Schlieren texture of Liquid Crystal nematic phase

Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. Schlieren (from German; singular "schliere" are optical inhomogeneities in transparent material not visible to the human eye Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional Liquid, and those of a Solid A chemical substance is a Material with a definite chemical composition. In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating For instance, a liquid crystal (LC) may flow like a liquid, but have the molecules in the liquid arranged and/or oriented in a crystal-like way. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by There are many different types of LC phases, which can be distinguished based on their different optical properties (such as birefringence). Birefringence, or double refraction, is the decomposition of a ray of Light into two rays (the ordinary ray and the extraordinary ray When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have a distinct texture. A microscope ( Greek: ( micron) = small + ( skopein) = to look or see is an instrument for viewing objects that are Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 In Materials science, texture is the distribution of crystallographic orientations of a sample Each "patch" in the texture corresponds to a domain where the LC molecules are oriented in a different direction. Within a domain, however, the molecules are well ordered. Liquid crystal materials may not always be in an LC phase (just as water is not always in the liquid phase: it may also be found in the solid and gas phase). Water is a common Chemical substance that is essential for the survival of all known forms of Life. In the Physical sciences a phase is a Set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties

Liquid crystals can be divided into thermotropic and lyotropic LCs. A Liquid crystal (LC is thermotropic if the order of its components is determined or changed by temperature A material is called lyotropic if it forms Liquid crystal phases because of the addition of a Solvent. Thermotropic LCs exhibit a phase transition into the LC phase as temperature is changed, whereas lyotropic LCs exhibit phase transitions as a function of concentration of the mesogen in a solvent (typically water) as well as temperature ^{[1][2] [3] [4] [5] [6] [7] [8]}. In Thermodynamics, phase transition or phase change is the transformation of a thermodynamic system from one phase to another Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance Mesogen is the fundamental unit of a Liquid crystal that induces structural order in the crystals A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature

History

In 1888 Austrian botanical physiologist Friedrich Reinitzer (1858-1927), working at the German University of Prague, was extracting cholesterol from carrots to establish its chemical formula. Plant physiology is a subdiscipline of Botany concerned with the function or Physiology, of Plants Closely related fields include Plant morphology Friedrich Richard Reinitzer ( February 25, 1857 in Prague - February 16, 1927 in Graz) was an Austrian botanist Charles University in Prague (also simply Charles University; Univerzita Karlova v Praze Universitas Carolina Karls-Universität zu Prag is the oldest and largest Cholesterol is a Lipid found in the Cell membranes and transported in the Blood plasma of all Animals It is an essential component of mammalian The carrot ( Daucus carota subsp sativus, Etymology: Middle French carotte, from Late Latin carōta, from Greek karōton Reinitzer examined the physico-chemical properties of various derivatives of the cholesterol. Other researchers had observed distinct color effects on cooling cholesterol derivatives just above the solidification temperature. For freezing as a method of food preservation see Frozen food. Reinitzer himself found the same phenomenon in cholesteryl benzoate, but the colors near the solidification of cholesteryl benzoate were not the most peculiar feature. Cholesteryl benzoate, also called 5-cholesten-3-yl benzoate, is an Organic chemical, an Ester of Cholesterol and Benzoic acid. Reinitzer found that cholesteryl benzoate does not melt like other compounds, but had two melting points. Melting is a process that results in the phase change of a substance from a Solid to a Liquid. The melting point of a solid is the temperature range at which it changes state from solid to Liquid. At 145. 5°C it melted into a cloudy liquid, and at 178. 5°C it melted again and the cloudy liquid became clear. The phenomenon was reversible. Seeking help from a physicist, on March 14, 1888, he wrote a letter to Otto Lehmann, at that time a Privatdozent in Aachen. Otto Lehmann (born January 13, 1855 in Konstanz, Germany; died June 17, 1922 in Karlsruhe) was a German Private docent (abbreviates PD or Priv-Doz) is a title conferred in some European university systems especially in German -speaking countries ( Ripuarian: Oche, Dutch: Aken, Spanish: Aquisgrán, Italian: Aquisgrana, French, They exchanged letters and samples. Lehmann examined the intermediate cloudy fluid, and reported seeing crystallites. A crystallite is a domain of solid-state matter that has the same structure as a single Crystal. Reinitzer's Viennese colleague von Zepharovich also indicated that the intermediate "fluid" was crystalline. The exchange of letters with Lehmann ended on April 24, with many questions unanswered. Reinitzer presented his results, with credits to Lehmann and von Zepharovich, at a meeting of the Vienna Chemical Society on May 3, 1888 ^[9].

By this time Reinitzer had discovered and described three important features of cholesteric liquid crystals (the name coined by Georges Friedel in 1922): the existence of two melting points, the reflection of circularly polarized light, and the ability to rotate the polarization direction of light. Georges Friedel (born 19 July 1865 in Mulhouse; died 11 December 1933 in Strasbourg) was a French mineralogist and crystallographer

After his accidental discovery, Reinitzer disappeared from the stage. Research was continued by Lehmann, who realised that he had encountered a new phenomenon and was in a position to launch an investigation of it. In his postdoctoral years he had acquired expertise in crystallography. The principal weapon in his scientific arsenal was experimental microscopy in which Lehmann was adept. It was Lehmann's microscope which had attracted Reinitzer's attention. Lehmann started a systematic study, first of cholesteryl benzoate, and then of related compounds which exhibited the double-melting phenomenon. He was able to make observations in polarised light, and his microscope possessed a hot stage enabling high temperature observations. The intermediate cloudy phase clearly sustained flow, but other features, particularly the signature under a microscope, convinced Lehmann that he was dealing with a solid. By the end of August 1889 he had his article ready for submission to the Zeitschrift für Physikalische Chemie ^[10].

Liquid crystals were not popular among scientists in the early 20th century and the material remained a pure scientific curiosity for about 80 years.

For more details on the history of liquid crystals see: Tim Sluckin, Ueber die Natur der kristallinischen Flüssigkeiten und flüssigen Kristalle (The early history of liquid crystals), Bunsen-Magazin, 7. Jahrgang, 5/2005 ^[11] and the History of Liquid Crystals website.

In 1969, Hans Kelker succeeded in synthesizing a substance that has a nematic phase at room temperature, MBBA (p-Methoxybenzyliden-p’-n-butylanilin), the well known "fruit-fly" of liquid crystal research ^[12]. The next step to commercialization of liquid crystal displays was the synthesis of further chemically stable substances (cyanobiphenyls) with low melting temperatures by George Gray ^[13].

In 1991, when liquid crystal displays were already well established in our everyday life, Pierre-Gilles de Gennes (1932-2007) received the Nobel Prize in physics "for discovering that methods developed for studying order phenomena in simple systems can be generalized to more complex forms of matter, in particular to liquid crystals and polymers". Pierre-Gilles de Gennes ( October 24, 1932 in Paris &ndash May 18, 2007 in Orsay) was a French Physicist An illustrative introduction into liquid crystals, their history and unique physical properties is available on Nobelprize.org.

Liquid crystal phases

The various LC phases (called mesophases) can be characterized by the type of ordering that is present. One can distinguish positional order (whether molecules are arranged in any sort of ordered lattice) and orientational order (whether molecules are mostly pointing in the same direction), and moreover order can be either short-range (only between molecules close to each other) or long-range (extending to larger, sometimes macroscopic, dimensions). Macroscopic is commonly used to describe physical objects that are measurable and observable by the Naked eye. Most thermotropic LCs will have an isotropic phase at high temperature. Isotropy is uniformity in all directions Precise definitions depend on the subject area That is, heating will eventually drive them into a conventional liquid phase characterized by random and isotropic molecular ordering (little to no long-range order), and fluid-like flow behavior. FLUID ( F ast L ight '''U'''ser '''I'''nterface D esigner is a graphical editor that is used to produce FLTK Source code Under other conditions (for instance, lower temperature), an LC might inhabit one or more phases with significant anisotropic orientational structure and short-range orientational order while still having an ability to flow. Anisotropy (pronounced with stress on the third syllable ˌænaɪˈsɒtrəpi is the property of being directionally dependent as opposed to Isotropy, which means homogeneity Structure is a fundamental and sometimes Intangible notion covering the Recognition, Observation, nature, and Stability of

The ordering of liquid crystalline phases is extensive on the molecular scale. This order extends up to the entire domain size, which may be on the order of micrometres, but usually does not extend to the macroscopic scale as often occurs in classical crystalline solids. Macroscopic is commonly used to describe physical objects that are measurable and observable by the Naked eye. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. However, some techniques (such as the use of boundaries or an applied electric field) can be used to enforce a single ordered domain in a macroscopic liquid crystal sample. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can The ordering in a liquid crystal might extend along only one dimension, with the material being essentially disordered in the other two directions. In mathematics the dimension of a Space is roughly defined as the minimum number of Coordinates needed to specify every point within it

Thermotropic liquid crystals

Thermotropic phases are those that occur in a certain temperature range. If the temperature is raised too high, thermal motion will destroy the delicate cooperative ordering of the LC phase, pushing the material into a conventional isotropic liquid phase. At too low a temperature, most LC materials will form a conventional (though anisotropic) crystal. Many thermotropic LCs exhibit a variety of phases as temperature is changed. For instance, a particular mesogen may exhibit various smectic and nematic (and finally isotropic) phases as temperature is increased. An example of a compound displaying thermotropic LC behavior is para-azoxyanisole. para -Azoxyanisole (PAA is a Zwitterionic Liquid crystal. In a solid state it appears as a white powder but when heated it forms a liquid crystal

Nematic phase

Schematic of mesogen ordering in a nematic phase.

One of the most common LC phases is the nematic, where the molecules have no positional order, but they have long-range orientational order. Thus, the molecules flow and their center of mass positions are randomly distributed as in a liquid, but they all point in the same direction (within each domain). Most nematics are uniaxial: they have one axis that is longer and preferred, with the other two being equivalent (can be approximated as cylinders). Some liquid crystals are biaxial nematics, meaning that in addition to orienting their long axis, they also orient along a secondary axis. A biaxial nematic is a spatially homogeneous Liquid crystal with three distinct optical axes

The word nematic comes from the Greek νημα, which means 'thread. ' This term originates from the thread-like topological defects observed in nematics, which are formally called 'disclinations. Also see base concepts Topology, Differential equations Quantum theory & Condensed matter physics. A Disclination is a Line defect in which Rotational symmetry is violated ' Nematics also exhibit so-called hedgehog topological defects. In Mathematics, a hedgehog space is a Topological space, consisting of a set of spines joined at a point

Nematics have fluidity similar to that of ordinary (isotropic) liquids but they can be easily aligned by an external magnetic or electric field. An aligned nematic has the optical properties of a uniaxial crystal and this makes them extremely useful in liquid crystal displays (LCD).

Smectic phases

Schematic of mesogen alignment in the smectic phases. The smectic A phase (left) has mesogens organized into layers. In the smectic C phase (right), the mesogens are tilted inside the layers.

The smectic phases, which are found at lower temperatures than the nematic, form well-defined layers that can slide over one another like soap. The smectics are thus positionally ordered along one direction. In the Smectic A phase, the molecules are oriented along the layer normal, while in the Smectic C phase they are tilted away from the layer normal. These phases are liquid-like within the layers. There is a very large number of different smectic phases, all characterized by different types and degrees of positional and orientational order.

Chiral phases

Schematic of mesogen ordering in chiral liquid crystal phases. The chiral nematic phase (left), also called the cholesteric phase, and the smectic C* phase (right).

The chiral nematic phase exhibits chirality (handedness). The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional Liquid, and those of a Solid The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image This phase is often called the cholesteric phase because it was first observed for cholesterol derivatives. A cholesteric liquid crystal is a type of Liquid crystal with a Helical structure and which is therefore chiral. Cholesterol is a Lipid found in the Cell membranes and transported in the Blood plasma of all Animals It is an essential component of mammalian Only chiral molecules (i. The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image e. : those that lack inversion symmetry) can give rise to such a phase. This phase exhibits a twisting of the molecules perpendicular to the director, with the molecular axis parallel to the director. The finite twist angle between adjacent molecules is due to their asymmetric packing, which results in longer-range chiral order. In the smectic C* phase (an asterisk denotes a chiral phase), the molecules have positional ordering in a layered structure (as in the other smectic phases), with the molecules tilted by a finite angle with respect to the layer normal. The chirality induces a finite azimuthal twist from one layer to the next, producing a spiral twisting of the molecular axis along the layer normal.

The chiral pitch, p, refers to the distance over which the mesogens undergo a full 360° twist (but note that the structure of the chiral nematic phase repeats itself every half-pitch, since in this phase directors at 0° and +/-180° are equivalent). The pitch, p, typically changes when the temperature is altered or when other molecules are added to the LC host (an achiral LC host material will form a chiral phase if doped with a chiral material), allowing the pitch of a given material to be tuned accordingly. In some liquid crystal systems, the pitch is of the same order as the wavelength of visible light. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. This causes these systems to exhibit unique optical properties, such as selective reflection, and these properties are exploited in a number of optical applications.

Blue Phases

Blue phases are special types of liquid crystal phases that appear in the temperature range between a chiral nematic phase and an isotropic liquid phase. The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional Liquid, and those of a Solid Isotropy is uniformity in all directions Precise definitions depend on the subject area Blue phases have a regular three-dimensional cubic structure of defects with lattice periods of several hundred nanometers, and thus they exhibit selective Bragg reflections in the wavelength range of light (visible part of electromagnetic radiation) corresponding to the cubic lattice. In Physics, Bragg's law is the result of experiments into the Diffraction of X-rays or neutrons off Crystal surfaces at certain angles Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 The cubic crystal system (or isometric) is a Crystal system where the Unit cell is in the shape of a Cube. Although blue phases are of interest for fast light modulators or tunable photonic crystals, the very narrow temperature range within which blue phases exist, usually less than a few kelvin, has always been a problem. Photonic crystals are periodic Optical (nanostructures that are designed to affect the motion of Photons in a similar way that periodicity of a Semiconductor The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Recently the stabilization of blue phases over a temperature range of more than 60 K including room temperature (260–326 K) could be demonstrated ^[14][15].

Furthermore, electro-optical switching with response times of the order of 10^-4 s for the stabilized blue phases at room temperature has been shown ^[16].

In May 2008 it was announced that the first Blue Phase Mode LCD panel had been developed. History In Reinitzer 's reports from 1888 on the melting behaviour of Cholesteryl benzoate there is a note that the substance briefly turned blue as it changed

Discotic phases

Disk-shaped mesogens can orient themselves in a layer-like fashion known as the discotic nematic phase. If the disks pack into stacks, the phase is called a discotic columnar. The columnar phase is a class of liquid-crystalline phases in which molecules assemble into cylindrical structures to act as Mesogens Originally these kinds of liquid The columns themselves may be organized into rectangular or hexagonal arrays. Chiral discotic phases, similar to the chiral nematic phase, are also known.

Lyotropic liquid crystals

A lyotropic liquid crystal consists of two or more components that exhibit liquid-crystalline properties in certain concentration ranges. A liquid crystalline material is called lyotropic if phases having long-ranged orientational order are induced by the addition of a Solvent. In the lyotropic phases, solvent molecules fill the space around the compounds to provide fluidity to the system. A material is called lyotropic if it forms Liquid crystal phases because of the addition of a Solvent. A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. In contrast to thermotropic liquid crystals, these lyotropics have another degree of freedom of concentration that enables them to induce a variety of different phases.

A compound which has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule. Hydrophile, from the Greek (hydros "water" and φιλια (philia "friendship" refers to a physical property of a Molecule In Chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) refers to the physical property of Amphiphile (from the Greek αμφις amphis both and φιλíα Philia: love friendship is a term describing a Chemical compound possessing both Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences depending on the volume balances between the hydrophilic part and hydrophobic part. These structures are formed through the micro-phase segregation of two incompatible components on a nanometer scale. Soap is an everyday example of a lyotropic liquid crystal.

The content of water or other solvent molecules changes the self-assembled structures. At very low amphiphile concentration, the molecules will be dispersed randomly without any ordering. At slightly higher (but still low) concentration, amphiphilic molecules will spontaneously assemble into micelles or vesicles. A micelle (rarely micella, plural micelles) is an aggregate of Surfactant molecules dispersed in a liquid Colloid. A vesicle is a small bubble of liquid within a cell A more formal definition in Cell biology, would be that a vesicle is a relatively small intracellular membrane-enclosed This is done so as to 'hide' the hydrophobic tail of the amphiphile inside the micelle core, exposing a hydrophilic (water-soluble) surface to aqueous solution. These spherical objects do not order themselves in solution, however. At higher concentration, the assemblies will become ordered. A typical phase is a hexagonal columnar phase, where the amphiphiles form long cylinders (again with a hydrophilic surface) that arrange themselves into a roughly hexagonal lattice. This is called the middle soap phase. At still higher concentration, a lamellar phase (neat soap phase) may form, wherein extended sheets of amphiphiles are separated by thin layers of water. For some systems, a cubic (also called viscous isotropic) phase may exist between the hexagonal and lamellar phases, wherein spheres are formed that create a dense cubic lattice. These spheres may also be connected to one another, forming a bicontinuous cubic phase.

The objects created by amphiphiles are usually spherical (as in the case of micelles), but may also be disc-like (bicelles), rod-like, or biaxial (all three micelle axes are distinct). These anisotropic self-assembled nano-structures can then order themselves in much the same way as liquid crystals do, forming large-scale versions of all the thermotropic phases (such as a nematic phase of rod-shaped micelles).

For some systems, at high concentration, inverse phases are observed. That is, one may generate an inverse hexagonal columnar phase (columns of water encapsulated by amphiphiles) or an inverse micellar phase (a bulk liquid crystal sample with spherical water cavities).

A generic progression of phases, going from low to high amphiphile concentration, is:

• Discontinuous cubic phase (micellar cubic phase)
• Hexagonal phase (hexagonal columnar phase) (middle phase)
• Bicontinuous cubic phase
• Lamellar phase
• Bicontinuous cubic phase
• Reverse hexagonal columnar phase
• Inverse cubic phase (Inverse micellar phase)

Even within the same phases, their self-assembled structures are tunable by the concentration: for example, in lamellar phases, the layer distances increase with the solvent volume. A micellar cubic phase is a Lyotropic liquid crystal phase formed when the concentration of Micelles dispersed in a solvent (usually water is sufficiently high that A hexagonal phase of Lyotropic liquid crystal is formed by some amphiphilic molecules when they are mixed with water or another polar solvent The cubic crystal system (or isometric) is a Crystal system where the Unit cell is in the shape of a Cube. The cubic crystal system (or isometric) is a Crystal system where the Unit cell is in the shape of a Cube. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals.

Similar phases and characteristics can be observed in immiscible diblock copolymers. A heteropolymer or copolymer is a Polymer derived from two (or more Monomeric species as opposed to a Homopolymer where only one monomer is

Metallotropic liquid crystals

Liquid crystal phases can also be based on low-melting inorganic phases like ZnCl₂ that have a structure formed of linked tetrahedra and easily form glasses. The addition of long chain soaplike molecules leads to a series of new phases that show a variety of liquid crystalline behavior both as a function of the inorganic-organic composition ratio and of temperature. This class of materials has been named metallotropic ^{J.D. Martin et al.}

Biological liquid crystals

Lyotropic liquid-crystalline phases are abundant in living systems, the study of which is referred to as polymorphism. Polymorphism in biophysics is the aspect of the behaviour of Lipids that influences their long-range order i Accordingly, lyotropic liquid crystals attract particular attention in the field of biomimetic chemistry. In particular, biological membranes and cell membranes are a form of liquid crystal. MembraneA biological membrane or biomembrane is an enclosing or separating Amphipathic layer that acts as a barrier within or around a cell. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Their constituent molecules (e. g. , phospholipids) are perpendicular to the membrane surface, yet the membrane is capable of a range of elastic stress, leading to some aspects of elastic behaviour to be exhibited. Phospholipids are a class of Lipids and are a major component of all Biological membranes All phospholipids contain a Diglyceride, a Phosphate These lipids vary in shape (see page on lipid polymorphism). Polymorphism in biophysics is the aspect of the behaviour of Lipids that influences their long-range order i The constituent molecules can inter-mingle easily, but tend not to leave the membrane due to the high energy requirement of this process. Lipid molecules can flip from one side of the membrane to the other, this process being catalysed by flippases and floppases (depending on the direction of movement). These liquid crystal membrane phases can also host important proteins such as receptors freely "floating" inside, or partly outside, the membrane, e. g. CCT.

Many other biological structures exhibit LC behaviour. For instance, the concentrated protein solution that is extruded by a spider to generate silk is, in fact, a liquid crystal phase. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Spider silk, also known as Gossamer, is a Protein Fiber spun by Spiders Spiders use their silk to make webs or other structures which function The precise ordering of molecules in silk is critical to its renowned strength. DNA and many polypeptides can also form LC phases and this too forms an important part of current academic research. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino

Pattern formation in liquid crystals

Anisotropy of liquid crystals is a property not observed in other fluids. The science of pattern formation deals with the visible ( statistically) orderly outcomes of Self-organisation and the common principles behind similar This anisotropy makes flows of liquid crystals to behave much differentially than ordinary fluids. For example, injecting a flux of a liquid crystal between two close parallel plates (viscous fingering), orientation of molecules couples with the flow, with the result of emergence of dendritic patterns. Viscous fingering is the formation of patterns in a morphologically unstable interface between two fluids in a porous medium or in a Hele-Shaw cell ^[17]. This anisotropy is also manifested in the interfacial energy (surface tension) between different liquid crystal phases. For the work of fiction see Surface Tension (short story. Surface tension is a property of the surface of a Liquid that causes it to This anisotropy determines the equilibrium shape at the coexistence temperature, and is so strong that usually facets appear. When temperature is changed one of the phases grows, forming different morphologies depending on the temperature change. ^[18]

Theoretical treatment of liquid crystals

Microscopic theoretical treatment of fluid phases can become quite involved, owing to the high material density, which means that strong interactions, hard-core repulsions, and many-body correlations cannot be ignored. In the case of liquid crystals, anisotropy in all of these interactions further complicates analysis. There are a number of fairly simple theories, however, that can at least predict the general behavior of the phase transitions in liquid crystal systems.

Order parameter

The local nematic director which is also the local optical axis is given by the spatial and temporal average of the long molecular axes over a small element of volume and time.

The description of liquid crystals involves an analysis of order. To make this quantitative, an orientational order parameter is usually defined based on the average of the second Legendre polynomial:

where θ is the angle between the mesogen molecule axis and the local director (which is the 'preferred direction' in a volume element of a liquid crystal sample, also representing its local optical axis). Note People sometimes refer to the more general Associated Legendre polynomials as simply Legendre polynomials. This definition is convenient, since for a completely random and isotropic sample, S=0, whereas for a perfectly aligned sample S=1. For a typical liquid crystal sample, S is on the order of 0. 3 to 0. 8, and generally decreases as the temperature is raised. In particular, a sharp drop of the order parameter to 0 is observed when the system undergoes a phase transition from an LC phase into the isotropic phase. The order parameter can be measured experimentally in a number of ways. For instance, diamagnetism, birefringence, Raman scattering, NMR and EPR can also be used to determine S. Diamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied Magnetic field, thus causing a repulsive effect Birefringence, or double refraction, is the decomposition of a ray of Light into two rays (the ordinary ray and the extraordinary ray Raman scattering or the Raman effect (pronounced — is the inelastic scattering of a Photon. Electron paramagnetic resonance (EPR or electron spin resonance (ESR Spectroscopy is a technique for studying Chemical species that have one or more unpaired

One could also characterize the order of a liquid crystal using other even Legendre polynomials (all the odd polynomials average to zero since the director can point in either of two antiparallel directions). These higher-order averages are more difficult to measure, but can yield additional information about molecular ordering.

Onsager hard-rod model

A very simple model which predicts lyotropic phase transitions is the hard-rod model proposed by Lars Onsager. Lars Onsager ( November 27, 1903 &ndash October 5, 1976) was a Norwegian – American physical chemist and This theory considers the volume excluded from the center-of-mass of one idealized cylinder as it approaches another. Specifically, if the cylinders are oriented parallel to one another, there is very little volume that is excluded from the center-of-mass of the approaching cylinder (it can come quite close to the other cylinder). If, however, the cylinders are at some angle to one another, then there is a large volume surrounding the cylinder where the approaching cylinder's center-of-mass cannot enter (due to the hard-rod repulsion between the two idealized objects). Thus, this angular arrangement sees a decrease in the net positional entropy of the approaching cylinder (there are fewer states available to it). In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy

The fundamental insight here is that, while parallel arrangements of anisotropic objects leads to a decrease in orientational entropy, there is an increase in positional entropy. Thus in some case greater positional order will be entropically favorable. This theory thus predicts that a solution of rod-shaped objects will undergo a phase transition, at sufficient concentration, into a nematic phase. Recently this theory is used to observe the phase transition between nematic and smectic-A at very high concentration also ^{Hanif et al.}. Although this model is conceptually helpful, its mathematical formulation makes several assumptions that limit its applicability to real systems.

Maier-Saupe mean field theory

This statistical theory includes contributions from an attractive intermolecular potential. The anisotropic attraction stabilizes parallel alignment of neighboring molecules, and the theory then considers a mean-field average of the interaction. A Many-body system with interactions is generally very difficult to solve exactly except for extremely simple cases ( Gaussian field theory, 1D Ising model. Solved self-consistently, this theory predicts thermotropic phase transitions, consistent with experiment.

Elastic continuum theory

In this formalism, a liquid crystal material is treated as a continuum; molecular details are entirely ignored. Rather, this theory considers perturbations to a presumed oriented sample. One can identify three types of distortions that could occur in an oriented sample: (1) twists of the material, where neighboring molecules are forced to be angled with respect to one another, rather than aligned; (2) splay of the material, where bending occurs perpendicular to the director; and (3) bend of the material, where the distortion is parallel to the director and mesogen axis. All three of these types of distortions incur an energy penalty. They are defects that often occur near domain walls or boundaries of the enclosing container. The response of the material can then be decomposed into terms based on the elastic constants corresponding to the three types of distortions. Elastic continuum theory has been shown to be a particularly powerful tool for modelling liquid crystal devices.

Effect of chirality

As already described, chiral mesogens usually give rise to chiral mesophases. The term chiral (pronounced /ˈkaɪɹ(əl̩/ is used to describe an object that is non- superimposable on its mirror image For molecular mesogens, this means that the molecule must possess some form of asymmetry, usually a stereogenic center. A stereocenter, or stereogenic centre, is any Atom in a Molecule bearing groups such that an interchanging of any two groups leads to a Stereoisomer An additional requirement is that the system not be racemic: a mixture of right- and left-handed versions of the mesogen will cancel the chiral effect. In Chemistry, a racemic mixture, or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral Due to the cooperative nature of liquid crystal ordering, however, a small amount of chiral dopant in an otherwise achiral mesophase is often enough to select out one domain handedness, making the system overall chiral.

Chiral phases usually have a helical twisting of the mesogens. If the pitch of this twist is on the order of the wavelength of visible light, then interesting optical interference effects can be observed. The chiral twisting that occurs in chiral LC phases also makes the system respond differently to right- and left-handed circularly polarized light. These materials can thus be used as polarization filters.

It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana liquid crystals) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. The appearance mechanism of this macroscopic chirality is not yet entirely clear. It appears that the molecules stack in layers and orient themselves in a tilted fashion inside the layers. These liquid crystals phases may be ferroelectric or anti-ferroelectric, both of which are of interest for applications. Ferroelectricity is a physical property of a material whereby it exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent Ferroelectricity is a physical property of a material whereby it exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent

Chirality can also be incorporated into a phase by adding a chiral dopant, which needs not necessarily be mesogenic itself. A dopant, also called doping agent and dope, is an impurity element added to a crystal or semiconductor lattice in low concentrations in order to alter the optical/electrical TN or STN mixtures often contain a small amount of such dopants.

Applications of liquid crystals

Liquid crystals find wide use in liquid crystal displays, which rely on the optical properties of certain liquid crystalline substances in the presence or absence of an electric field. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can In a typical device, a liquid crystal layer (typically 10 μm thick) sits between two polarizers that are crossed (oriented at 90° to one another). Polarization ( ''Brit'' polarisation) is a property of Waves that describes the orientation of their oscillations The liquid crystal alignment is chosen so that its relaxed phase is a twisted one (hence: Twisted nematic field effect). The twisted nematic effect ( TN-effect) in Liquid crystals is claimed to be first discovered by James Fergason in 1970 at the International Liquid Xtal This twisted phase reorients light that has passed through the first polarizer, allowing it to be transmitted through the second polarizer (and reflected back to the observer if a reflector is provided). The device thus appears transparent. When an electric field is applied to the LC layer, the long molecular axes tend to align parallel to the electric field thus gradually untwisting in the center of the liquid crystal layer. In this state, the mesogens do not reorient light, so the light polarized at the first polarizer is absorbed at the second polarizer, and the device loses transparency with increasing voltage. In this way, the electric field can be used to make a pixel switch between transparent or opaque on command. Color LCD systems use the same technique, with color filters used to generate red, green, and blue pixels. Similar principles can be used to make other liquid crystal based optical devices.

Thermotropic chiral LCs whose pitch varies strongly with temperature can be used as crude thermometers, since the color of the material will change as the pitch is changed. Liquid crystal color transitions are used on many aquarium and pool thermometers. Other liquid crystal materials change color when stretched or stressed. Thus, liquid crystal sheets are often used in industry to look for hot spots, map heat flow, measure stress distribution patterns, and so on. Liquid crystal in fluid form is used to detect electrically generated hot spots for failure analysis in the semiconductor industry. Failure analysis is the process of collecting and analyzing data to determine the cause of a Failure and how to prevent it from recurring A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Liquid crystal memory units with extensive capacity were used in Space Shuttle navigation equipment.

It is also worth noting that many common fluids are in fact liquid crystals. Soap, for instance, is a liquid crystal, and forms a variety of LC phases depending on its concentration in water.

See also

• Chromonic
• Liquid crystal display
• Liquid crystal polymer
• Lyotropic liquid crystal
• Molecular modelling
• Plastic crystallinity
• Smart glass
• Thermochromics

References

1. ^ de Gennes, P.G. and Prost, J (1993). In a chromonic, relatively flat molecules form linear aggregates Background Liquid crystallinity in polymers may occur either by solving a polymer in a solvent (lyotropic liquid crystal A liquid crystalline material is called lyotropic if phases having long-ranged orientational order are induced by the addition of a Solvent. Molecular modelling is a collective term that refers to theoretical methods and computational techniques to model or mimic the behaviour of Molecules The techniques Plastic crystallinity is a phenomenon exhibited by many materials that form molecular solids with relatively weak interaction between the molecules usually just van der Waals forces Smart glass or switchable glass, also called smart windows or switchable windows in its application to Windows or Skylights refers Thermochromism is the ability of Substance to change Color due to a change in Temperature. Pierre-Gilles de Gennes ( October 24, 1932 in Paris &ndash May 18, 2007 in Orsay) was a French Physicist The Physics of Liquid Crystals. Oxford: Clarendon Press. ISBN 0-19-852024-7.  
2. ^ Chandrasekhar, S. (1992). Liquid Crystals, 2^nd edition, Cambridge: Cambridge University Press. ISBN 0-521-41747-3 (hardback) ISBN 0-521-42741-X (pbk. ).  
3. ^ T. J. Sluckin, D. A. Dunmur and H. Stegemeyer (2004). Crystals That Flow - classic papers from the history of liquid crystals. London: Taylor & Francis. ISBN 0-415-25789-1.  
4. ^ I. Dierking (2003). Textures of Liquid Crystals. Weinheim: Wiley-VCH. ISBN 3-527-30725-7.  
5. ^ Kleinert, H. and Maki, K (1981). Hagen Kleinert (born 1941 is Professor of Theoretical Physics at the Free University of Berlin, Germany (since 1968 Honorary Professor at the Kyrgyz-Russian "Lattice Textures in Cholesteric Liquid Crystals". Fortschritte Physik 29 (1).  
6. ^ Collings, P. J. and Hird, M (1997). Introduction to Liquid Crystals. Bristol, PA: Taylor & Francis. ISBN 0-7484-0643-3 (hardback) ISBN 0-7484-0483-X (pbk. ).  
7. ^ James D. Martin, Cristin L. Keary, Todd A. Thornton, Mark P. Novotnak, Jeremey W. Knutson and Jacob C. W. Folmer (2006). "Metallotropic liquid crystals formed by surfactant templating of molten metal halides". Nature Materials 5: 271–275.  
8. ^ Hanif Bayat Movahed, Raul Cruz Hidalgo and D. E. Sullivan (March 2006). "The phase transitions of semiflexible hard sphere chains liquid". Phys. Rev. E 73, 032701.  
9. ^ F. Reinitzer, Beiträge zur Kenntniss des Cholesterins, Monatshefte für Chemie (Wien) 9, 421-441 (1888)
10. ^ O. Lehmann, Über fliessende Krystalle, Zeitschrift für Physikalische Chemie 4, 462-72 (1889)
11. ^ Tim Sluckin, "Ueber die Natur der kristallinischen Flüssigkeiten und flüssigen Kristalle" (The early history of liquid crystals), Bunsen-Magazin, 7. Jahrgang, 5/2005
12. ^ H. Kelker, B. Scheurle, Angew. Chem. Int. Ed. 8 (1969), 884
13. ^ G. W. Gray, K. J. Harrison, J. A. Nash, Electronics Lett. 9 (1973) 130
14. ^ Harry J. Coles, Mikhail N. Pivnenko, Liquid crystal 'blue phases' with a wide temperature range, Nature 436, 997-1000 (18, August 2005)
15. ^ Jun Yamamoto, Isa Nishiyama1, Miyoshi Inoue and Hiroshi Yokoyama, Optical isotropy and iridescence in a smectic 'blue phase', Nature 437, 525-528 (22, September 2005)
16. ^ Kikuchi H, Yokota M, Hisakado Y, Yang H, Kajiyama T. , Polymer-stabilized liquid crystal blue phases, Nature Materials, Volume 1, Issue 1, pp. 64-68 (2002)
17. ^ A. Buka et al., "Viscous fingering in liquid crystals", Phys. Rev. A 36, 3984 - 3989 (1987)]
18. ^ "Phase-field simulations and experiments of faceted growth in liquid crystal" R. Gonzalez-Cinca et al., Physica D 99, 359-368 (1996)

External links

• History and Properties of Liquid Crystals by Nobelprize.org
• Definitions of basic terms relating to low-molar-mass and polymer liquid crystals (IUPAC Recommendations 2001)
• An intelligible introduction to liquid crystals from Case Western Reserve University
• Liquid Crystal Physics tutorial from the Liquid Crystals Group, University of Colorado
• Introduction to liquid crystals from the Liquid Crystal Technology Group, Oxford University
• Liquid Crystals & Photonics Group - Ghent University (Belgium), good tutorial
• Liquid crystals Liquid Crystals Interactive Online (not updated since 1999)
• Liquid Crystal Institute Kent State University
• Liquid Crystals a journal by Taylor&Francis
• Molecular Crystals and Liquid Crystals a journal by Taylor & Francis
• Hot-spot detection techniques for ic's
• What are liquid crystals? from Chalmers University of Technology, Sweden