| 1. |
- Alexe-Ionescu, Anca-Luiza, et al.
(författare)
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Anchoring of nematic liquid crystals on a thin polymeric film
- 2008
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Ingår i: PHYSICAL REVIEW E. - 1539-3755. ; 77
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the influence of a thin polymer layer with lateral side groups on the anchoring of a nematic liquid crystal. We show that the effective anisotropic part of the anchoring energy depends on the coupling of the nematic with the polymer side groups, as well as on the coupling of the polymer side groups with the surface. The relaxation time for the nematic orientation induced by an external field is evaluated by considering the surface dissipation connected to the relative motion of the nematic director with respect to the polymer side groups.
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| 2. |
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| 3. |
- Ata Alla, Rasha Mohamed, et al.
(författare)
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Light-control of liquid crystal alignment from vertical to planar
- 2013
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 102:23
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Tidskriftsartikel (refereegranskat)abstract
- Continuous or discontinuous transition from vertical to planar alignment was found in nematic cells, containing polyimide alignment layer exposed to linear polarized UV light, depending on whether the alignment layer was mechanically rubbed or not prior the illumination. The polyimide material is designed originally to promote vertical alignment when deposited onto the inner cell substrates' surface. The mechanism behind these observations is complicated. It includes inclination of the alkyl side chains of the polyimide alignment layer towards the rubbing direction and the bond breaking process due to UV illumination which are considered as the key factors for occurrence of such alignment transition.
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| 4. |
- Ata Alla, Rasha Mohamed, et al.
(författare)
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Thickness dependence of the anchoring energy of a nematic cell
- 2013
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 113:16
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Tidskriftsartikel (refereegranskat)abstract
- The cell gap thickness dependence of the anchoring energy strength of a nematic cell is investigated. The relaxation time of the elastic deformation of a nematic liquid crystal layer imposed by an external electric field is analyzed as a function of the thickness of the nematic layer. The measurements have been performed with conventional sandwich cells with cell gap ranging from 1 μm to 10 μm. According to our data, the anchoring energy strength is not only a characteristic of the interface substrate-nematic liquid crystal but it also depends strongly on the cell thickness.
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| 5. |
- Barbero, Giovanni, et al.
(författare)
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Continuum description of the interfacial layer of nematic liquid crystals in contact with solid surfaces
- 2009
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Ingår i: J. Chem. Phys.. - : AIP Publishing. - 0021-9606. ; 130, s. 174902, 1-10
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Tidskriftsartikel (refereegranskat)abstract
- We investigate when it is possible to introduce surface physical parameters characterizing the nematic/substrate interface. The analysis is performed by solving the problem assuming that the presence of the surface introduces a spatial variation, mainly localized close to the limiting surfaces, of the bulk properties of the nematic (delocalized model). The results of the calculation are compared to the prediction of a model in which the presence of the surface is taken into account by means of new physical parameters, localized to the surface (localized model). We show that if the viscous dissipative effects or the surface alignment effects are considered, the two models predict the same relaxation times and the same threshold for the Freedericksz transition is obtained. From these results we deduce that the localized models are equivalent to the delocalized ones. A continuum description of the interfacial layer of nematic liquid crystals in contact with solid surface in terms of surface properties is then correct, which makes the solution of this kind of problems simpler. Also a softening of the elastic constants near the surfaces can be represented by a localized surface energy term.
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| 6. |
- Barbero, G., et al.
(författare)
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Effect of a surface polarized layer on the optical properties of a nematic cell
- 2023
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Ingår i: Journal of Molecular Liquids. - 0167-7322. ; 384
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Tidskriftsartikel (refereegranskat)abstract
- The optical properties of a cell of nematic liquid crystal presenting a tilted surface polarization submitted to an electric field are theoretically investigated. The origin of the surface polarization is assumed to be related to the interaction of the nematic molecules with the substrate and to the polar order due to the cell filling procedure. The analysis shows that the coupling of the surface polarization with the external field has different effects on the two surfaces limiting the sample. On one of them, the coupling gives rise, in an unbounded sample, to a continuous variation of the easy angle, whereas on the other one it is responsible for a first-order like transition of the easy axis orientation. For a sample of finite thickness our analysis shows that in the presence of a coupling between the two surfaces, operated by the elastic properties of the liquid crystal, the abrupt variation of the easy axis on one of the surfaces induces also on the other surface a discontinuity on the nematic surface orientation. The optical transmission of the cell versus the amplitude of the applied voltage, for an impinging monochromatic linearly polarized light, presents a discontinuity for the critical voltage at which the first-order transition of the surface tilt angles appears.
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| 7. |
- Barbero, Giovanni, et al.
(författare)
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Nonmonotonic behavior of the nematic tilt angle in a temperature-induced surface transition
- 2008
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Ingår i: PHYSICAL REVIEW E. ; 77
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Tidskriftsartikel (refereegranskat)abstract
- We report on a nonmonotonic temperature behavior of the pretilt in cells containing polymers as alignment layers. The latter are very promising since they enable control of the anchoring of the liquid crystal. We show that such a behavior cannot be interpreted by the standard Landau or mean-field theory. We propose a generalization of the mean-field model, including the temperature dependence of the anchoring due to the polymer layer that enables a description of the nonlinear behavior. The agreement between the predictions of the model and the experimental data is good.
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| 8. |
- Barbero, Giovanni, et al.
(författare)
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Temperature-induced tilt transition in the nematic phase of liquid crystal
- 2009
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Ingår i: JOURNAL OF APPLIED PHYSICS. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 105, s. 064516, 1-4
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Tidskriftsartikel (refereegranskat)abstract
- A temperature-induced transition from homeotropic to tilted alignment in the nematic phase of a liquid crystal with smectic C-nematic phase sequence was observed. The homeotropic alignment of the nematic phase at temperatures near the nematic-isotropic transition was found to undergo a continuous change to tilted one on decreasing the temperature. The appearance of smectic-C-like order at the solid surface on approaching the smectic C-nematic transition is assumed to be the origin of this temperature-induced alignment transition.
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| 9. |
- Bubnov, Alexej, et al.
(författare)
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Polar liquid crystalline monomers with two or three lactate groups for the preparation of side chain polysiloxanes
- 2006
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Ingår i: Liquid Crystals. - : Informa UK Limited. - 0267-8292 .- 1366-5855. ; 33:5, s. 559-566
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Tidskriftsartikel (refereegranskat)abstract
- Two new chiral liquid crystalline monomers with bilactate or trilactate chiral units were synthesized and studied. The monomers show the paraelectric SmA and ferroelectric SmC* phases. The antiferroelectric phase was detected only for the monomer possessing the bilactate unit. The temperature dependences of the spontaneous polarization and spontaneous tilt angle were measured. Real and imaginary parts of the complex permittivity were studied as a function of frequency and temperature. The synthesized monomers contain a double bond at the end of the achiral terminal chain, and were used to prepare liquid crystalline polysiloxanes.
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| 10. |
- Drevensek-Olenik, I., et al.
(författare)
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Ferroelectric switching of electrically commanded alignment layers for liquid crystal displays
- 2006
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Ingår i: J.Appl.Phys.. - : AIP Publishing. - 0021-8979. ; 100
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Tidskriftsartikel (refereegranskat)abstract
- Optical second harmonic generation (SHG) was used to probe the structural and dynamic properties of electrically commanded alignment layers for liquid crystal displays. The layers were 200 nm thick and were made of siloxane-based side chain ferroelectric liquid crystal polymer (FLCP). The effect of an external dc electric field in the range of 0–2 V/µm on the SHG signal was probed in an empty sandwich cell and in a cell filled with an isotropic liquid (hexadecane). In both cases the nonlinear induced polarization shows characteristics of the "V-shaped" switching, which is associated with a relatively minor reorientational perturbation of the film. This signifies a presence of domain structure, which is strongly pinned to the substrates. The values of switching times deduced from the SHG response are two orders of magnitude larger from the values obtained by optical birefringence measurements. This observation suggests that in the top surface layer of the FLCP, which is mainly responsible for the birefringence and also for the so-called electrically commanded surface effect, the reorientation of the polymer side chains takes place much faster than in the regions close to the substrate. ©2006 American Institute of Physics
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