Smectic

Smectic is a term used in the field of liquid crystals to describe a family of mesophases in which elongated molecules exhibit both orientational order and one-dimensional positional order. In smectic phases, molecules tend to align with a common direction, and they form evenly spaced layers. Within a layer, the molecules can move laterally as in a two-dimensional liquid, while movement perpendicular to the layer is constrained by the layer structure.

The best-known smectic subphases are Smectic A (SmA), where molecules on average stand perpendicular to the layer planes, and Smectic C (SmC), where the molecules are tilted at a fixed angle relative to the layer normal. There are other, more ordered subphases as well, such as Smectic B (SmB), which can display hexatic ordering within layers. In some chiral compounds, ferroelectric smectic C* (SmC*) phases occur, in which molecular tilt is coupled to a spontaneous polarization, enabling distinctive fast electro-optic switching.

Formation and properties: Smectic phases typically form upon cooling from the isotropic or nematic phase. Layer spacing is on the order of a few nanometers, roughly corresponding to molecular length. Smectics exhibit optical anisotropy and birefringence, and their elastic behavior is strongly influenced by layer compression and bending, while interlayer sliding imparts a degree of fluidity.

Applications and significance: Smectic phases underpin certain liquid-crystal devices, including displays. SmC* ferroelectric materials are of particular interest for fast, low-voltage electro-optic applications. Beyond technology, smectics are studied for insights into phase transitions, defects, and soft matter physics.