Three-dimensional director structures of defects in Grandjean-Cano wedges of cholesteric liquid crystals studied by fluorescence confocal polarizing microscopy.
Three-dimensional director structures of defects in Grandjean-Cano wedges of cholesteric liquid crystals studied by fluorescence confocal polarizing microscopy.
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通过荧光共焦偏振显微镜研究胆甾型液晶 Grandjean-Cano 楔形缺陷的三维导向结构。
DOI：
10.1103/physreve.66.051703
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发表时间：
2002
期刊：
影响因子：
--
通讯作者：
O. Lavrentovich
中科院分区：
文献类型：
--
作者：
I. Smalyukh;O. Lavrentovich
我们使用一种非破坏性的荧光共聚焦偏振显微镜技术可视化三维导演图案的缺陷，在Grandjean-Cano楔填充与pitch p=5 microm的液晶。指向矢的强表面锚定导致体中位错的稳定晶格。光学切片的垂直截面的楔形，使我们能够建立详细的结构的位错和他们的扭结。Burgers矢量B=p/2的位错位于样品的薄部分，非常接近平分面。它们的核心分裂成一对τ（-1/2）和λ（+1/2）向错。当B=p/2位错形成扭结时，观察到一对λ（-1/2）和τ（+1/2）向错。沿着B=p/2位错的扭结使位错的水平改变+/-p/4和+/-p/2;这些扭结被限制在滑移平面内，并且非常长，（5-10）p。在楔形样品的某个临界厚度h（c）以上，位错具有伯格斯矢量B= p。它们经常被发现远离平分面。B=p位错的核心分裂成一对非奇异的λ（-1/2）和λ（+1/2）向错。沿着B=p位错的扭结具有典型的尺寸p，并在垂直于滑移面的方向上形成尖点。在尖点处，λ（-1/2）和λ（+1/2）向错交换末端。其他缺陷结构包括“莱曼簇”，即，由两个λ（-1/2）和两个λ（+1/2）向错形成的零Burgers矢量位错和具有分裂成两个以上向错的非零Burgers矢量位错。我们采用粗粒度的Lubensky-de Gennes模型来描述一些观察到的功能。我们计算的弹性能量的位错远离核心，估计的能量的核心分裂成不同类型的向错，研究有限的样品厚度上的位错能量的影响，并计算Peach-Koehler弹性力时发生的位错是从其平衡位置移动。楔形体中的扩张/压缩能量与位错能量的平衡定义了h（c）的值，并允许估计位错的核心能量。最后，我们考虑Peierls-Nabarro机制阻碍滑移的位错穿过的层。由于核的分裂向错特征，滑移与攀移相比是困难的，特别是对于B=p位错。
We use a nondestructive technique of fluorescence confocal polarizing microscopy to visualize three-dimensional director patterns of defects in Grandjean-Cano wedges filled with a cholesteric liquid crystal of pitch p=5 microm. Strong surface anchoring of the director causes a stable lattice of dislocations in the bulk. Optical slicing in the vertical cross sections of the wedges allows us to establish the detailed structure of dislocations and their kinks. Dislocations of Burgers vector b=p/2 are located in the thin part of the sample, very close to the bisector plane. Their cores are split into a pair of tau(-1/2) and lambda(+1/2) disclinations. Pairs of lambda(-1/2) and tau(+1/2) disclinations are observed when the b=p/2 dislocation forms a kink. The kinks along the b=p/2 dislocations change the level of dislocations by +/-p/4 and +/-p/2; these kinks are confined to the glide plane and are very long, (5-10) p. Above some critical thickness h(c) of the wedge sample, the dislocations are of Burgers vector b=p. They are often found away from the bisector plane. The core of b=p dislocations is split into a pair of nonsingular lambda(-1/2) and lambda(+1/2) disclinations. The kinks along the b=p dislocation are of a typical size p and form cusps in the direction perpendicular to the glide plane. At the cusp, lambda(-1/2) and lambda(+1/2) disclinations interchange ends. Other defect structures inlude "Lehmann clusters," i.e., dislocations of zero Burgers vector formed by two lambda(-1/2) and two lambda(+1/2) disclinations and dislocations of nonzero Burgers vector with a core split into more than two disclinations. We employ the coarse-grained Lubensky-de Gennes model of the cholesteric phase to describe some of the observed features. We calculate the elastic energy of a dislocation away from the core, estimate the energy of the core split into disclinations of different types, study the effect of finite sample thickness on the dislocations energy, and calculate the Peach-Koehler elastic forces that occur when a dislocation is shifted from its equilibrium position. Balance of the dilation/compression energy in the wedge and the energy of dislocations defines the value of h(c) and allows to estimate the core energy of the dislocations. Finally, we consider the Peierls-Nabarro mechanisms hindering glide of dislocations across the cholesteric layers. Because of the split disclination character of the core, glide is difficult as compared to climb, especially for b=p dislocations.