Universal law for the rotational viscosity of nematic liquid crystals

-Temperature dependence of dielectric properties of nematic liquid crystals are investigated for two types of devices: wide temperature range liquid crystal (LC) displays and capacitor temperature sensor. Both real and imaginary components of the dielectric constant have been measured in wide frequency and temperature range including t < 0°C as well as versus the angle θ between the directions of both magnetic and electric field in the measuring scheme. Some physical parameters of the nematic LC (NLC) dielectric relaxation have been determined. Effective values of the LC dielectric permittivity with different values of the LC pretilt angles in different parts of a complex LC cell have been simulated.

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“…It corresponds to W values for other substances . The relaxation time τ = ( f r ) − 1 values for ZhK‐1282 agree well with data on 5CB …”

Section: Measurement Of Both Real and Imaginary Components Of The Diesupporting

confidence: 79%

Dielectric properties of liquid crystals for display and sensor applications

et al. 2015

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“…In fragile supercooled liquids, the MCT critical temperature T C is typically 20%-30% higher than the experimental glass transition temperature T g . Dielectric relaxation experiments on 5-CB indicate that the glass transition temperature is 210 K. 72,73 Comparing T g with T CL for 5-CB, one finds that T CL ϭ1.28T g , a typical value for fragile supercooled liquids. ͑Values of T g for the other liquid crystals are not available.͒ Recent analysis of the available data on the ␣ relaxation time ␣ (T) in various glass forming liquids showed that at T C , ␣ (T) is of the order of 10 Ϫ6 -10 Ϫ7.5 s. 74 The dielectric relaxation spectra of 5-CB 73 and 8-CB 75 give for these materials ␣ (T CL )ϭ10 Ϫ6.6 s. Again, these values are consistent with the view that for the liquid crystals T CL is akin to T C for supercooled liquids.…”

Section: Discussionmentioning

confidence: 99%

Dynamical signature of two “ideal glass transitions” in nematic liquid crystals

Cang

Novikov

et al. 2003

The Journal of Chemical Physics

113

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A temperature scaling analysis using the same mode coupling theory ͑MCT͒ scaling relationships employed for supercooled liquids is applied to optical heterodyne detected optical Kerr effect data for four liquid crystals. The data cover a range of times from ϳ1 ps to 100 ns and a range of temperatures from ϳ50 K above the isotropic to nematic phase transition temperature T NI down to ϳT NI. The slowest exponential component of the data obeys the Landau-de Gennes ͑LdG͒ theory for the isotropic phase of liquid crystals. However, it is also found that the liquid crystal data obey MCT scaling relationships, but, instead of a single scaling temperature T C as found for supercooled liquids, in the liquid crystals there are two scaling temperatures T CL ͑L for low temperature͒ and T CH ͑H for high temperature͒. T CH is very close to T*, which results from LdG scaling, just below the isotropic to nematic phase transition temperature, T NI , but is 30-50 K higher than T CL. The liquid crystal time dependent data have the identical functional form as supercooled liquid data, that is, a fast power law decay with temperature independent exponent, followed by a slower power law decay with temperature independent exponent, and on the longest time scales, an exponential decay with highly temperature dependent decay constant. For each liquid crystal, the amplitudes of the two power laws scale with expressions that involve T CL , but the exponential decay time constant ͑long time dynamics͒ scales with an expression that involves T CH. The existence of two scaling temperatures can be interpreted as a signature of two ''glass transitions'' in liquid crystals. In ideal MCT developed for spheres, T C is the ''ideal glass transition temperature,'' although it is found experimentally to be ϳ20%-30% above the experimental glass transition temperature, T g. The transition in nematic liquid crystals at T CL corresponds to the conventional ideal MCT glass transition, while the transition at T CH can occur for nonspherical molecules, and may correspond to the freezing in of local nematic order.

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“…mais bien au contraire, elle n'a cess6 de s'6tendre progressivement aux milieux condenses les plus divers, et recouvre actuellement les polym6res amorphes , les electrolytes [34], les verres metalliques les cristaux liquides [37][38][39][40], vitreux [41], ferro6lectriques [42]. Elle a aussi ete utiJis6e par Angell [43] pour 6tablir une classification des liquides fond6e sur leurs propri6t6s thermodynamiques au voisinage de la transition vitreuse.…”

unclassified

Etude de la viscosité de cisaillement des liquides sous-refroidis jusqu'à leur température de transition vitreuse. Analyse des variations thermiques des coefficients de Vogel-Fulcher. - I. Carbonate de propylène

Bondeau

Huck

1985

J. Phys. France

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Universal law for the rotational viscosity of nematic liquid crystals

Cited by 28 publications

References 8 publications

Dielectric properties of liquid crystals for display and sensor applications

Dielectric properties of liquid crystals for display and sensor applications

Dynamical signature of two “ideal glass transitions” in nematic liquid crystals

Etude de la viscosité de cisaillement des liquides sous-refroidis jusqu'à leur température de transition vitreuse. Analyse des variations thermiques des coefficients de Vogel-Fulcher. - I. Carbonate de propylène

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