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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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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
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Surface-induced transition of nematic liquid crystals on graphene/SiC substrate
摘要： A nematic liquid-crystal director aligns along the armchair direction of graphene grown on a SiC substrate. The temperature-dependent textural change in the nematic phase is di?erent from the usual texture on the alignment layer. The isotropic-nematic phase transition temperature decreases over time after cell fabrication. Tiny domains occur with the phase transition and appear to be merged from a critical temperature with decreasing temperature in the nematic phase. This is thought to be due to the transition occurring at the interface. On the other hand, the nematic liquid crystal in graphene grown on a Cu foil does not show textural changes, and the phase transition temperature does not decrease even after a long time has elapsed. X-ray ?uorescence measurements indicate that silicon atoms exist in the liquid crystal possibly extracted from the SiC substrate. A model of ?rst-order phase transition on the graphene surface has been proposed. This transition is accompanied by an inhomogeneous distribution of scalar order parameters and the transformation from a small multi-grained structure to a large domain distribution in the director ?eld. This structural transformation takes place at a temperature di?erent from that corresponding to the bulk transition. Such behavior may be explained by the adsorption process of silicon atoms in contact with the SiC-graphene interface.
关键词： surface-induced transition,phase transition,SiC substrate,nematic liquid crystals,graphene
更新于2025-09-04 15:30:14
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Geometric stabilisation of topological defects on micro-helices and grooved rods in nematic liquid crystals
摘要： We demonstrate how the geometric shape of a rod in a nematic liquid crystal can stabilise a large number of oppositely charged topological defects. A rod is of the same shape as a sphere, both having genus g = 0, which means that the sum of all topological charges of defects on a rod has to be (cid:2)1 it usually shows only one hyperbolic hedgehog or a Saturn ring defect with negative unit charge. Multiple unit charges can be stabilised either by friction or large length, which screens the pair-interaction of unit charges. Here we show that the curved shape of helical colloids or the grooved surface of a straight rod create energy barriers between neighbouring defects and prevent their annihilation. The experiments also clearly support the Gauss–Bonnet theorem and show that topological defects on helices or grooved rods always appear in an odd number of unit topological charges with a total topological charge of (cid:2)1.
关键词： topological defects,geometric stabilisation,micro-helices,nematic liquid crystals,grooved rods
更新于2025-09-04 15:30:14