We have performed the first measurements on surface-aligned samples of the blue phase III of chiral liquid crystals and have tested proposed icosahedral, cubic-domain, and double-twist structures for blue phase III. Angular-reflection studies on surface-aligned samples show no evidence of expected icosahedral structure; uv reflection measurements on unaligned samples show no evidence of expected cubic-domain structure; and attempts to observe Cano fringes show no evidence for any spatial periodicity.PACS numbers: 61.30. Eb, 61.30.Gd Although the blue phases of chiral liquid crystals have been intensively studied,^ the structure of the highertemperature blue phase III (BP III) remains one of the outstanding problems of liquid-crystal physics.^ The lower-temperature blue phases-blue phase I (BP I) and blue phase II (BP II)-evidently have body-centered cubic (bcc) and simple-cubic structure, respectively. These phases are examples of condensed matter structures in which frustration modifies, but does not disrupt, the long-range order by creating a regular lattice of disclinations.^ For BP III, the role of frustration is not clear, but it may actually cause destruction of the long-range order by creating a tangle of disclinations while still allowing considerable order on the scale of the chiral pitch.BP III is visually amorphous, optically active, and has a broad reflection peak which is only weakly aff'ected by moderate electric fields. Its order is thought to be closer to the cubic blue phases than to the isotropic phase. Proposed structures include the quasicrystal model,^"^ in which the structure is quasiperiodic with reciprocallattice vectors derived from the twelve vertex and thirty edge vectors of an icosahedron; the double-twist structure,^ which suggests that BP III may be a spaghettilike tangle of "double-twist" cylinders (the order parameter in a double-twist cylinder is uniaxial, with a principal axis which spatially rotates about any diameter of the cylinder and whose uniaxiality decreases with increasing distance from the cylinder axis); and a class of structures we collectively call cubic-domain structures in which BP III consists of small domains or correlated regions having cubic symmetry-either bcc, like BP I,^ or simple cubic, like BP 11.^ Optical studies should be able to test the validity of these structures. For unaligned BP III, the wavelengths of higher-order reflection peaks give information on various crystal symmetries. Thus if the longest observable reflection wavelength is XQ and we ignore polarization selection rules, the cubic-domain structure predicts higher-order Bragg peaks at X/X^^ -X/Jl, 1/V37. . .. The quasicrystal model predicts higher-order peaks at XAo = 1.62, 1.05, 1, 0.85, 0.73, 0.71, 0.62, 0.59, and 0.58, where XQ is now the wavelength corresponding to the reflection from an icosahedral edge vector.'* Filev^ has claimed, however, that the only reflections with significant amplitude are for A,Ao=1.05, 1, and 0.62. Finally, the double-twist structure need not have higheror...
This study has investigated experimentally the effects of hydrogen addition and turbulence on the ignition and the flame-kernel development characteristics in isotropic and homogeneous turbulence for propane mixtures. In this study, the lean-and rich-fueled hydrogen added propane mixtures with different equivalence ratios (=0.5~1.4) and hydrogen additional rates ( H =0.0~1.0) are prepared, while maintaining the laminar burning velocity (S L0 =25 or 15 cm/s). First, in order to investigate the ignition and flame-kernel development in quiescence, the minimum ignition energy Ei min and the relationship between the flame radius and the burning velocity of meso-scale laminar flames in the range of flame radii r f approximately from 1 to 5 mm are examined quantitatively by using sequential schlieren photography in a constant volume vessel. Then, the experiments in isotropic and homogeneous turbulence are carried out for two turbulence level with the turbulence intensity u' being approximately 0.35 and 1.76 m/s. Ei min for each mixture is also defined experimentally at each u'. It is found that the effects of hydrogen addition and turbulence on the ignition and the burning velocity of meso-scale flame characteristics are much different between the lean and the rich hydrogen added propane mixtures. It also becomes clear that there is a good relationship between Ei min characteristics in turbulence and Lewis number or the Markstein number. Additionally, the transition region of the minimum ignition energy could be summarized regardless of , H and u'/S L0 by using the proposed turbulent Karlovitz number based on the burning velocity of the meso-scale flame in quiescence.
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