Free-standing films of twist grain boundary TGB_Aand UTGB_C*liquid crystals studied by fluorescence confocal polarizing microscopy

Abstract

The director structures, meniscus profile, and defects in free-standing films of the twist grain boundary TGB A and UTGB C* liquid crystals were studied. The films were characterized by a combination of polarizing microscopy and fluorescence confocal polarizing microscopy. Five principal regions of meniscus were distinguished. When film thickness in the meniscus is much smaller then the TGB pitch, there is no difference between the free-standing films of TGB and ordinary smectic A liquid crystals. When the film thickness is larger than the TGB pitch, filamentary texture is observed. The 3D director pattern of the filaments are similar to the ground state director fields of TGB A and UTGB C* liquid crystals. In the intermediate thickness region of the meniscus, when the film thickness and TGB pitch are commensurate, a unique radial pattern is observed. Based on the fluorescence confocal polarizing microscopy studies of the director field, we propose a model for the 3D director structure in this part of the meniscus.

Structures in the meniscus of smectic membranes: the role of dislocations?

“…To end this section, let us mention that very similar striped patterns, with a varying periodicity, were also reported in the meniscus of twist grain boundary (TGB) LCs featuring SmA and SmC* blocks. 13 3.1.2 Focal conic domains. As we go further away from the film, the stripes give way to a two-dimensional (2D) ordered array of focal conic domains (FCDs).…”

“…Seemingly similar splay domains, together with additional features, were also noticed by Maclennan in the meniscus of smectic ferroelectric compounds. 11 Dhara et al 12 and Smalyukh et al 13 further explored the behavior of FSFs made of twist grain boundary TGB A and undulating twist grain boundary UTGB C* compounds. Optical microscopy observations reveal that the meniscus of these LCs also exhibits radial stripes with a varying periodicity that depends on the meniscus thickness.…”

“…14 As in ref. 13, the authors attributed the modulation to bulk layer undulations due to dilative strain caused by the meniscus curvature, C, which induces a transverse pressure gradient equal to Dp ¼ p air À p smectic ¼ g sa C 4 0, where g sa is the smectic-air surface tension. And it is well-known that smectics can minimize their energy through a buckling of the layers above a critical dilative strain.…”

Alignment and electrooptic effects in nanoparticle-doped nematic liquid crystals

“…[8-10, 12, 32] Samples, without or with only few of these birefringent stripes show even an inverted electrooptic switching characteristic. First, conventional polarizing optical microscopy (POM) and fluorescence confocal polarizing microscopy (FCPM) [33][34][35][36] were applied to clarify the origin and director field topology of the birefringent stripes observed in these homeotropic samples. The latter method allows three-dimensional imaging of the local director orientation by measuring the fluorescence intensity of a dichroic dye dissolved in the liquid crystal.…”

Enhanced confocal microscopy imaging of the in-plane switching of cholesteric liquid crystal cells

“…To ensure that only light emitted from the focal region of the confocal beam is collected, a pinhole is used at the focal point of the light emitted from the voxel of interest to allow a spatial resolution of better then 1 µm in all three dimensions. This technique has previously been used to examine such structures as twist-grain boundaries in free-standing liquid crystal films [16], defects in biaxial smectic-A materials [17] and focal-conic domains in self-assembled structures [18]. It has also been shown that FCPM can be used to probe the cholesteric structure (of pitch p) in a Grandjean-Cano wedge cell [19] provided that the parameter p∆n / 2λ is small enough to limit the influence of the Maugin effect on the resulting images.…”