Invited Lecture. New results on the electrohydrodynamic instability in nematics

Kramer, Lorenz; Bodenschatz, Eberhard; Pesch, Werner; Thom, W.; Zimmermann, Walter

doi:10.1080/02678298908045420

Cited by 80 publications

(44 citation statements)

References 33 publications

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“…The motion of isolated dislocations toward the horizontal grain boundaries and the confinement of the dislocations to grain boundaries is expected on general arguments from amplitude equations [21]. However, existing theoretical work focused on the horizontal domain walls and individual dislocations [21]. A more detailed theoretical and experimental study is required to understand the full range of observed behavior.…”

Section: Discussionmentioning

confidence: 99%

“…The dislocations exhibit highly non-trivial behavior, forming coherent grain boundaries, exhibiting motion that is neither climb nor glide, and executing interesting dynamics, including oscillations. The motion of isolated dislocations toward the horizontal grain boundaries and the confinement of the dislocations to grain boundaries is expected on general arguments from amplitude equations [21]. However, existing theoretical work focused on the horizontal domain walls and individual dislocations [21].…”

Section: Discussionmentioning

confidence: 99%

“…The work with diblock copolymers has already made important contributions along these lines for disclinations. In this paper, we focus on the dynamics of confined dislocations found in the oblique roll regime of electroconvection [21]. Figure 1b is a top view of a pattern in an electroconvection cell that illustrates the defects of interest in this paper.…”

Section: Introductionmentioning

confidence: 99%

“…The main defect in the normal roll regime is isolated dislocations. The dynamics of these dislocations have been studied both experimentally and theoretically [18,19,20,21], though not in the context of domain growth. Oblique rolls correspond to stripes in which the wavevector forms a nonzero angle with respect to the undistorted director field.…”

Section: Introductionmentioning

confidence: 99%

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Dislocation dynamics in an anisotropic stripe pattern

2004

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The dynamics of dislocations confined to grain boundaries in a striped system are studied using electroconvection in the nematic liquid crystal N4. In electroconvection, a striped pattern of convection rolls forms for sufficiently high driving voltages. We consider the case of a rapid change in the voltage that takes the system from a uniform state to a state consisting of striped domains with two different wavevectors. The domains are separated by domain walls along one axis and a grain boundary of dislocations in the perpendicular direction. The pattern evolves through dislocation motion parallel to the domain walls. We report on features of the dislocation dynamics. The kinetics of the domain motion are quantified using three measures: dislocation density, average domain wall length, and the total domain wall length per area. All three quantities exhibit behavior consistent with power law evolution in time, with the defect density decaying as t −1/3 , the average domain wall length growing as t 1/3 , and the total domain wall length decaying as t −1/5 . The two different exponents are indicative of the anisotropic growth of domains in the system.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Dislocation dynamics in an anisotropic stripe pattern

2004

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“…It was found theoretically that in the case of d.c. applied voltage for planarly aligned MBBA the flexoelectric effect leads to a reduction of the threshold and to the appearance of oblique or possibly parallel rolls [6,7,8] depending on the choice of the flexoelectric coefficients.…”

Section: Introductionmentioning

confidence: 99%

Crossover Between Flexoelectric Stripe Patterns and Electroconvection in Hybrid Aligned Nematics

Delev

Krekhov

Kramer

2001

Molecular Crystals and Liquid Crystals Science and Technology.

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We report experimental and theoretical results on the flexoelectric instability and crossover between flexoelectric domains and electroconvection in a hybrid aligned nematic MBBA under d.c. voltage. At threshold a spatially periodic flexoelectric deformation in the form of longitudinal domains (along the planar director) was observed. With increasing voltage the director deviation out of the initial plane becomes clearly detectable. Observations of tracers show that hydrodynamic flow develops inside the flexoelectric stripe pattern in the form of circular orbits perpendicular to the domain stripes. With further increasing d.c. voltage electroconvection sets in in the form of drifting oblique rolls inside the flexoelectric domains.

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Defects in Continuous Media

1991

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Whenever one deals with an extended system where longrange order is induced by short-range interactions one may find topological defects. These are imperfections in the ordering that cannot appear or disappear individually inside the material, like, for example, interstitials and vacancies in a crystal. Defects are important because they determine to a large extent the mechanical and other properties of real materials.""] The theory of defects in crystals is quite complicated, mainly due to the three-dimensional nature and the discreteness of the underlying lattice. As a result, one has a considerable number of different topological point-and linedefects. The discreteness also makes the detailed description very complicated and quantitative results about the energy and other properties are therefore very difficult to obtain. A simple continuum description['"] is only valid far from the defect core and often does not adequately explain important properties. From the experimental &de, the detailed structure of single defects cannot be observed, so there is considerable room for controversy.The situation is more promising for systems where the order can be described genuinely by a continuum theory since the spatial variations connected with the ordering are slow at the molecular level. Good candidates for a continuum description are systems undergoing a second-order phase transition near to the transition point. However, even more appropriate are fluids undergoing a transition to a macroscopically ordered state via a symmetry-breaking instability. Here, hydrodynamic equations can describe the phenomena to virtually arbitrary precision and moreover, the defects are usually easily visible due to their large size.We here consider extended fluid layers where the longrange order is two-dimensional. The most prominent example is the Rayleigh-BCnard instability, where a simple fluid, like water, is driven into a state with permanent convection due to buoyancy effects by heating from below (in the usual case of a positive thermal expansion coefficient).['b1 When the fluid layer is bounded by rigid plates and the experiment

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Invited Lecture. New results on the electrohydrodynamic instability in nematics

Cited by 80 publications

References 33 publications

Dislocation dynamics in an anisotropic stripe pattern

Dislocation dynamics in an anisotropic stripe pattern

Crossover Between Flexoelectric Stripe Patterns and Electroconvection in Hybrid Aligned Nematics

Defects in Continuous Media

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