liquid crystals

liquid crystals. They are a state of matter that exhibits properties between those of conventional liquids and solid crystals. A key characteristic is their anisotropy, meaning their physical properties, such as refractive index and electrical conductivity, depend on direction. This unique behavior arises from the partial ordering of their constituent molecules, which can be rod-like or disc-like in shape, allowing them to flow like a liquid while maintaining some long-range orientational order.

Properties and phases

The distinct phases are defined by the type and degree of molecular order. In the isotropic phase, molecules are randomly oriented as in a normal liquid. Upon cooling, a transition often occurs to a nematic phase, where molecules exhibit long-range orientational order, aligning their long axes in a common direction described by a director, but have no positional order. Further ordering leads to smectic phases, where molecules arrange into layers; within these layers, molecules may have varying degrees of positional order, as seen in smectic A and smectic C phases. Cholesteric phases, also called chiral nematics, exhibit a helical twist of the director, leading to unique optical properties like selective reflection of light. The transitions between these phases are studied within the framework of Landau theory and are often sensitive to external fields from devices like a Kerr cell.

Types and classification

Liquid crystals are broadly classified as lyotropic or thermotropic. Lyotropic phases are formed by dissolving amphiphilic compounds, such as soaps or lipids, in a solvent like water; their structure depends on concentration and temperature, with common phases being lamellar, hexagonal, and cubic. Thermotropic phases, which change with temperature, are further divided by molecular shape. Calamitic liquid crystals have rod-like molecules and include most common nematic and smectic materials. Discotic liquid crystals, composed of disc-shaped molecules like those in triphenylene derivatives, can form columnar phases where stacks of discs organize into hexagonal arrays. Bent-core molecules, or banana liquid crystals, often exhibit polar order and complex phases like B7 phase. Polymer-based and liquid crystal elastomers combine orientational order with rubber elasticity.

History and discovery

The discovery is credited to the Austrian botanist and chemist Friedrich Reinitzer, who in 1888 observed two distinct melting points in cholesteryl benzoate while working at the Charles University in Prague. He noted the material first became a cloudy liquid and then cleared at a higher temperature, a phenomenon he discussed with the German physicist Otto Lehmann of the University of Karlsruhe. Lehmann conducted extensive optical microscopy studies, coining the term "flüssige Kristalle" (liquid crystals). Early theoretical work was advanced by the Soviet physicist Vsevolod Frederiks, who discovered the Freedericksz transition, a fundamental realignment effect in magnetic fields. The modern era began with George William Gray's synthesis of stable room-temperature materials at the University of Hull, which were crucial for the work of James Fergason and the team at RCA Laboratories under George Heilmeier, leading to the first practical liquid crystal display.

Applications and technology

The most widespread application is in liquid crystal displays used in devices from digital watches and calculators to televisions and smartphones, leveraging the electrically controlled birefringence of nematic materials in configurations like the twisted nematic field effect. Polymer dispersed liquid crystal films are used in smart windows and privacy glass. Beyond displays, cholesteric materials are used in thermography for temperature-sensitive coatings and in laser applications due to their photonic band gap. Lyotropic systems are fundamental to the soap industry and the study of biological membranes, with structures like the lipid bilayer being a key example. Ferroelectric liquid crystals, discovered by Robert Meyer, offer fast switching for microdisplays. Other uses include optical phased arrays for LIDAR and sensors in devices like the Mauguin parameter waveplate.

Physical theory and modeling

The continuum theory is predominantly described by the Frank–Oseen elastic continuum theory, which quantifies the energy costs of splay, twist, and bend deformations through elastic constants. The dynamics of director reorientation are governed by the Ericksen–Leslie equations, which include viscosity coefficients. For phase transitions and order parameters, Landau–de Gennes theory uses a tensorial order parameter, expanding on the simpler Maier–Saupe mean field theory for nematics. Computer simulations, such as Monte Carlo methods and molecular dynamics, are employed to study these materials at the atomistic level, often using Gay–Berne model potentials for anisotropic interactions. Key experimental probes include nuclear magnetic resonance spectroscopy, X-ray scattering at facilities like the Advanced Photon Source, and observations of topological defects like disclinations.

Category:Condensed matter physics Category:Phases of matter Category:Optical materials