Electroconvection in homeotropic nematic liquid crystals

Éber, Nándor; Buka, Ágnes

doi:10.1080/01411590500185278

Cited by 8 publications

(4 citation statements)

References 28 publications

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“…Its threshold U c has weak, linear frequency dependence; therefore it can be detected at high frequencies. At increasing f a sequence of transitions from 'conductive' to 'dielectric' rolls and then to the 'prewavy' pattern has been reported [18]. A higher conductivity has been found to promote the formation of the 'prewavy' pattern by reducing its threshold; then a direct transition from 'conductive' rolls to the 'prewavy' pattern is often observed [15].…”

Section: Introductionmentioning

confidence: 99%

Electroconvection in nematic mixtures of bent-core and calamitic molecules

et al. 2009

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The onset of electroconvection in binary mixtures of a bent-core and a rodlike nematic has been characterized by measuring the threshold voltage U(c) and the critical wave number of the pattern in a wide range of frequencies f. In the mixtures rich in bent-core molecules, a "conductive-prewavy2-patternless-prewavy1" morphological sequence has been detected with an unusual negative slope of U(c)(f) at high frequencies. This latter scenario seems to be related to the bent-core component, as it disappears with increasing the concentration of rodlike molecules. In addition, one of the parameters most relevant for electroconvection, the electrical conductivity, has also been varied by ionic salt doping. It has been found that the above effect of the banana-shaped molecules on the electroconvection scenarios can be suppressed by the conductivity.

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Section: Introductionmentioning

confidence: 99%

Electroconvection in nematic mixtures of bent-core and calamitic molecules

et al. 2009

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“…2,3 Unlike modern liquid crystal displays, first electro-optic applications of liquid crystals were based on electrically induced instabilities ͑dynamic scattering mode͒. [9][10][11][12] During the past few years, the role of flexoelectric contributions has been recognized as well. [5][6][7] In uniformly parallel aligned liquid crystals, the application of ac voltages with a low frequency is known to induce convection rolls; for example, normal rolls, vertical rolls, oblique rolls, or undulating rolls ͑conductive regime͒.…”

Section: Introductionmentioning

confidence: 99%

Electroconvection of liquid crystals: Tool for fabricating modulated polymer surfaces

Hoischen

Benning

Kitzerow

2009

Journal of Applied Physics

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Reversible optical control of transmittance in polymer/liquid crystal composite films by photoinduced phase transitionElectrically induced dissipative structures in liquid crystals can be stored by the photocuring of reactive mesogens. Thus, spontaneous pattern formation can be used to fabricate phase gratings or polymer films with a periodic surface modulation from an initially uniform sample. Different patterns, both in the conductive and in the dielectric regimes, have been stored. Photopolymerization and subsequent investigation by atomic force microscopy provide a useful tool in analyzing dissipative structures, even if the lattice constants are smaller than 1 m. This method indicates surface modulations with amplitudes of up to 12 nm and periodicities between 800 nm and several tens of micrometers. The influences of composition and curing conditions on the topography have been studied.

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“…The standard model by Carr and Helfrich,[46][47][48] which describes the behavior of a uniformly planar oriented (-, +) material (Δε < 0, Δσ > 0) was shown to explain also the possibility of pattern formation in (+, -) nematic liquid crystals (Δε > 0 and Δσ < 0) in the case of perpendicular (homeotropic) anchoring. [43][44][45] Electroconvection was also observed in many non-standard cases, for example (-, -) [49,50], (+, +) [51], or (-, +) under homeotropic anchoring conditions (⇒ "pre-wavy pattern") [52,53] and for bent-core liquid crystal compounds. [54][55][56] Most recently, the influence of flexoelectric contributions on the pattern formation was described [50] as well as domain patterns in polymer-dispersed nematic liquid crystal (PDLC) cells doped with minute amounts of multi-walled carbon nanotubes.…”

Section: Electrooptic Measurementsmentioning

confidence: 90%

Alignment and electrooptic effects in nanoparticle-doped nematic liquid crystals

Kinkead

Urbanski

et al. 2010

SPIE Proceedings

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It is well known that doping nematic liquid crystals with nanoparticles can alter the electrooptic response of the nematic host as well as the alignment of the liquid crystal molecules on various substrates. In addition, nanoparticles dispersed in a nematic matrix often induce defects and defect patterns justifying the necessity for more detailed optical and electrooptic investigations including effects of nanoparticle size, coating, concentration and core material. We studied the local alignment of nematic LC molecules in such dispersions by means of fluorescence confocal polarizing microscopy. The results of two-and three-dimensional imaging indicate that frequently observed birefringent stripes, which are induced by the presence of metal nanoparticles and semiconductor quantum dots, correspond to twist disclinations located at the LC/substrate interface. The luminescence of dispersed quantum dots shows that the ends of these disclination threads are pinned to conglomerates of nanoparticles that stabilize these line defects. By performing (x,z)-scans, it can be shown that the defects are not walls extending through the entire cell gap, but lines that are located at the substrate surface. Our experiments also confirm, as hypothesized before, that the nanoparticles preferably reside at the liquid crystal/substrate interfaces. Finally, detailed electrooptic investigations also revealed that a contrast inversion observed earlier is initiated by a change from parallel to homeotropic anchoring, thereby causing an instability, which in turn leads to the appearance of convection rolls (Kapustin-Williams domains). This electrohydrodynamic instability is likely an example for the behavior of (+, -) systems predicted by de Gennes, which was only recently experimentally observed for the first time.

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Electroconvection in homeotropic nematic liquid crystals

Cited by 8 publications

References 28 publications

Electroconvection in nematic mixtures of bent-core and calamitic molecules

Electroconvection in nematic mixtures of bent-core and calamitic molecules

Electroconvection of liquid crystals: Tool for fabricating modulated polymer surfaces

Alignment and electrooptic effects in nanoparticle-doped nematic liquid crystals

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