| 1. |
- Elamain, Omaima Abubakr, et al.
(författare)
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Alignment and alignment transition of bent core nematics
- 2013
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on the alignment of nematics consisting of bimesogen bent core molecules of chlorine substituent of benzene derivative and their binary mixture with rod like nematics. It was found that the alignment layer made from polyimide material, which is usually used for promoting vertical (homeotropic) alignment of rod like nematics, promotes instead a planar alignment of the bent core nematic and its nematic mixtures. At higher concentration of the rod like nematic component in these mixtures, a temperature driven transition from vertical to planar alignment was found near the transition to isotropic phase. (C) 2013 AIP Publishing LLC.
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| 2. |
- Elamain, Omaima Abubakr, et al.
(författare)
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Field-induced optically isotropic state in bent core nematic liquid crystals: unambiguous proof of field-induced optical biaxiality
- 2013
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Ingår i: Journal of Physics D-Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 46:45
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Tidskriftsartikel (refereegranskat)abstract
- The behaviour of bent core (BC) nematic liquid crystals was investigated under dc applied electric field. The optically isotropic state of a sample containing BC nematic was observed under application of low dc electric fields. The quality of the dark state when the sample was inserted between two crossed polarizers was found to be superb and it did not change when rotating the sample between the polarizers. The coupling between the net molecular dipole moment and the applied dc electric field was considered as the origin of the out-of-plane switching of the BC molecules resulting in switching from the field-off bright state to the field-on dark state. The field-induced optically isotropic state is an unambiguous proof of the field-induced biaxiality in the BC nematic liquid crystal. A simple model explaining the appearance of the isotropic optical state in BC nematics and the switching of the sample slow axis between three mutually orthogonal directions under dc applied electric field is proposed.
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| 3. |
- Elamain, Omaima Abubakr
(författare)
-
Natural science specialising in physics
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A new class of liquid crystal materials has been recently discovered. These mate- rials are constituting of bent core (BC) molecules. Being initially only of scientific interest, now they attract the interest not only of scientists but also of engineers because of their great potential for applications in liquid crystal displays (LCDs) and photonics devices. The present study focused mainly on pure BC nematics and their binary mixtures (BC/RL) with a rod-like (RL) nematic related to their a) anchoring properties with respect to a solid surface in contact with these materials b) general behavior under an applied dc electric field, and particular electro-optic response due to the linear coupling with the applied field c) polar electro-optic response due to flexoelectricity. The conventional methods for obtaining vertical alignment (VA) of the RL nematics were unable to promote VA of the BC and their binary (BC/RL) mixtures, for the concentration less than 20wt%, instead they adopted planar alignment (PA). In concentration higher than 20wt%, the binary mixtures adopted VA in a very narrow temperature interval below the transition to the isotropic phase; where anchoring transition from VA to PA was observed upon cooling. Interestingly, field-induced optically isotropic state and switching of the sample optic axis between three mutually orthogonal directions were observed in BC and BC/RL, with concentration less than 40wt%. Due to the similarity in the molecular structures, the above observations were compared with the one observed in anti-ferroelectric liquid crystals (AFLC) with molecular tilt close or equal to 45◦. Periodic domain pattern and helical molecular order, with helix axis parallel to the cell substrate and normal to the domains, were found to be formed under an applied dc electric field, as a result of flexoelectricity of the BC materials. Polar flexoelectric response was detected in these materials when subjected to an out-of plane or in-plane low frequency electric field. Keywords: Liquid Crystal, Nematics, Bent Core Molecules, Electro-Optics, Biaxial, Flexoelectricity
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| 4. |
- Elamain, Omaima Abubakr, et al.
(författare)
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Optically isotropic state in bent core nematic mixtures with rod like molecules induced by direct current electric field
- 2013
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:16
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Tidskriftsartikel (refereegranskat)abstract
- One of the most important characteristic of the liquid crystal displays is the contrast of the generated images. The dark state of the display is a prerequisite for a high image contrast. Optically isotropic state was observed in bent core nematic mixtures containing rod like molecules induced by low applied dc electric field. It is found that the switching properties of these nematic mixtures depend on the concentration of the rod like molecules. Comparatively high concentrations of rod like molecules in the mixtures resulted in an improvement of the contrast and switching properties of the bent core nematic mixtures, thus demonstrating their potential for display applications.
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| 5. |
- Elamain, Omaima Abubakr, et al.
(författare)
-
Periodic pattern formation in an achiral bent core nematic
- 2018
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Ingår i: Aip Advances. - : AIP Publishing. - 2158-3226. ; 8:9
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Tidskriftsartikel (refereegranskat)abstract
- We investigated a periodic flexoelectric domain pattern, which appeared as regular parallel stripes in an achiral bent core nematic liquid crystal when dc electric field was applied. We found that such a pattern was first formed at the substrate surface and took place in sandwich cells with a gap larger than 2 mu m. The field-induced periodic pattern was preserved in the field-off state by a polymer network formed in the cell and was found to exhibit a polar as well as a linear electro-optic response due to in-plane switching of the sample optic axis. A comparison between this response and the one obtained in short cholesteric liquid crystals, aligned in ULH (uniform lying helix) texture, and short pitch ferroelectric liquid crystals, respectively, suggested that a heliconical molecular order is most probably formed in the field-induced periodic stripe pattern, with the helix axis orthogonal to the stripes. To our knowledge, this is the first experimental evidence of field-induced chiral-symmetry breaking in the flexoelectric periodic stripe pattern in achiral bent core nematic, resulting in heliconical molecular order, resembling the one of twist-bend (TB) nematic phase in this kind of nematics. (C) 2018 Author(s).
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| 6. |
- Elamain, Omaima Abubakr, et al.
(författare)
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Polar flexoelectric in-plane and out-of-plane switching in bent core nematic mixtures
- 2016
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Ingår i: Japanese Journal of Applied Physics. - : IOP Publishing. - 0021-4922 .- 1347-4065. ; 55:7
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Tidskriftsartikel (refereegranskat)abstract
- Polar electro-optic response, arising from the coupling between an applied in-plane and out-of-plane dc electric field, respectively, and the flexoelectric polarization of bent core nematic liquid crystal mixtures with hybrid alignment is studied in conventional sandwich cells with homeotropic anchoring at one of the cell substrates and planar at the other. Such a hybrid alignment, however, results in a splay/bend elastic deformation of the nematic giving rise of a flexoelectric polarization. It was found that a pronounced polar electro-optic response, both in-plane and out of plane, took place in the bent core nematic mixtures at very low voltages due to the high flexoelectric polarization in these mixtures, compared with the one observed in calamitic liquid crystals.
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