Electroconvection in a Homeotropic Nematic under the Influence of a Magnetic Field

Richter, H.; Klöpper, N.; Hertrich, A.; Buka, Ágnes

doi:10.1209/0295-5075/30/1/007

Cited by 58 publications

(28 citation statements)

References 10 publications

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“…The primary purpose of this is to provide information about the homogeneity of our samples above V F where EC first occurs and to determine the direction of symmetry breaking by the Freedericksz domain. We then present results for the EC onset voltage V c , the critical wave number k c , the obliqueness angle c , and the traveling-wave ͑Hopf͒ frequency at onset, c , over a range of sample conductances ʈ and drive frequencies f. Our results expand the findings of Richter et al ͓19,30͔,Tóth et al ͓31͔,and Buka et al ͓5͔ to larger parameter ranges. To the extent possible comparison with theoretical predictions by Hertrich et al ͓28͔ will be made.…”

Section: Introductionsupporting

confidence: 74%

Onset of electroconvection of homeotropically aligned nematic liquid crystals

et al. 2006

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We present experimental measurements near the onset of electroconvection (EC) of homeotropically aligned nematic liquid crystals Phase 5A and MBBA. A voltage of amplitude square root 2V0 and frequency f was applied. With increasing V0, EC occurred after the bend Freedericksz transition. We found supercritical bifurcations to EC that were either stationary bifurcations or Hopf bifurcations to traveling convection rolls, depending on the sample conductances. Results for the onset voltages Vc, the critical wave numbers kc, the obliqueness angles thetac, and the traveling-wave (Hopf) frequencies at onset omegac over a range of sample conductances and driving frequencies are presented and compared, to the extent possible, with theoretical predictions. For the most part good agreement was found. However, the experiment revealed some unusual results for the orientations of the convection rolls relative to the direction selected by the Freedericksz domain.

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Section: Introductionsupporting

confidence: 74%

Onset of electroconvection of homeotropically aligned nematic liquid crystals

et al. 2006

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“…Homeotropically aligned nematic LCs, on the other hand, where EC appears above a bend Fréedericksz transition, offer the possibility to control the anisotropy by an additional planar magnetic field H [11]. In fact, the preferred axis can be varied by rotating H with respect to the sample after the pattern has formed.…”

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confidence: 99%

Dynamics of defects in electroconvection patterns

et al. 2002

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PACS. 61.30.Gd -Orientational order of liquid crystals; electric and magnetic field effects on order. PACS. 61.72.Cc -Kinetics of defect formation and annealing. PACS. 61.72.Ff -Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, X-ray topography, etc.).Abstract. -In homeotropically aligned nematics with negative dielectric anisotropy the electrohydrodynamic instability occurs above a bend Fréedericksz transition. In the presence of a magnetic field H parallel to the liquid crystal slab, ordered roll patterns with a welldefined uniform wave vector k id appear above the onset of convection. By rotating the cell around an axis perpendicular to the slab by a small angle α, one can manipulate the system into a state with wave vector k = k id + ∆ k, where ∆ k is roughly perpendicular to k id . We have studied experimentally the motion of defects, which then move essentially perpendicular to the rolls. The direction as well as the magnitude of the velocity as a function of ∆ k agrees with predictions of the weakly nonlinear theory. In particular, we obtain evidence for the nonanalyticity for ∆ k → 0.

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“…The occurrence of chaos at onset can be easily blocked if one breaks the degeneracy of the orientation by applying a small magnetic field H parallel to the surfaces [15]. Then the director will tilt towards H at the Freedericksz transition and the EC rolls appearing at V >V c will be nicely ordered too, exhibiting the same pattern morphologies found in planar samples (see figures 1b and 1c).…”

Section: Pattern Characteristics At Onsetmentioning

confidence: 76%

Electroconvection in homeotropic nematic liquid crystals

Éber

Buka

2005

Phase Transitions

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Electroconvection is a classical example of pattern-forming phenomena in liquid crystals, typically observed in nematics with negative dielectric and positive conductivity anisotropies. This article focuses on how electroconvection in the homeotropic geometry differs from that in planar alignment. The influence of an additional magnetic field on the pattern characteristics and on secondary instabilities (the normal roll-abnormal roll transition) is discussed. The homeotropic alignment offers unique possibilities also for studying defect motion. Basic characteristics of some patterns of large wavelength are presented and compared with those of the classical Carr-Helfrich structures. Finally, electroconvection in substances with negative conductivity anisotropy is addressed.

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Electroconvection in a Homeotropic Nematic under the Influence of a Magnetic Field

Cited by 58 publications

References 10 publications

Onset of electroconvection of homeotropically aligned nematic liquid crystals

Onset of electroconvection of homeotropically aligned nematic liquid crystals

Dynamics of defects in electroconvection patterns

Electroconvection in homeotropic nematic liquid crystals

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