reduction in temperature for the three compounds. For C6 K 11 > K 33 by a factor up to ∼2 depending on the temperature, for C4 it is greater by a factor up to 1.3, and for C7 it is greater by a factor of ∼2.5. These results suggest that the clusters do not have any effect on K 11 . The magnitude of the effective flexoelectric coefficient e = (|e 1 − e 3 |) is measured by creating a uniform lying helix (ULH) configuration in a planar cell. By doping the bent-core system with a small wt% of a chiral dopant, the ULH is obtained by cooling planar cells to the cholesteric phase under weak electric field. The effective flexoelectric coefficient is greater for the bent-core systems than for calamatics but it is much lower than would otherwise have been expected for such systems. |e 1 − e 3 | for C4 > C6 ≈ C7 is greater by 20% to 25% than C6 and C7 at the same reduced temperature. These differences in the effective flexoelectric coefficient can easily arise from a difference in the chain lengths among the members of the series but if the presence of clusters were to have an influence on |e 1 − e 3 |, then these would reduce it, contrary to the expectations for the bent-core systems.

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“…The equality K 1 = 3K 2 appears in some theories with hard molecules [124,125]. Recently, the elastic constants and orientational viscosities of bent-core nematic liquid crystals have been determined [126] and the unusual anisotropies of these parameters have been discussed in terms of short-range, smectic-C correlations among molecules. The relation (93) was not satisfied and the viscosity ratios also contrasted with ordinary calamitics.…”

Section: The Equilibrium Distortion In the Chiral Biaxial Phasementioning

confidence: 99%

Statistical Theory of Biaxial Nematic and Cholesteric Phases

Kapanowski

2011

Acta Phys. Pol. A

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A statistical theory of biaxial nematic and cholesteric phases is presented. It is derived in the thermodynamic limit at small density and small distortions. The considered phases are composed of short rigid biaxial or less symmetric molecules. The expressions for various macroscopic parameters involve the one-particle distribution function and the potential energy of two-body short-range interactions. The cholesteric phase is regarded as a distorted form of the nematic phase. Exemplary calculations are shown for several systems of molecules interacting via Corner-type potential based on the Lennard-Jones 12-6 functional dependence. The temperature dependence of the order parameters, the elastic constants, the phase twists, the dielectric susceptibilities, and the flexoelectric coefficients are obtained. The flow properties and defects in nematics are also discussed.

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Elastic constants and orientational viscosities of a bent-core nematic liquid crystal

Cited by 105 publications

References 33 publications

The limits of flexoelectricity in liquid crystals

The limits of flexoelectricity in liquid crystals

Effect of cybotactic clusters on the elastic and flexoelectric properties of bent-core liquid crystals belonging to the same homologous series

Statistical Theory of Biaxial Nematic and Cholesteric Phases

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