Determination of Splay and Twist Relaxation Modes in Nematic Liquid Crystals from Dynamic Light Scattering Experiments

Yoon, Do Y.; Pecora, R.

doi:10.1021/jp9813645

Cited by 15 publications

(7 citation statements)

References 19 publications

(40 reference statements)

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“…predicts do not propagate". Similar statements appear in the textbook derivations [1,2] and this view has been supported by experiment over subsequent years [14][15][16][17].…”

supporting

confidence: 82%

Propagating Director Bend Fluctuations in Nematic Liquid Crystals

Humpert

Allen

2015

Phys. Rev. Lett.

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We show, by molecular simulation, that for a range of standard, coarse-grained, nematic liquid crystal models, the director bend fluctuation is a propagating mode. This is in contrast to the generally-accepted picture of nematic hydrodynamics, in which all the director modes (splay, twist, bend, and combinations thereof) are overdamped. By considering the various physical parameters that enter the equations of nematodynamics, we propose an explanation of this effect, and conclude that propagating bend fluctuations may be observable in some experimental systems.

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“…predicts do not propagate". Similar statements appear in the textbook derivations [1,2] and this view has been supported by experiment over subsequent years [14][15][16][17].…”

supporting

confidence: 82%

Propagating Director Bend Fluctuations in Nematic Liquid Crystals

Humpert

Allen

2015

Phys. Rev. Lett.

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“…The viscosities ( ) q α η v are combinations of the Leslie and Miesowicz viscosities of the nematic. From the scattering geometries described above, values of the elastic moduli, and the fitted relaxation rates, we calculate the orientational viscosities for pure bend and splay scattering geometries as [36]:…”

Section: Diamagnetic Anisotropymentioning

confidence: 99%

Elastic and viscous properties of the nematic dimer CB7CB

et al. 2017

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We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic, and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4'-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rodlike cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist-bend nematic, N_{TB}, and the oblique helicoidal cholesteric, Ch_{OH}, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pretransitional regions near the N-N_{TB} and N-to-isotropic (I) transitions. The splay constant K_{11} is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K_{33} and twist K_{22} constants are measured by DLS. K_{33}, being the smallest of the three constants, shows a strong nonmonotonous temperature dependence with a negative slope in both N-I and N-N_{TB} pretransitional regions. The measured ratio K_{11}/K_{22} is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist, and bend fluctuations in the N phase are comparable to those of nematics formed by rodlike molecules. All three show strong temperature dependence, increasing sharply near the N-N_{TB} transition.

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“…As a function of increasing temperature, they observe a non-linear increase in the value of k 22 /γ 1 . Gu et al [15] and recently Borsali et al [16] have also reported measurements of k 22 /γ 1 of the same nematic liquid crys-tal, 5CB. Though our system is cholesteric, the values of k 22 /γ 1 are of the same order of magnitude reported in nematics [14][15][16].…”

Section: Resultsmentioning

confidence: 84%

“…There have been a few reports on the measurements of the twist viscoelastic coefficient k 22 /γ 1 for nematic liquid crystals in the literature [14][15][16]. Sefton et al have carried out dynamic light scattering experiments to determine k 22 /γ 1 for pure nematic pentylcyanobiphenyl (5CB) and for the system doped with different amounts of a polymeric side chain liquid crystal.…”

Section: Resultsmentioning

confidence: 99%

A dynamic light scattering study of the viscoelastic twist mode in cholesteric liquid crystals

Giridhar

Suresh

2002

Eur. Phys. J. E

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Although viscoelastic properties of liquid crystals are very important for applications like display technology, there are not many direct techniques to study them. We have used dynamic light scattering to obtain dispersion curves for the twist mode of director fluctuations in a thermotropic cholesteric liquid crystal (cholesteric) mixture, in a wave vector range comparable to the inverse pitch of the cholesteric. In the experiment, light scattered in the vicinity of Bragg reflection is investigated using a hitherto unused scattering geometry where the scattering wave vector q is parallel to the cholesteric helix axis. By fitting the experimental data to a parabolic function of the scattering wave vector, we have determined the twist viscoelastic coefficient k 22/γ1. We have studied the temperature dependence of k22/γ1 and found that the viscoelastic coefficient increases rapidly as the cholesteric-to-isotropic transition temperature is approached.

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Determination of Splay and Twist Relaxation Modes in Nematic Liquid Crystals from Dynamic Light Scattering Experiments

Cited by 15 publications

References 19 publications

Propagating Director Bend Fluctuations in Nematic Liquid Crystals

Propagating Director Bend Fluctuations in Nematic Liquid Crystals

Elastic and viscous properties of the nematic dimer CB7CB

A dynamic light scattering study of the viscoelastic twist mode in cholesteric liquid crystals

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