Undulations in a confined lamellar system with surface anchoring

Ishikawa, Taro; Lavrentovich, Oleg D.

doi:10.1103/physreve.63.030501

Cited by 34 publications

(46 citation statements)

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“…For a large rate of strain in the strips of reduced dimension L z rectangular domains occur, in contrast to chevron structures observed [1][2][3][4][5] in and predicted [12][13][14] for extended systems. The undulation period behaves analogously to theoretical predictions, [12][13][14] and hints of a compressed induced buckling could be found.…”

Section: Resultscontrasting

confidence: 57%

“…Such buckling or undulation instabilities, forming sinusoidal or chevron patterns, could be found in smectic liquid crystals, 1 ferromagnetic garnet films, 2,3 magnetic fluids, 4 or in cholesteric mixtures. 5 We report on the observation of undulation instabilities in nanostructured strips of a ferromagnetic Fe/Gd multilayer by magnetic transmission x-ray microscopy ͑MTXM͒.…”

Section: Introductionmentioning

confidence: 99%

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Undulation instabilities in laterally structured magnetic multilayers

Eimüller

Scholz

Köhler

et al. 2002

Journal of Applied Physics

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Undulation instabilities of magnetic domains have been observed in nanostructured strips of a ferromagnetic Fe/Gd multilayer. The novel technique of magnetic transmission x-ray microscopy, which is based on the x-ray magnetic circular dichroism, was used for imaging. Below a critical magnetic field, sinus-like modulations of the magnetic domains could be observed. At a higher rate of field induced strain rectangular patterns occur. They seem to be characteristic for the reduced lateral width of the magnetic system and are in contrast to chevron patterns observed in extended systems. The domain morphologies found in different magnetic fields H, and in nanowires of various widths Lz, have been summarized in a H–Lz “phase” diagram. An analogy with theoretical predictions for extended systems could be found.

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Section: Resultscontrasting

confidence: 57%

Section: Introductionmentioning

confidence: 99%

Undulation instabilities in laterally structured magnetic multilayers

Eimüller

Scholz

Köhler

et al. 2002

Journal of Applied Physics

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“…Experimental data, figure 5 (c), suggest that Fluorescence confocal microscopy 885 the undulations are less symmetric: two adjacent shoulders of the undulation wave are of different length. Similar asymmetric undulations have been observed in lyotropic 'rippled' phases [25], undulating chevron tips of chiral SmC * liquid crystals [26] and near the substrates of a cell filled with undulating cholesteric liquid crystals [27]. The asymmetry can be caused by different reasons, including finite anchoring at the interface [27] and the chiral nature of the material.…”

Section: Radial Structuresmentioning

confidence: 71%

“…Similar asymmetric undulations have been observed in lyotropic 'rippled' phases [25], undulating chevron tips of chiral SmC * liquid crystals [26] and near the substrates of a cell filled with undulating cholesteric liquid crystals [27]. The asymmetry can be caused by different reasons, including finite anchoring at the interface [27] and the chiral nature of the material. The latter mechanism has been discussed for lyotropic phases in [28].…”

Section: Radial Structuresmentioning

confidence: 71%

Free-standing films of twist grain boundary TGB_Aand UTGB_C*liquid crystals studied by fluorescence confocal polarizing microscopy

et al. 2003

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The director structures, meniscus profile, and defects in free-standing films of the twist grain boundary TGB A and UTGB C* liquid crystals were studied. The films were characterized by a combination of polarizing microscopy and fluorescence confocal polarizing microscopy. Five principal regions of meniscus were distinguished. When film thickness in the meniscus is much smaller then the TGB pitch, there is no difference between the free-standing films of TGB and ordinary smectic A liquid crystals. When the film thickness is larger than the TGB pitch, filamentary texture is observed. The 3D director pattern of the filaments are similar to the ground state director fields of TGB A and UTGB C* liquid crystals. In the intermediate thickness region of the meniscus, when the film thickness and TGB pitch are commensurate, a unique radial pattern is observed. Based on the fluorescence confocal polarizing microscopy studies of the director field, we propose a model for the 3D director structure in this part of the meniscus.

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“…Experimental studies of undulations of two dimensional and three dimensional systems were performed in [10] and [12], respectively. They used cholesteric liquid crystals with a pitch 5µm and 50 − 70µm cell thickness for the optical study.…”

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confidence: 99%

Layer Undulations in Smectic A Liquid Crystals

Garcı́a-Cervera¹,

Joo²

2010

Jnl of Comp & Theo Nano

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Abstract. We investigate instabilities of smectic A liquid crystals when a magnetic field is applied in the direction parallel to smectic layers. We use the Landau-de Gennes model of smectic A liquid crystals to characterize the critical magnetic field. When smectic A liquid crystals are confined between parallel plates, we derive analytic estimates for the magnetic field strength, at which the undeformed state loses its stability. We also present numerical simulations to confirm the Helfrich-Hurault effect due to the applied magnetic field.1. Introduction. We consider smectic A liquid crystals confined in two flat plates and uniformly aligned in a way that the smectic layers are parallel to the bounding plates. If a magnetic field is applied in a direction parallel to the smectic layers, the instability occurs above the threshold magnetic field. When the magnetic field reaches the critical threshold, one can see periodic perturbation of the layers. This phenomenon is called the Helfrich-Hurault effect. (See [8] and [9].) We present the analytical estimate of the critical field and perform numerical simulations describing this effect by using the Landau-de Gennes model.Liquid crystal phases form when a material has a degree of positional or orientational ordering yet stays in a liquid state. In the nematic state, molecules tend to align themselves along a preferred direction with no positional order of the centers of mass. The unit vector field n, nematic director, represents the average direction of molecular alignment. Moreover, if the liquid crystal is chiral, n follows a helical pattern, with temperature-dependent pitch. Upon lowering the temperature, or increasing concentration, according to whether the liquid crystal is thermotropic or lyotropic, the nematic liquid crystal experiences a transition to the smectic A phase with molecules arranged along equally spaced layers. The molecules tend to align themselves along the direction perpendicular to the layers.The Helfrich-Hurault effect in a lamellar system can be caused by magnetic/electric field or by mechanical tension [4]. In this paper, we study the magnetic field driven instabilities in smectic A liquid crystals. Helfrich and Hurault proposed the model that can explain the periodic perturbations in cholesteric liquid crystals under a magnetic field or an electric field applied parallel to the helical axis ([8], [9]). They assumed that the layers are fixed at the cell boundaries, i.e., the undulations vanish at the boundaries. Still with this assumption, Stewart extended the classic Helfrich-Hurault theory to three dimensional finite samples of smectic A liquid crystals in [13]. However, he made an ansatz to simplify the problem.Experimental studies of undulations of two dimensional and three dimensional systems were performed in [10] and [12], respectively. They used cholesteric liquid crystals with a pitch 5µm and 50 − 70µm cell thickness for the optical study. Since the layer thickness of smectic A liquid crystals is in the nanometer range (nm), it i...

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Undulations in a confined lamellar system with surface anchoring

Cited by 34 publications

References 20 publications

Undulation instabilities in laterally structured magnetic multilayers

Undulation instabilities in laterally structured magnetic multilayers

Free-standing films of twist grain boundary TGB_Aand UTGB_C*liquid crystals studied by fluorescence confocal polarizing microscopy

Layer Undulations in Smectic A Liquid Crystals

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Undulations in a confined lamellar system with surface anchoring

Cited by 34 publications

References 20 publications

Undulation instabilities in laterally structured magnetic multilayers

Undulation instabilities in laterally structured magnetic multilayers

Free-standing films of twist grain boundary TGBAand UTGBC*liquid crystals studied by fluorescence confocal polarizing microscopy

Layer Undulations in Smectic A Liquid Crystals

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Free-standing films of twist grain boundary TGB_Aand UTGB_C*liquid crystals studied by fluorescence confocal polarizing microscopy