Liquid crystal phase structure visualization

We briefly discussed the origin and structure of liquid crystals in Section 4.13. The last decade has witnessed a surge of interest in liquid crystals because of their applications in display devices (devices that convert an electrical signal into visual information). The design of liquid crystal (LC) devices relies on the relation between the molecular structure and the phase behaviour (relative smectic-nematic tendency, NI etc.) as well as the physical properties of the liquid crystals (Chandrasekhar, 1994). 

Cholesteric liquid crystals are compounds that go through a transition phase in which they flow like a liquid, yet retain much of the molecular order of a crystalline solid. Liquid crystals are able to reflect iridescent colors, depending on the temperature of their environment. Because of this property they may be applied to the surfaces of bonded assemblies and used to project a visual color picture of minute thermal gradients associated with bond discontinuities. Cholesteric crystals are potentially a simple, reliable, and economical method for evaluating bond defects in metallic composite structures.f Materials with poor heat-transfer properties are difficult to test by this method. The joint must also be accessible from both sides. ... 

Liquid crystal materials may exhibit birefringence (discussed in Section 2.5), so polarized optical microscopy is an ideal technique for visualization of liquid crystal textures. Samples are prepared as a thin film ( 2-20 pm thick) between glass plates. By observing the interesting defect textures that may form in each phase, it is often possible to make accurate phase identifications by microscopy alone, even though the microscope is unable to resolve the actual molecular packing structure. 

Visual observation using a microscope is valuable both for monitoring a sample and as an analytical tool. Phases such as liquid, vapor, and solid are clearly identifiable. Even different solid phases usually have distinctive appearances that allow them to be distinguished from each other. In many cases, visual observation is all that is required for determining phase relationships of a chosen system over a range of P and T. Not only can relative sizes of the phases be used to measure their relative abundances, but optical properties such as refractive index can be used as an indication of changes in crystal structure (e.g., quartz) and composition (e.g., albite melt). Visual observation in conjunction with other analytical techniques is important as well. For instance, when fluorescent diamonds are selected for use in studies using an intense X-ray beam, visual observation can provide information about the location of the X-ray beam with respect to the sample. 

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