Liquidcrystal

Liquid crystals are materials that exhibit properties between those of conventional liquids and solid crystals. In these substances, molecules retain mobility like a liquid but display some long-range order, such as a preferred orientation or layered structure. This combination produces anisotropic properties, notably directional optical and dielectric behavior.

The most common mesophases include nematic, smectic, and cholesteric (chiral nematic) phases. In the nematic phase, molecules align on average along a director but lack positional order. The smectic phase forms layered structures with varying in-layer order. Cholesteric liquid crystals are nematic in character but have a helical twist due to molecular chirality, giving selective reflection of light. Discotic liquid crystals consist of disk-like molecules that stack into columnar arrangements. Thermotropic liquid crystals change phase mainly with temperature, while lyotropic liquid crystals form ordered phases when dissolved in a solvent at sufficient concentration.

Properties of liquid crystals arise from their anisotropy, including refractive index differences (birefringence) and dielectric anisotropy. They respond strongly to electric and magnetic fields, enabling controlled reorientation of molecules. This makes them well suited for applications that rely on rapid, tunable optical changes.

Historically, the phenomenon was first observed by Friedrich Reinitzer in 1888 during studies of cholesterol derivatives. Otto Lehmann conducted foundational work and helped establish the field, with the term liquid crystal entering common use in the early 20th century. Today, liquid crystals underpin many technologies, most notably liquid-crystal displays used in screens and monitors, as well as a range of photonic and sensing applications.