Mesomorphism

Mesomorphism refers to the intermediate state of matter that exists between a crystalline solid and an isotropic liquid. Materials exhibiting mesomorphism are known as liquid crystals. Unlike conventional solids, which have molecules arranged in a fixed, ordered lattice, and liquids, where molecules are randomly oriented, liquid crystals possess a degree of molecular order while retaining fluidity. This partial order can manifest in various ways, leading to different types of mesophases.

The most common types of liquid crystalline phases include nematic, smectic, and cholesteric. In the nematic phase, molecules tend to align along a common direction, called the director, but have no positional order. Smectic phases exhibit both orientational order and some degree of positional order, typically forming layers. Cholesteric phases, also known as chiral nematic phases, are characterized by helical structures formed by molecules with chirality.

Mesomorphism arises from anisotropic molecular shapes, often rod-like or disc-like, and specific intermolecular interactions. These materials are highly responsive to external stimuli such as electric fields, magnetic fields, and temperature. This sensitivity is the basis for their widespread applications, most notably in liquid crystal displays (LCDs) used in televisions, computer monitors, and mobile devices, where their ability to change optical properties under an electric field is exploited. Other applications include optical sensors, thermometers, and in some advanced materials.