Smectics

Smectics are a class of liquid crystal phases characterized by long-range orientational order and one-dimensional translational order that forms layered structures. They flow like liquids within the layers but possess a periodic density modulation perpendicular to the layers, giving them a layered, semi-crystalline character.

The molecules in smectic phases are typically elongated. In Smectic A, the molecular long axes align on average perpendicular to the layer planes, while in Smectic C they are tilted with respect to the layer normal. Smectic B features more in-layer order, often with hexagonal packing and additional short-range translational order within the layers. Higher-order smectics (such as D and E) show increasingly complex layering and in-layer arrangements. The layer spacing is roughly equal to the molecular length but can vary with temperature and pressure. Smectics may host defects such as focal conic domains that reflect the layered structure.

Physically, smectic phases exhibit elastic properties tied to their layering, including layer bending and compression, described by elastic constants. They can display distinctive optical textures and birefringence. Some smectic phases, particularly chiral ones like SmC*, are ferroelectric, enabling fast electro-optic switching. Transitions between nematic and smectic phases can be direct or proceed via intermediate states and may be first- or second-order depending on material and conditions.

Smectics are encountered in various thermotropic liquid crystals and have been studied extensively in research. In practical display technologies, nematic and cholesteric phases are more common, but smectic materials are important for specialized optoelectronic and nonlinear optical applications.