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Hybrid photosensitive structures based on nematic liquid crystals and lithium niobate substrates
摘要： Liquid crystal cells based on lithium niobate substrates have recently been proposed as good candidates for optofluidic devices and for light-induced controlled generation of defects in liquid crystal films. The peculiarity of these structures lies in the possibility of using the bulk photovoltaic effect of lithium niobate to obtain an optically induced dc field able to affect the molecular liquid crystal director. Reversible fragmentation and self-assembling of liquid crystal droplets driven by the lithium niobate pyroelectric properties have also been reported. We review the basic results obtained so far with the aim of making the point and seeing what else can be done in the framework of the realization of hybrid structures combining lithium niobate with the electro-optical and nonlinear optical properties of liquid crystals.
关键词： pyroelectric effect,optofluidic devices,lithium niobate,liquid crystals,photovoltaic effect
更新于2025-09-09 09:28:46
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Thermal and electrical wavelength tuning of Bragg reflection with ultraviolet light absorbers in polymer-stabilized cholesteric liquid crystals
摘要： Autonomous stimulus-responsive polymer-stabilized cholesteric liquid crystals (PSCLCs) are promising materials for various applications. In this work, a PSCLC cell whose wavelength of selective reflection changes autonomously according to the temperature or can be controlled by applying an electric field was fabricated by using ultraviolet (UV) light absorbers. The UV light absorbers in the PSCLC cell are used not only for preventing the degradation of the substrate but also for stabilizing the polymeric network by converting the harmful UV radiation into harmless heat. To achieve the optimum conditions, different types of UV light absorbers were examined at various concentrations and UV light intensities. The results revealed that the benzophenone-type UV absorbers provide an extensive blue-shift tuning of the Bragg reflection in the PSCLC. By determining the molar extinction coefficient (e) values, which provide a measure of UV light absorption at a given wavelength, at three important wavelengths—the maximum absorption wavelength (lmax) of the UV absorbers, lmax of the photoinitiator (345 nm), and the UV lamp wavelength (365 nm)—the UV light intensity gradient throughout the cell was defined. Reduction of the e value indicates a sharp drop in the UV light intensity within a PSCLC cell. This phenomenon leads to the diffusion of reactive mesogens towards the UV lamp side of the cell to achieve a tight pitch gradient and extensive blue-shift tuning of the selective reflection.
关键词： thermal tuning,blue-shift tuning,electrical tuning,UV light absorbers,polymer-stabilized cholesteric liquid crystals
更新于2025-09-09 09:28:46
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Liquid crystalline droplets in aqueous environments: electrostatic effects
摘要： We demonstrate the strong impact of electrostatic properties on radial–bipolar structural transitions in nematic liquid crystal (LC) droplets dispersed in different aqueous environments. In the experimental part of the study, we systematically changed the electrostatic properties of both LC droplets and aqueous solutions. Mixtures of nematics were studied by combining LC materials with negative (azoxybenzene compounds) and strongly positive (cyanobiphenyl) dielectric anisotropy. The aqueous solutions were manipulated by introducing either polyvinyl alcohol, glycerol, electrolyte or amphiphilic anionic surfactant SDS into water. In the supporting theoretical study, we identified the key parameters influencing the dielectric constant and the electric field strength of aqueous solutions. We also estimated the impact of different electrolytes on the Debye length at the LC–aqueous interface. The obtained results are further analysed for chemical and biological sensing applications.
关键词： electrostatic effects,aqueous environments,nematic droplets,liquid crystals,structural transitions
更新于2025-09-09 09:28:46
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MAXWELL STRESS INDUCED FLOW-DEFORMATION AND OPTICAL NONLINEARITIES IN LIQUID CRYSTALS (Invited Paper)
摘要： We present a critical account of intense pulsed-laser ?eld induced refractive index changes caused by ?ow, crystalline axis reorientation and distortion and other high order photonic processes in transparent liquid crystals. In particular, the optical nonlinearity associated with Maxwell Stress induced ?ow-reorientation in nematic liquid crystals is explicitly calculated, and their possibility for all-optical switching application is experimentally demonstrated. Similar ?ows processes have also been observed in Blue-Phase liquid crystals with nanosecond and picosecond pulsed-lasers.
关键词： all-optical switching,liquid crystals,optical nonlinearities,Blue-Phase liquid crystals,nematic liquid crystals,Maxwell Stress
更新于2025-09-04 15:30:14
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Physical Properties of Pure and Nano Ag Doped Liquid Crystalline Compounds Containing 1,3,4-Oxadizole Unit
摘要： Dielectric properties and other physical properties such as electrical conductivity (AC) and relaxation time or activation energy have been studied for two systems pure LC [V]6,6, [V]7,6, [V]8,6, [V]6,7, [V]7,7 and [V]8,7 and their doped with silver nanoparticles. The results show the increasing in real dielectric permittivity ? with increasing length terminal chain. So the real dielectric permittivity increasing with raising temperature. To compare between the values of (400) Hz and (4000) Hz we observe these values at the low frequency are larger than that in high frequency. Generally, one can see that the doping of Ag nano particles effectively reduced the permittivity of the LC materials with its large electric dipole moment. The electrical conductivity σ value for pure LC samples in general increases with increasing temperature. So we observed increasing of electrical conductivity values at high frequency. The time scale is discussed in terms of the Arrhenius plot. Generally, with increasing the temperature the time period that spends by molecules at the transition state will increase. The activation energy Ea values show the increase in the activation energy to the doped systems.
关键词： Dielectric permittivity,Doped liquid crystals,Liquid crystals,Relaxation time,Electrical conductivity,Activation energy,Silver nanoparticles
更新于2025-09-04 15:30:14
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Fabrication of vertical alignment liquid crystal cell without forming a conventional polyimide-alignment layer
摘要： A novel preparation method of the vertical alignment polymer film (VAPF) was proposed by polymerisation of the monomers, 2,3-difluoro-4?-[2-(diacryloyl-oxy-hexyl)methyl ester-hexadecyl-oxy]-biphenyl (DFDAMHB) and 2,7-dimethacryloyl-oxy-phenanthrene (DMAPhen), being dissolved in the liquid crystal (LC) material. For obtaining the vertical alignment (VA), the ultraviolet exposure was carried out to the LC cell below the nematic to isotropic transition temperature of the LC material. The VA mode LC cell with the VAPF produced from the monomers DFDAMHB and DMAPhen exhibited enough level of electro-optical and response properties, high voltage holding ratio and low residual direct current voltage.
关键词： vertical alignment polymer film,Liquid crystals,nematic to isotropic transition temperature,monomers,polymerisation
更新于2025-09-04 15:30:14
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Light energy accumulation by cholesteric liquid crystal layer at oblique incidence
摘要： Oblique propagation of light through a planar layer of a cholesteric liquid crystal (CLC) is solved by Ambartsumian’s modified layer addition method. Two cases are considered, namely, the case when dielectric boundaries have a minimum influence on light transmission and the case when the CLC layer is in a vacuum. It is shown that in the first case a total internal reflection can happen for the fast eigen-mode at large incidence angles. New important features of light reflection (transmission) spectra, photonic density of states and accumulated energy density in the CLC layer are studied. The light localisation peculiarities in the CLC layer at oblique incidence is investigated too. It is shown that the light localisation for both the long-wavelength edge mode and the short-wavelength edge mode changes differently when the incidence angle changes. The obtained results can be used in the design of low-threshold lasers, in solar cell systems, in chiral photonics, in systems strongly absorbing light at certain wavelengths and when designing systems with absorption suppression created on the base of absorbing media, etc.
关键词： total internal reflection,Cholesteric liquid crystals,photonic band gap,light localisation
更新于2025-09-04 15:30:14
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[IEEE 2018 Global Smart Industry Conference (GloSIC) - Chelyabinsk, Russia (2018.11.13-2018.11.15)] 2018 Global Smart Industry Conference (GloSIC) - Mesophase Materials as Smart Media for Emerging Pressure Sensors: Capacitive Method of Measurement of DC Conductivity
摘要： Liquid crystals are a perspective type of materials for application in the field of tactile pressure sensors owning to their unique physical properties. One of such properties is the anisotropic electric conductivity, the value of which can be controlled by the external electric field. In this report we will consider a capacitive method of the measurement of the direct current conductivity in the liquid crystal layer, confined in the cell with blocking electrodes. It will be demonstrated that this technique allows estimating the direct current conductivity of a liquid crystal in a broad temperature range. The retrieved results can be used
关键词： blocking electrodes,impedance spectroscopy,liquid crystals,electric conductivity
更新于2025-09-04 15:30:14
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Effect of the anchoring strength on the phase behaviour of discotic liquid crystals under face-on confinement
摘要： In this study we have performed molecular dynamics simulations to study a Gay-Berne discotic fluid confined in a slab geometry for a fixed confinement length. Four different anchoring strengths with a homeotropic (face-on) configuration were studied. We found that changing the anchoring strength changes the normal component of the stress tensor, which in turn changes the density of the system’s bulk. This phenomenon leads to a shift in the isotropic-nematic transition temperature. We observe that the temperature regions where the nematic phase is present diminishes as the anchoring strength increases. The anchoring strength also affects the nematic-columnar coexistence temperature-region: it spans over more temperatures at higher anchoring strengths.
关键词： molecular dynamics simulations,discotic liquid crystals,anchoring strength,phase behaviour,face-on confinement
更新于2025-09-04 15:30:14
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short-range order on the activation energies of translational and rotational viscosities of nematic liquid crystals with highly polar molecules
摘要： We report experimental studies on the temperature-dependent translational and rotational viscosities of a few nematic liquid crystals made of highly polar molecules. Cyanobiphenyl (nCB) with longitudinal and cyanobicyclohexane molecules (CCN-mn) with transverse dipole moments were chosen and the measurements were made in the nematic phase. The translational viscosity (shear viscosity) is measured by using a rheometer and the rotational viscosity is measured from the dynamics of the Freedericksz transition by measuring the optical phase-retardation decay of the sample. The associated activation energies are obtained from the ?tting of the appropriate equation in the nematic phase. Only 8CB and CCN-47 exhibits N to Sm-A phase transition and their activation energies of translational and rotational viscosities are larger compared to the shortest homologs in the respective series. Both activation energies of 8CB are signi?cantly larger than that of CCN-47. The temperature-dependent steady states of the director distribution under shear are comprehensibly re?ected in the rheodielectric measurements of 8CB than that of CCN-47. The results are explained based on the differences in the Sm-A short-range orders and the critical slowing down of the order parameter ?uctuations. We present a simple physical model of molecular association that gives rise to different Sm-A short-range orders in both classes of compounds.
关键词： nematic liquid crystals,rotational viscosity,activation energies,Sm-A short-range order,translational viscosity
更新于2025-09-04 15:30:14