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Langmuir-Blodgett Techniques
Published in Günter Mahler, Volkhard May, Michael Schreiber, Molecular Electronics, 2020
The high-surface-pressure phase LS is optically isotropic in the plane, so that it is impossible to image its textures. However for the other phases it has proved possible to photograph the textures. Figure 9 shows computer-generated images of one of the domain textures observed in monolayer phases with tilted molecules using fluorescence (77) and Brewster-angle (78) microscopy. The contrast is due to the tilt-related optical anisotropy. The continuous change of tilt axis throughout the domain interior and its constant orientation to the boundary are characteristic for mesophases. This particular texture is also known in bulk liquid crystals (79) and is called a “boojum” because its morphology is identical to that of the defect of the same name in superfluid helium-3.
Effect of phase transitions on liquid crystal colloids: a short review
Published in Liquid Crystals Reviews, 2020
Kottoli Poyil Zuhail, Matjaž Humar, Surajit Dhara
Zuhail et al. studied the effect of N-SmA phase transition on boojum colloids [42]. The colloids are treated with N-Trimethoxysilylpropyl- N,N,N-trimethylammonium chloride (MAP) for planar anchoring. Figures 12(a–c) show the polarising optical microscope images of a boojum particle and the transformation of the defects across the N-SmA phase transition in 8CB LC. The elastic distortion increases along the rubbing direction as the N-SmA transition is approached (Figure 12(b)). In the SmA phase, the distortion is further extended and a high contrast tail with a straight dark-line on both poles of the particle along the rubbing direction is observed. This is somewhat similar to the focal line that was observed in the case of hyperbolic hedgehog defect in dipolar particles across the N-SmA transition (Figure 5(e)). One important difference is that, the point defect moves toward the surface of the dipolar particle just before the transition, whereas similar movement is not observed in the case of boojum colloid as the points are always residing on the surface of the particle. The brighter regions in the neighbourhood of the particle in the SmA phase indicate that the layers are strongly bent. For investigating the molecular orientation around the particle, a low birefringence LC (CCN-47, ) was chosen for analysing λ-plate images. In such samples, the second-order retardation effect due to increased birefringence of the SmA phase is easily avoided. It exhibits N-SmA phase transition at 28.2C. Figure 13 shows some snapshots across the N-SmA phase transition. The bluish and yellowish colours correspond to the clockwise and anti-clockwise rotation of the director from the rubbing direction as mentioned previously. By comparing Figure 13(a–c), a schematic representations of LC director around the particle in the N and SmA phases (with focal conic lines) are shown in Figure 12(d,e), respectively. These focal conic lines are parallel to the rubbing direction, being less and less visible far from the microsphere. In the simulation, when the surface anchoring is weak, two point-like defects are observed in the SmA phase [40]. At high anchoring strength, the defects in the SmA phase spread over a large range and look similar to focal conic lines. Here the focal lines are formed due to the angular discontinuity of the SmA layer orientation.